Vintage Synth Repair

----- Original Message -----

From: Adrian Corston

To: vintagesynthrepair@yahoogroups.com

Sent: Friday, July 19, 2013 11:44 AM

Subject: [vintagesynthrepair] CS-70m power supply issue, -15V rail is dead

My CS-70m (which last worked about 4-5 years ago) has a power supply fault. With the power supply board disconnected from all voice and control circuitry I see the following voltages on the various test pins:

+5 = +4.95V
-5 = -4.10V
+10 = +8.27V
-15 = +0.616V
+15 = +17.64V

The +15' and -15' voltages (used to drive the op-amp ICs in the supply that I measured on pins 4 and 8 of each of IC1, IC2 and IC3) test at:

-15' = -14.22V
+15' = +14.23V

I have checked voltages starting at the transformer secondaries and working my way in and it all looks good up to the collector of the regulator transistor Tr20 (2SB688) which measures -26V. On the emitter (output) it measures +0.616V, as compared to Tr15 which measures +26V on the collector and +15V on the emitter. So I replaced Tr20 with a 2SA1264N, which is nearly identical according to the data sheets and documented as an acceptable substitute. Unfortunately this resulted in no change, so presumably it is not the cause of the fault. All measurements were made on the transistor legs directly, so it's not the wiring between Tr20 (which is mounted on a electrically non-conductive heatsink) and the board at fault.

From that point on I'm starting to move out of my knowledge zone as far as power supply circuits are concerned - obviously the fault lies in some other component or components, but I'm not sure how to identify which ones (because I don't know how the rest of the circuit works). What do I do next, other than replace ALL of the components that make up the -15V regulator section of the supply?

The relevant schematic is here (sorry for the resolution and the poor splicing attempt, it's the best copy I could find): CS-70m PS schematic

Thanks in advance for any help you can offer!

Cheers,
A.

>
>Cheers,
>A.
>

>
> I'd also check the electrolytic on the emitter of TR20 isn't pulling anything to ground
> 
> 
> 
> 
> ________________________________
>  From: Peter Mörck <peter@...>
> To: vintagesynthrepair@yahoogroups.com 
> Sent: Friday, 19 July 2013, 18:20
> Subject: Re: [vintagesynthrepair] CS-70m power supply issue, -15V rail is dead
>  
> 
> 
> Â  
> Hi,
> Â 
> The -15' and +15' values are probably right as 
> they're derived from a zener diode + a PN drop, so 15-0.7 and -15+0.7, roughly. 
> Sounds about right.
> Â 
> From my limited knowledge I react on the voltage 
> 0.6V because it sounds like a PN-junction voltage drop.
> So I would guess that something, coming from 
> 0V/ground, via a reverse PN-junction, reaches the emitter of Tr20.
> Â 
> For example, its own base-emitter junction, if the 
> base of Tr20 is 0V.
> Is it?
> Â 
> Also switch it off and check for a short to ground 
> from Tr20 base.
> Â 
> If IC3 is socketed you could try replacing 
> that. Who knows what will happen if that's fried...
> Â 
> Also check for shorts between Collector-Emitter on 
> Tr22 and Tr23.
> Â 
> Not sure if all or any of that is actually 
> meaningful to do, but that's what I'd do at least...
> Â 
> Maybe some electronics wizard has better 
> suggestions :)
> Â 
> BR,
> Peter
> Â 
> Â 
> ----- Original Message ----- 
> >From: Adrian  Corston 
> >To: vintagesynthrepair@yahoogroups.com 
> >Sent: Friday, July 19, 2013 11:44 
> AM
> >Subject: [vintagesynthrepair] CS-70m  power supply issue, -15V rail is dead
> >
> >Â  
> >My CS-70m (which last worked about 4-5 years ago) has a power supply  fault.  With the power supply board disconnected from all voice and  control circuitry I see the following voltages on the various test  pins:
> >
> >+5 = +4.95V
> >-5 = -4.10V
> >+10 = +8.27V
> >-15 = 
>   +0.616V
> >+15 = +17.64V
> >
> >The +15' and -15' voltages (used to drive the 
>   op-amp ICs in the supply that I measured on pins 4 and 8 of each of IC1, IC2 
>   and IC3) test at:
> >
> >-15' = -14.22V
> >+15' = +14.23V
> >
> >I have 
>   checked voltages starting at the transformer secondaries and working my way in 
>   and it all looks good up to the collector of the regulator transistor Tr20 
>   (2SB688) which measures -26V.  On the emitter (output) it measures 
>   +0.616V, as compared to Tr15 which measures +26V on the collector and +15V on 
>   the emitter.  So I replaced Tr20 with a 2SA1264N, which is nearly 
>   identical according to the data sheets and documented as an acceptable 
>   substitute.  Unfortunately this resulted in no change, so presumably it 
>   is not the cause of the fault.  All measurements were made on the 
>   transistor legs directly, so it's not the wiring between Tr20 (which is 
>   mounted on a electrically non-conductive heatsink) and the board at 
>   fault.
> >
> >From that point on I'm starting to move out of my knowledge zone 
>   as far as power supply circuits are concerned - obviously the fault lies in 
>   some other component or components, but I'm not sure how to identify which 
>   ones (because I don't know how the rest of the circuit works).  What do I 
>   do next, other than replace ALL of the components that make up the -15V 
>   regulator section of the supply?
> >
> >The relevant schematic is here (sorry 
>   for the resolution and the poor splicing attempt, it's the best copy I could 
>   find): CS-70m PS  schematic
> >
> >Thanks in advance for any help you can 
>   offer!
> >
> >Cheers,
> >A.
> >
>

I somehow don't quite understand:

You're getting both +15V and -15V at the opamps, but you assume that
something is wrong with the power supply because the test point in the
supply itself doesn't give you -15V?
I would guess it's probably the wrong test point then.

I'll be having another look at the unit tonight and will try out the excellent suggestions received.

Change ANY electrolytic capacitors in that -15 circuit. *done*

Diode "check" any diodes ( make sure they conduct one way and NOT the other) *I don't see any in the -15V circuit*

If there are any tantalum capacitors ( usually teardrop shape) replace them too *not relevant - there are none*

The -15' and +15' values are probably right as they're derived from a zener diode + a PN drop, so 15-0.7 and -15+0.7, roughly.
Sounds about right. *OK*

From my limited knowledge I react on the voltage 0.6V because it sounds like a PN-junction voltage drop.
So I would guess that something, coming from 0V/ground, via a reverse PN-junction, reaches the emitter of Tr20.
For example, its own base-emitter junction, if the base of Tr20 is 0V. Is it? *no, base is > 0V*

Also switch it off and check for a short to ground from Tr20 base. *no short found*

If IC3 is socketed you could try replacing that. Who knows what will happen if that's fried... *IC3 replaced (and socketed for next time)*

Also check for shorts between Collector-Emitter on Tr22 and Tr23. *no shorts found*

I'd also check the electrolytic on the emitter of TR20 isn't pulling anything to ground *electrolytic replaced*

Also check for any shorted tantalum caps throughout the synth... *not relevant, rest of synth is disconnected*

5. No voltage is put out for -15V. Other voltage positions operate correctly.
Probably cause: Tr21 - Tr20 - 0.22ohms.1P *tr20 replaced, tr21 replacement pending, 0.22ohm resistor measures OK*

----- Original Message -----

From: Adrian Corston

To: vintagesynthrepair@yahoogroups.com

Sent: Saturday, July 20, 2013 5:31 PM

Subject: Re: [vintagesynthrepair] CS-70m power supply issue, -15V rail is dead

On 20/07/13 5:50 PM, Adrian Corston wrote:

I'll be having another look at the unit tonight and will try out the excellent suggestions received.

Here we go... my responses are in red in the quoted text below.

Change ANY electrolytic capacitors in that -15 circuit. *done*

Diode "check" any diodes ( make sure they conduct one way and NOT the other) *I don't see any in the -15V circuit*

If there are any tantalum capacitors ( usually teardrop shape) replace them too *not relevant - there are none*

The -15' and +15' values are probably right as they're derived from a zener diode + a PN drop, so 15-0.7 and -15+0.7, roughly.
Sounds about right. *OK*

From my limited knowledge I react on the voltage 0.6V because it sounds like a PN-junction voltage drop.
So I would guess that something, coming from 0V/ground, via a reverse PN-junction, reaches the emitter of Tr20.
For example, its own base-emitter junction, if the base of Tr20 is 0V. Is it? *no, base is > 0V*

Also switch it off and check for a short to ground from Tr20 base. *no short found*

If IC3 is socketed you could try replacing that. Who knows what will happen if that's fried... *IC3 replaced (and socketed for next time)*

Also check for shorts between Collector-Emitter on Tr22 and Tr23. *no shorts found*

I'd also check the electrolytic on the emitter of TR20 isn't pulling anything to ground *electrolytic replaced*

Also check for any shorted tantalum caps throughout the synth... *not relevant, rest of synth is disconnected*

5. No voltage is put out for -15V. Other voltage positions operate correctly.
Probably cause: Tr21 - Tr20 - 0.22ohms.1P *tr20 replaced, tr21 replacement pending, 0.22ohm resistor measures OK*

None of the above things that I tried had any effect; the -15V rail is still sitting at around 0.6V.

I have no 2SB560 (or well-documented substitute) handy for tr21, but I did find a 2SA1534A (in another old power supply) which has a similar spec. The 2SA1534A is only 60V rather than 100V, but I assume that won't matter in a -15V circuit (I only measured 26V for Vce in the circuit with the 2SB560 in place). Pinouts are the same for the two parts. However according to my wonderfully-useful transistor comparison book it is not "lo-sat" (low collector-emitter saturation voltage) whereas the 2SB560 is. According to data sheets:

2SB560 Vce(sat) = -0.3V typ, -0.8V max
2SA1534A Vce(sat) = -0.2V typ, -0.4V max

Datasheets:
2SB560: http://www.futurlec.com/Transistors/2SB560.shtml
2SA1534A: http://www.alldatasheet.com/datasheet-pdf/pdf/12706/PANASONIC/2SA1534.html

This looked near enough to me, so I tried substituting it. Unfortunately the 1K resistor between tr20/tr21 collectors and tr22/23 collectors immediately started getting too hot to touch, so I removed power straight away. I will try again next week after I find a real 2SB560.

Thanks again,
A.

>
> Thanks - yes, this is what I use now, but it is so fiddly, and often I  
> need to sit there for a while to make them come loose.
> Maybe I am too afraid to break the plasctic sockets.
> 
> Terje
> 
> 
> Den 22. juli. 2013 kl. 21.50 skrev John Rose:
> 
> >
> > If they are the one's I think they are I use a very small flat blade  
> > screwdriver, jewellers screwdrivers are ideal as you can get very  
> > close to the ideal size.
> >
> >
> >
> > Many thanks
> >
> >
> >
> > John Rose
> >
> >
> >
> > (www.astrodevelopments.com)
> >
> >
> >
> > From: vintagesynthrepair@yahoogroups.com [mailto:vintagesynthrepair@yahoogroups.com 
> > ] On Behalf Of Terje Winther
> > Sent: 22 July 2013 18:22
> > To: vintagesynthrepair@yahoogroups.com
> > Subject: [vintagesynthrepair] moog flag tool
> >
> >
> >
> >
> >
> > Hi,
> >
> > I don´t suppose anyone have an extra "moog flag tool"?
> >
> > This is the minimoog "flag tool" that you use to release the small  
> > metal "flags" that are inside those white plastic AMP multicontacts  
> > that holds all the PCBs. Used in some other synths as well, but the  
> > minimoog is infamous for these, since you need to remove them and  
> > solder the cables (originally they are only crimped).
> >
> > So what are people using these days? It says in the service manual  
> > that "if you are careful, you can use a paper clip".
> >
> > Well, I tried, but I never seem to get the feeling of to do this.
> >
> > Any hints?
> >
> >
> >
> > Terje Winther
> >
> > terje.winther@...
> >
> > http://wintherstormer.no/
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > 
> 
> Terje Winther
> terje.winther@...
> http://wintherstormer.no/
>

On 22/07/13 4:39 AM, Peter Mörck wrote:

I would expect it to be around -15.7V or so when the circuit is operating properly. Exactly how much is it?

How much is base voltage of Tr21, Tr22 and Tr23?

Check if D7 is good.

On 22/07/13 6:31 AM, Hugh Vartanian wrote:

Hi,

A little about the operation of this circuit when operating normally. Hopefully there is a hint in there that will get to the bottom of this! Note, no distinction is made between what is obvious and what is less so in the description. Also, the ideas of ;'more' or 'less' voltage or current are a little funny when talking about positive and negative values. Generally the conversation uses absolute values or magnitudes when using such language.:

1. Tr20 is the main pass transistor, across which is the difference between the unregulated voltage at the collector and the regulated side at the emitter. When it's base is pulled negative, current flows through from the emitter turning on the transistor and pulling the (amplified) current through the collector, driving the emitter to a negative voltage. If there is an open circuit in this transistor, there will be no output. (see below)

2. Tr21 functions as another amplifier (a 'darlington' connected transistor pair with Tr 20) to pull base current out of Tr20, with the low currents being pulled by the rest of the circuit. So, pull Tr21 base negative, amplify that to pull current through Tr20 base, and give us the juice for the -15V supply.

3. If one takes Tr22 and Tr23 out of the circuit (disconnect their collectors), then Tr21 turns on with the current through the 1K resistor, which turns on Tr20. In this situation the emitter of Tr 20 will be at the unregulated supply voltage (minus a few tenths of a volt.). (IF the rest of the synth is disconnected from the -15V supply, then one could do this test to check out Tr20 and Tr21, but I don't think we need to try this just yet and -26V cannot get to the rest of the synth circuitry safely!.)

4. We want regulation of the output voltage, so we must have a way of draining off the current from the 1K resistor and not sending it into the pass transistor(s). Tr22 or Tr 23 do this.

5. First off, Tr22 is there to keep the -15V supply off until the +15V supply is turned on. If negative supply is on and the positive supply is off then Tr22 is turned on through the 47K resistor on its base. This pulls the current from the 1K resistor to ground, keeping it from turning on Tr21. When the positive supply comes up, then Tr22 is shut off by the slight positive voltage on the D7/27K resistor junction. If the voltage at the D7/27K/47K/Tr22Base junction is not positive several tenths of a volt when power is up, then something is the matter in the upper circuit, probably an open 27K resistor (if the 1K above it was open, then the positive output wouldn't be working.).

6. Next is the regulation function. The regulator operates to keep the -15V output a mirror of the +15V output. Pin 3 of IC3 is connected to the junction of 2x 10K resistors, one to +15V and one to -15V. IC 3 will drive through Tr23/Tr21/Tr20 until the junction at the 10K resistors is almost exactly 0V, halfway between + and -15V. If that voltage is positive, meaning +15 is bigger than -(-15V), then we need 'more' -15V. The positive voltage on pin 3 is amplified by the op-amp, making pin 1 more positive, turning off Tr23 and letting more current from the 1K go into the Tr21/Tr20 transistors, giving us more -15V. If the voltage on pin 3 is too negative, i.e. -(-15V) is greater than +15V, then the output of IC3 goes negative, which turns on Tr23 more and shuts off Tr21/Tr20 more, giving us less drive and a 'lower' -15V. By this action, the -15V is kept at a mirror of +15V.

7. It looks like the .22 ohm resistor ought to be part of a current limiting circuit, but I don't see that represented in the schematic. Measured as not open, so it is fine.

8. If something was dragging down -15V, Tr20 would be very hot. Somehow we are just not getting juice through Tr20.

9. The +.6V measured on the -15V rail is probably leak through from the + supply through the circuitry in the absence of any -15V drive. If there was -0.6V on the -15V rail, I'd say the upper 10K resistor is open.

10. Interesting voltages to know, once the transistor thing is straightened out, are pin 3 of IC3 (should be nearly 0V), pin 2 of IC3 (also 0V), pin 1 of IC 3 (somewhere slightly negative, maybe a volt or so), Tr23 collector (-16 or 17V, about 1.5V less than -(+15V)). Also the E/B/C voltages on Tr20 and Tr21. Measure these and report them.

11. Interesting that the 1K resistor is only hot when the bad transistor is installed. I would expect it to be hot if there is no -15V output. Otherwise there is an open circuit in here somewhere. Possible that Tr20 is bad???? An open circuit in the collector would do this. If you pull Tr20, there should be a diode measured with the meter from the Emitter to the Base and from the Collector to the Base. If either of these is open circuit (or shorted), in both directions with the meter on 'ohms' or preferably the 'diode' scale, then this transistor is bad. No hot resistor, no output. Same measurements on Tr21, same outcome (cold 1K and no output). Measure these transistors between EB, CB, and might as well CE with the meter on the diode scale, meter probes in both directions, and publish the results.

12. BTW, small Vce sat differences don't matter when substituting transistors for function is this circuit (even most circuits unless they are RF). Most important is that the right transistor type is used ( for bipolar transistors PNP or NPN (FETs more complicated)), the PINOUT is confirmed for the E/B/C connections, voltage margin is OK (60 is fine for this circuit), and the current capability is OK (only an issue here for the Tr20 pass transistor. Everything else is small signal and almost any substitution will work fine to test this circuit. The transistor substitution listed above (2sb590/2sa1534a) should be fine. Check between the pins with the meter as listed above, both transistors, with the diode range, between each pair of pins. Maybe the substitute was good and the resistor got hot because both Tr20 and Tr21 are bad(!).

Looking forward to voltages and transistor readings!

Hugh

Thank you both for your suggestions.

Unfortunately, one thing I did tonight was to accidentally short out pins on Tr9 to the case of Tr5. I was being so careful, so of course the very first time I forgot to check them before applying power they shorted out *sigh*. So after checking and replacing F2 and it now seems to be mostly working as before, with the exception that the -5V rail is now reading almost exactly +5.00V (!?), and the base and emitter of Tr9 which appear to be shorted to the unregulated supply voltage (-26V) on the collector. I will get a replacement for Tr9; I am pretty sure it's toast. Should I replace other components in that area (maybe the 220uF electrolytic or the 10K and 20K resistors and IC2?) I feel pretty stupid, and now have the five off-board power transistors all sealed up in insulated snap-lock bags to avoid it happening again :-(

As far as the -15V circuit is concerned, I took the following readings. I also tried to work out where the fault may be coming from and did some extra voltage measurements along that path, so hopefully this will help with identifying the root cause.

The 1K resistor across the collector and base of Tr21 gets very hot even with the original Tr21 (2SB560) so that was a red herring, sorry. I've put the original transistor back in now.

All component value measurements below were made with the component under test removed from the circuit.

Tr20 base = +0.180V
Tr21 base = -0.280V
Tr22 base (D7/27K junction) = -0.783V [This looks wrong! Focus on that circuit then]

27K measures 27.0K
D7 tests OK with a 0.626V voltage drop - I replaced it anyway, with a new, similarly tested 1N4148
47K (from Tr22base to the unregulated negative supply) measures 46.9K
1K (from Tr16 base to unregulated positive supply) measures 1.00K

[So the components of the voltage divider look OK, something else must be driving the D7/27K junction negative. Is it Tr19/Tr17?]

27K/1K junction (Tr16 base) = +1.326V
Tr17 base = +0.136V
Tr19 base = +0.700V

[That comes all the across from Tr13, Tr14]

Tr13 collector = +0.693V
Tr13 base = +0.054V
Tr14 base = +0.690V
ZD3 should be a 6.8V zener (marked on the PCB but not in the service manual - I didn't test it though)
ZD3 cathode = +7.48V

Hopefully that gives some more insight - should I check more or is that enough to identify the faulty part(s)?

I feel like I can now almost understand what's happening, but I'm not quite there yet. I think Tr14/Tr13/Tr19/Tr17 might be there to stop both the +15V and -15V rails from operating unless the +10V is OK, or something along those lines.

Due to the rapid heating of the 1K resistor I was hesistant to leave power on on for longer than a few minutes at a time, and I noticed that some of the values above were not constant and changed for about the first minute or so after power was applied. For example Tr20 base started at around +0.190V and kept dropping about 2mV/sec. Also the -15V rail reads about -12V for one refresh of my digital multimeter when power is first applied, then it starts increasing fairly rapidly until it reaches the +0.6V mark where it then remains, again taking a little less than a minute to do so.

Hugh, your breakdown of aspects of the circuit's function and the process of debugging what's going wrong is fantastic and I found it incredibly helpful. Thank you so much! I really appreciate it.

Peter, sorry I forgot to measure the base of Tr23. I'll make sure I remember to do it tomorrow night.

Cheers,
A.

On Sat, Jul 20, 2013 at 3:09 PM, Peter Mörck <peter@...> wrote:

>From my limited knowledge I react on the voltage 0.6V because it sounds like a PN-junction voltage drop.
>So I would guess that something, coming from 0V/ground, via a reverse PN-junction, reaches the emitter of Tr20.
>For example, its own base-emitter junction, if the base of Tr20 is 0V. Is it? *no, base is > 0V*

I would expect it to be around -15.7V or so when the circuit is operating properly. Exactly how much is it?

How much is base voltage of Tr21, Tr22 and Tr23?

Check if D7 is good.

BR,

Peter

----- Original Message -----

From: Adrian Corston

To: vintagesynthrepair@yahoogroups.com

Sent: Saturday, July 20, 2013 5:31 PM

Subject: Re: [vintagesynthrepair] CS-70m power supply issue, -15V rail is dead

On 20/07/13 5:50 PM, Adrian Corston wrote:

I'll be having another look at the unit tonight and will try out the excellent suggestions received.

Here we go... my responses are in red in the quoted text below.

Change ANY electrolytic capacitors in that -15 circuit. *done*

Diode "check" any diodes ( make sure they conduct one way and NOT the other) *I don't see any in the -15V circuit*

If there are any tantalum capacitors ( usually teardrop shape) replace them too *not relevant - there are none*

The -15' and +15' values are probably right as they're derived from a zener diode + a PN drop, so 15-0.7 and -15+0.7, roughly.
Sounds about right. *OK*

From my limited knowledge I react on the voltage 0.6V because it sounds like a PN-junction voltage drop.
So I would guess that something, coming from 0V/ground, via a reverse PN-junction, reaches the emitter of Tr20.
For example, its own base-emitter junction, if the base of Tr20 is 0V. Is it? *no, base is > 0V*

Also switch it off and check for a short to ground from Tr20 base. *no short found*

If IC3 is socketed you could try replacing that. Who knows what will happen if that's fried... *IC3 replaced (and socketed for next time)*

Also check for shorts between Collector-Emitter on Tr22 and Tr23. *no shorts found*

I'd also check the electrolytic on the emitter of TR20 isn't pulling anything to ground *electrolytic replaced*

Also check for any shorted tantalum caps throughout the synth... *not relevant, rest of synth is disconnected*

5. No voltage is put out for -15V. Other voltage positions operate correctly.
Probably cause: Tr21 - Tr20 - 0.22ohms.1P *tr20 replaced, tr21 replacement pending, 0.22ohm resistor measures OK*

None of the above things that I tried had any effect; the -15V rail is still sitting at around 0.6V.

I have no 2SB560 (or well-documented substitute) handy for tr21, but I did find a 2SA1534A (in another old power supply) which has a similar spec. The 2SA1534A is only 60V rather than 100V, but I assume that won't matter in a -15V circuit (I only measured 26V for Vce in the circuit with the 2SB560 in place). Pinouts are the same for the two parts. However according to my wonderfully-useful transistor comparison book it is not "lo-sat" (low collector-emitter saturation voltage) whereas the 2SB560 is. According to data sheets:

2SB560 Vce(sat) = -0.3V typ, -0.8V max
2SA1534A Vce(sat) = -0.2V typ, -0.4V max

Datasheets:
2SB560: http://www.futurlec.com/Transistors/2SB560.shtml
2SA1534A: http://www.alldatasheet.com/datasheet-pdf/pdf/12706/PANASONIC/2SA1534.html

This looked near enough to me, so I tried substituting it. Unfortunately the 1K resistor between tr20/tr21 collectors and tr22/23 collectors immediately started getting too hot to touch, so I removed power straight away. I will try again next week after I find a real 2SB560.

Thanks again,
A.

On 22/07/13 11:47 PM, Adrian Corston wrote:

On 22/07/13 4:39 AM, Peter Mörck wrote:

I would expect it to be around -15.7V or so when the circuit is operating properly. Exactly how much is it?

How much is base voltage of Tr21, Tr22 and Tr23?

Check if D7 is good.

On 22/07/13 6:31 AM, Hugh Vartanian wrote:

Hi,

A little about the operation of this circuit when operating normally. Hopefully there is a hint in there that will get to the bottom of this! Note, no distinction is made between what is obvious and what is less so in the description. Also, the ideas of ;'more' or 'less' voltage or current are a little funny when talking about positive and negative values. Generally the conversation uses absolute values or magnitudes when using such language.:

1. Tr20 is the main pass transistor, across which is the difference between the unregulated voltage at the collector and the regulated side at the emitter. When it's base is pulled negative, current flows through from the emitter turning on the transistor and pulling the (amplified) current through the collector, driving the emitter to a negative voltage. If there is an open circuit in this transistor, there will be no output. (see below)

2. Tr21 functions as another amplifier (a 'darlington' connected transistor pair with Tr 20) to pull base current out of Tr20, with the low currents being pulled by the rest of the circuit. So, pull Tr21 base negative, amplify that to pull current through Tr20 base, and give us the juice for the -15V supply.

3. If one takes Tr22 and Tr23 out of the circuit (disconnect their collectors), then Tr21 turns on with the current through the 1K resistor, which turns on Tr20. In this situation the emitter of Tr 20 will be at the unregulated supply voltage (minus a few tenths of a volt.). (IF the rest of the synth is disconnected from the -15V supply, then one could do this test to check out Tr20 and Tr21, but I don't think we need to try this just yet and -26V cannot get to the rest of the synth circuitry safely!.)

4. We want regulation of the output voltage, so we must have a way of draining off the current from the 1K resistor and not sending it into the pass transistor(s). Tr22 or Tr 23 do this.

5. First off, Tr22 is there to keep the -15V supply off until the +15V supply is turned on. If negative supply is on and the positive supply is off then Tr22 is turned on through the 47K resistor on its base. This pulls the current from the 1K resistor to ground, keeping it from turning on Tr21. When the positive supply comes up, then Tr22 is shut off by the slight positive voltage on the D7/27K resistor junction. If the voltage at the D7/27K/47K/Tr22Base junction is not positive several tenths of a volt when power is up, then something is the matter in the upper circuit, probably an open 27K resistor (if the 1K above it was open, then the positive output wouldn't be working.).

6. Next is the regulation function. The regulator operates to keep the -15V output a mirror of the +15V output. Pin 3 of IC3 is connected to the junction of 2x 10K resistors, one to +15V and one to -15V. IC 3 will drive through Tr23/Tr21/Tr20 until the junction at the 10K resistors is almost exactly 0V, halfway between + and -15V. If that voltage is positive, meaning +15 is bigger than -(-15V), then we need 'more' -15V. The positive voltage on pin 3 is amplified by the op-amp, making pin 1 more positive, turning off Tr23 and letting more current from the 1K go into the Tr21/Tr20 transistors, giving us more -15V. If the voltage on pin 3 is too negative, i.e. -(-15V) is greater than +15V, then the output of IC3 goes negative, which turns on Tr23 more and shuts off Tr21/Tr20 more, giving us less drive and a 'lower' -15V. By this action, the -15V is kept at a mirror of +15V.

7. It looks like the .22 ohm resistor ought to be part of a current limiting circuit, but I don't see that represented in the schematic. Measured as not open, so it is fine.

8. If something was dragging down -15V, Tr20 would be very hot. Somehow we are just not getting juice through Tr20.

9. The +.6V measured on the -15V rail is probably leak through from the + supply through the circuitry in the absence of any -15V drive. If there was -0.6V on the -15V rail, I'd say the upper 10K resistor is open.

10. Interesting voltages to know, once the transistor thing is straightened out, are pin 3 of IC3 (should be nearly 0V), pin 2 of IC3 (also 0V), pin 1 of IC 3 (somewhere slightly negative, maybe a volt or so), Tr23 collector (-16 or 17V, about 1.5V less than -(+15V)). Also the E/B/C voltages on Tr20 and Tr21. Measure these and report them.

11. Interesting that the 1K resistor is only hot when the bad transistor is installed. I would expect it to be hot if there is no -15V output. Otherwise there is an open circuit in here somewhere. Possible that Tr20 is bad???? An open circuit in the collector would do this. If you pull Tr20, there should be a diode measured with the meter from the Emitter to the Base and from the Collector to the Base. If either of these is open circuit (or shorted), in both directions with the meter on 'ohms' or preferably the 'diode' scale, then this transistor is bad. No hot resistor, no output. Same measurements on Tr21, same outcome (cold 1K and no output). Measure these transistors between EB, CB, and might as well CE with the meter on the diode scale, meter probes in both directions, and publish the results.

12. BTW, small Vce sat differences don't matter when substituting transistors for function is this circuit (even most circuits unless they are RF). Most important is that the right transistor type is used ( for bipolar transistors PNP or NPN (FETs more complicated)), the PINOUT is confirmed for the E/B/C connections, voltage margin is OK (60 is fine for this circuit), and the current capability is OK (only an issue here for the Tr20 pass transistor. Everything else is small signal and almost any substitution will work fine to test this circuit. The transistor substitution listed above (2sb590/2sa1534a) should be fine. Check between the pins with the meter as listed above, both transistors, with the diode range, between each pair of pins. Maybe the substitute was good and the resistor got hot because both Tr20 and Tr21 are bad(!).

Looking forward to voltages and transistor readings!

Hugh

Thank you both for your suggestions.

Unfortunately, one thing I did tonight was to accidentally short out pins on Tr9 to the case of Tr5. I was being so careful, so of course the very first time I forgot to check them before applying power they shorted out *sigh*. So after checking and replacing F2 and it now seems to be mostly working as before, with the exception that the -5V rail is now reading almost exactly +5.00V (!?), and the base and emitter of Tr9 which appear to be shorted to the unregulated supply voltage (-26V) on the collector. I will get a replacement for Tr9; I am pretty sure it's toast. Should I replace other components in that area (maybe the 220uF electrolytic or the 10K and 20K resistors and IC2?) I feel pretty stupid, and now have the five off-board power transistors all sealed up in insulated snap-lock bags to avoid it happening again :-(

As far as the -15V circuit is concerned, I took the following readings. I also tried to work out where the fault may be coming from and did some extra voltage measurements along that path, so hopefully this will help with identifying the root cause.

The 1K resistor across the collector and base of Tr21 gets very hot even with the original Tr21 (2SB560) so that was a red herring, sorry. I've put the original transistor back in now.

All component value measurements below were made with the component under test removed from the circuit.

Tr20 base = +0.180V
Tr21 base = -0.280V
Tr22 base (D7/27K junction) = -0.783V [This looks wrong! Focus on that circuit then]

27K measures 27.0K
D7 tests OK with a 0.626V voltage drop - I replaced it anyway, with a new, similarly tested 1N4148
47K (from Tr22base to the unregulated negative supply) measures 46.9K
1K (from Tr16 base to unregulated positive supply) measures 1.00K

[So the components of the voltage divider look OK, something else must be driving the D7/27K junction negative. Is it Tr19/Tr17?]

27K/1K junction (Tr16 base) = +1.326V
Tr17 base = +0.136V
Tr19 base = +0.700V

[That comes all the across from Tr13, Tr14]

Tr13 collector = +0.693V
Tr13 base = +0.054V
Tr14 base = +0.690V
ZD3 should be a 6.8V zener (marked on the PCB but not in the service manual - I didn't test it though)
ZD3 cathode = +7.48V

Hopefully that gives some more insight - should I check more or is that enough to identify the faulty part(s)?

I feel like I can now almost understand what's happening, but I'm not quite there yet. I think Tr14/Tr13/Tr19/Tr17 might be there to stop both the +15V and -15V rails from operating unless the +10V is OK, or something along those lines.

Due to the rapid heating of the 1K resistor I was hesistant to leave power on on for longer than a few minutes at a time, and I noticed that some of the values above were not constant and changed for about the first minute or so after power was applied. For example Tr20 base started at around +0.190V and kept dropping about 2mV/sec. Also the -15V rail reads about -12V for one refresh of my digital multimeter when power is first applied, then it starts increasing fairly rapidly until it reaches the +0.6V mark where it then remains, again taking a little less than a minute to do so.

Hugh, your breakdown of aspects of the circuit's function and the process of debugging what's going wrong is fantastic and I found it incredibly helpful. Thank you so much! I really appreciate it.

Peter, sorry I forgot to measure the base of Tr23. I'll make sure I remember to do it tomorrow night.

Cheers,
A.

On Sat, Jul 20, 2013 at 3:09 PM, Peter Mörck <peter@...> wrote:

>From my limited knowledge I react on the voltage 0.6V because it sounds like a PN-junction voltage drop.
>So I would guess that something, coming from 0V/ground, via a reverse PN-junction, reaches the emitter of Tr20.
>For example, its own base-emitter junction, if the base of Tr20 is 0V. Is it? *no, base is > 0V*

I would expect it to be around -15.7V or so when the circuit is operating properly. Exactly how much is it?

How much is base voltage of Tr21, Tr22 and Tr23?

Check if D7 is good.

BR,

Peter

----- Original Message -----

From: Adrian Corston

To: vintagesynthrepair@yahoogroups.com

Sent: Saturday, July 20, 2013 5:31 PM

Subject: Re: [vintagesynthrepair] CS-70m power supply issue, -15V rail is dead

On 20/07/13 5:50 PM, Adrian Corston wrote:

I'll be having another look at the unit tonight and will try out the excellent suggestions received.

Here we go... my responses are in red in the quoted text below.

Change ANY electrolytic capacitors in that -15 circuit. *done*

Diode "check" any diodes ( make sure they conduct one way and NOT the other) *I don't see any in the -15V circuit*

If there are any tantalum capacitors ( usually teardrop shape) replace them too *not relevant - there are none*

The -15' and +15' values are probably right as they're derived from a zener diode + a PN drop, so 15-0.7 and -15+0.7, roughly.
Sounds about right. *OK*

From my limited knowledge I react on the voltage 0.6V because it sounds like a PN-junction voltage drop.
So I would guess that something, coming from 0V/ground, via a reverse PN-junction, reaches the emitter of Tr20.
For example, its own base-emitter junction, if the base of Tr20 is 0V. Is it? *no, base is > 0V*

Also switch it off and check for a short to ground from Tr20 base. *no short found*

If IC3 is socketed you could try replacing that. Who knows what will happen if that's fried... *IC3 replaced (and socketed for next time)*

Also check for shorts between Collector-Emitter on Tr22 and Tr23. *no shorts found*

I'd also check the electrolytic on the emitter of TR20 isn't pulling anything to ground *electrolytic replaced*

Also check for any shorted tantalum caps throughout the synth... *not relevant, rest of synth is disconnected*

5. No voltage is put out for -15V. Other voltage positions operate correctly.
Probably cause: Tr21 - Tr20 - 0.22ohms.1P *tr20 replaced, tr21 replacement pending, 0.22ohm resistor measures OK*

None of the above things that I tried had any effect; the -15V rail is still sitting at around 0.6V.

I have no 2SB560 (or well-documented substitute) handy for tr21, but I did find a 2SA1534A (in another old power supply) which has a similar spec. The 2SA1534A is only 60V rather than 100V, but I assume that won't matter in a -15V circuit (I only measured 26V for Vce in the circuit with the 2SB560 in place). Pinouts are the same for the two parts. However according to my wonderfully-useful transistor comparison book it is not "lo-sat" (low collector-emitter saturation voltage) whereas the 2SB560 is. According to data sheets:

2SB560 Vce(sat) = -0.3V typ, -0.8V max
2SA1534A Vce(sat) = -0.2V typ, -0.4V max

Datasheets:
2SB560: http://www.futurlec.com/Transistors/2SB560.shtml
2SA1534A: http://www.alldatasheet.com/datasheet-pdf/pdf/12706/PANASONIC/2SA1534.html

This looked near enough to me, so I tried substituting it. Unfortunately the 1K resistor between tr20/tr21 collectors and tr22/23 collectors immediately started getting too hot to touch, so I removed power straight away. I will try again next week after I find a real 2SB560.

Thanks again,
A.

On 22/07/13 11:47 PM, Adrian Corston wrote:

On 22/07/13 4:39 AM, Peter Mörck wrote:

I would expect it to be around -15.7V or so when the circuit is operating properly. Exactly how much is it?

How much is base voltage of Tr21, Tr22 and Tr23?

Check if D7 is good.

On 22/07/13 6:31 AM, Hugh Vartanian wrote:

Hi,

A little about the operation of this circuit when operating normally. Hopefully there is a hint in there that will get to the bottom of this! Note, no distinction is made between what is obvious and what is less so in the description. Also, the ideas of ;'more' or 'less' voltage or current are a little funny when talking about positive and negative values. Generally the conversation uses absolute values or magnitudes when using such language.:

1. Tr20 is the main pass transistor, across which is the difference between the unregulated voltage at the collector and the regulated side at the emitter. When it's base is pulled negative, current flows through from the emitter turning on the transistor and pulling the (amplified) current through the collector, driving the emitter to a negative voltage. If there is an open circuit in this transistor, there will be no output. (see below)

2. Tr21 functions as another amplifier (a 'darlington' connected transistor pair with Tr 20) to pull base current out of Tr20, with the low currents being pulled by the rest of the circuit. So, pull Tr21 base negative, amplify that to pull current through Tr20 base, and give us the juice for the -15V supply.

3. If one takes Tr22 and Tr23 out of the circuit (disconnect their collectors), then Tr21 turns on with the current through the 1K resistor, which turns on Tr20. In this situation the emitter of Tr 20 will be at the unregulated supply voltage (minus a few tenths of a volt.). (IF the rest of the synth is disconnected from the -15V supply, then one could do this test to check out Tr20 and Tr21, but I don't think we need to try this just yet and -26V cannot get to the rest of the synth circuitry safely!.)

4. We want regulation of the output voltage, so we must have a way of draining off the current from the 1K resistor and not sending it into the pass transistor(s). Tr22 or Tr 23 do this.

5. First off, Tr22 is there to keep the -15V supply off until the +15V supply is turned on. If negative supply is on and the positive supply is off then Tr22 is turned on through the 47K resistor on its base. This pulls the current from the 1K resistor to ground, keeping it from turning on Tr21. When the positive supply comes up, then Tr22 is shut off by the slight positive voltage on the D7/27K resistor junction. ;If the voltage at the D7/27K/47K/Tr22Base junction is not positive several tenths of a volt when power is up, then something is the matter in the upper circuit, probably an open 27K resistor (if the 1K above it was open, then the positive output wouldn't be working.).

6. Next is the regulation function. The regulator operates to keep the -15V output a mirror of the +15V output. Pin 3 of IC3 is connected to the junction of 2x 10K resistors, one to +15V and one to -15V. IC 3 will drive through Tr23/Tr21/Tr20 until the junction at the 10K resistors is almost exactly 0V, halfway between + and -15V. If that voltage is positive, meaning +15 is bigger than -(-15V), then we need 'more' -15V. The positive voltage on pin 3 is amplified by the op-amp, making pin 1 more positive, turning off Tr23 and letting more current from the 1K go into the Tr21/Tr20 transistors, giving us more -15V. If the voltage on pin 3 is too negative, i.e. -(-15V) is greater than +15V, then the output of IC3 goes negative, which turns on Tr23 more and shuts off Tr21/Tr20 more, giving us less drive and a 'lower' -15V. By this action, the -15V is kept at a mirror of +15V.

7. It looks like the .22 ohm resistor ought to be part of a current limiting circuit, but I don't see that represented in the schematic. Measured as not open, so it is fine.

8. If something was dragging down -15V, Tr20 would be very hot. Somehow we are just not getting juice through Tr20.

9. The +.6V measured on the -15V rail is probably leak through from the + supply through the circuitry in the absence of any -15V drive. If there was -0.6V on the -15V rail, I'd say the upper 10K resistor is open.

10. Interesting voltages to know, once the transistor thing is straightened out, are pin 3 of IC3 (should be nearly 0V), pin 2 of IC3 (also 0V), pin 1 of IC 3 (somewhere slightly negative, maybe a volt or so), Tr23 collector (-16 or 17V, about 1.5V less than -(+15V)). Also the E/B/C voltages on Tr20 and Tr21. Measure these and report them.

11. Interesting that the 1K resistor is only hot when the bad transistor is installed. I would expect it to be hot if there is no -15V output. Otherwise there is an open circuit in here somewhere. Possible that Tr20 is bad???? An open circuit in the collector would do this. If you pull Tr20, there should be a diode measured with the meter from the Emitter to the Base and from the Collector to the Base. If either of these is open circuit (or shorted), in both directions with the meter on 'ohms' or preferably the 'diode' scale, then this transistor is bad. No hot resistor, no output. Same measurements on Tr21, same outcome (cold 1K and no output). Measure these transistors between EB, CB, and might as well CE with the meter on the diode scale, meter probes in both directions, and publish the results.

12. BTW, small Vce sat differences don't matter when substituting transistors for function is this circuit (even most circuits unless they are RF). Most important is that the right transistor type is used ( for bipolar transistors PNP or NPN (FETs more complicated)), the PINOUT is confirmed for the E/B/C connections, voltage margin is OK (60 is fine for this circuit), and the current capability is OK (only an issue here for the Tr20 pass transistor. Everything else is small signal and almost any substitution will work fine to test this circuit. The transistor substitution listed above (2sb590/2sa1534a) should be fine. Check between the pins with the meter as listed above, both transistors, with the diode range, between each pair of pins. Maybe the substitute was good and the resistor got hot because both Tr20 and Tr21 are bad(!).

Looking forward to voltages and transistor readings!

Hugh

Thank you both for your suggestions.

Unfortunately, one thing I did tonight was to accidentally short out pins on Tr9 to the case of Tr5. I was being so careful, so of course the very first time I forgot to check them before applying power they shorted out *sigh*. So after checking and replacing F2 and it now seems to be mostly working as before, with the exception that the -5V rail is now reading almost exactly +5.00V (!?), and the base and emitter of Tr9 which appear to be shorted to the unregulated supply voltage (-26V) on the collector. I will get a replacement for Tr9; I am pretty sure it's toast. Should I replace other components in that area (maybe the 220uF electrolytic or the 10K and 20K resistors and IC2?) I feel pretty stupid, and now have the five off-board power transistors all sealed up in insulated snap-lock bags to avoid it happening again :-(

As far as the -15V circuit is concerned, I took the following readings. I also tried to work out where the fault may be coming from and did some extra voltage measurements along that path, so hopefully this will help with identifying the root cause.

The 1K resistor across the collector and base of Tr21 gets very hot even with the original Tr21 (2SB560) so that was a red herring, sorry. I've put the original transistor back in now.

All component value measurements below were made with the component under test removed from the circuit.

Tr20 base = +0.180V
Tr21 base = -0.280V
Tr22 base (D7/27K junction) = -0.783V [This looks wrong! Focus on that circuit then]

27K measures 27.0K
D7 tests OK with a 0.626V voltage drop - I replaced it anyway, with a new, similarly tested 1N4148
47K (from Tr22base to the unregulated negative supply) measures 46.9K
1K (from Tr16 base to unregulated positive supply) measures 1.00K

[So the components of the voltage divider look OK, something else must be driving the D7/27K junction negative. Is it Tr19/Tr17?]

27K/1K junction (Tr16 base) = +1.326V
Tr17 base = +0.136V
Tr19 base = +0.700V

[That comes all the across from Tr13, Tr14]

Tr13 collector = +0.693V
Tr13 base = +0.054V
Tr14 base = +0.690V
ZD3 should be a 6.8V zener (marked on the PCB but not in the service manual - I didn't test it though)
ZD3 cathode = +7.48V

Hopefully that gives some more insight - should I check more or is that enough to identify the faulty part(s)?

I feel like I can now almost understand what's happening, but I'm not quite there yet. I think Tr14/Tr13/Tr19/Tr17 might be there to stop both the +15V and -15V rails from operating unless the +10V is OK, or something along those lines.

Due to the rapid heating of the 1K resistor I was hesistant to leave power on on for longer than a few minutes at a time, and I noticed that some of the values above were not constant and changed for about the first minute or so after power was applied. For example Tr20 base started at around +0.190V and kept dropping about 2mV/sec. Also the -15V rail reads about -12V for one refresh of my digital multimeter when power is first applied, then it starts increasing fairly rapidly until it reaches the +0.6V mark where it then remains, again taking a little less than a minute to do so.

Hugh, your breakdown of aspects of the circuit's function and the process of debugging what's going wrong is fantastic and I found it incredibly helpful. Thank you so much! I really appreciate it.

Peter, sorry I forgot to measure the base of Tr23. I'll make sure I remember to do it tomorrow night.

Cheers,
A.

On Sat, Jul 20, 2013 at 3:09 PM, Peter Mörck <peter@...> wrote:

>From my limited knowledge I react on the voltage 0.6V because it sounds like a PN-junction voltage drop.
>So I would guess that something, coming from 0V/ground, via a reverse PN-junction, reaches the emitter of Tr20.
>For example, its own base-emitter junction, if the base of Tr20 is 0V. Is it? *no, base is > 0V*

I would expect it to be around -15.7V or so when the circuit is operating properly. Exactly how much is it?

How much is base voltage of Tr21, Tr22 and Tr23?

Check if D7 is good.

;

BR,

Peter

----- Original Message -----

From: Adrian Corston

To: vintagesynthrepair@yahoogroups.com

Sent: Saturday, July 20, 2013 5:31 PM

Subject: Re: [vintagesynthrepair] CS-70m power supply issue, -15V rail is dead

On 20/07/13 5:50 PM, Adrian Corston wrote:

I'll be having another look at the unit tonight and will try out the excellent suggestions received.

Here we go... my responses are in red in the quoted text below.

Change ANY electrolytic capacitors in that -15 circuit. *done*

Diode "check" any diodes ( make sure they conduct one way and NOT the other) *I don't see any in the -15V circuit*

If there are any tantalum capacitors ( usually teardrop shape) replace them too *not relevant - there are none*

The -15' and +15' values are probably right as they're derived from a zener diode + a PN drop, so 15-0.7 and -15+0.7, roughly.
Sounds about right. *OK*

From my limited knowledge I react on the voltage 0.6V because it sounds like a PN-junction voltage drop.
So I would guess that something, coming from 0V/ground, via a reverse PN-junction, reaches the emitter of Tr20.
For example, its own base-emitter junction, if the base of Tr20 is 0V. Is it? *no, base is > 0V*

Also switch it off and check for a short to ground from Tr20 base. *no short found*

If IC3 is socketed you could try replacing that. Who knows what will happen if that's fried... *IC3 replaced (and socketed for next time)*

Also check for shorts between Collector-Emitter on Tr22 and Tr23. *no shorts found*

I'd also check the electrolytic on the emitter of TR20 isn't pulling anything to ground *electrolytic replaced*

Also check for any shorted tantalum caps throughout the synth... *not relevant, rest of synth is disconnected*

5. No voltage is put out for -15V. Other voltage positions operate correctly.
Probably cause: Tr21 - Tr20 - 0.22ohms.1P *tr20 replaced, tr21 replacement pending, 0.22ohm resistor measures OK*

None of the above things that I tried had any effect; the -15V rail is still sitting at around 0.6V.

I have no 2SB560 (or well-documented substitute) handy for tr21, but I did find a 2SA1534A (in another old power supply) which has a similar spec. The 2SA1534A is only 60V rather than 100V, but I assume that won't matter in a -15V circuit (I only measured 26V for Vce in the circuit with the 2SB560 in place). Pinouts are the same for the two parts. However according to my wonderfully-useful transistor comparison book it is not "lo-sat" (low collector-emitter saturation voltage) whereas the 2SB560 is. According to data sheets:

2SB560 Vce(sat) = -0.3V typ, -0.8V max
2SA1534A Vce(sat) = -0.2V typ, -0.4V max

Datasheets:
2SB560: http://www.futurlec.com/Transistors/2SB560.shtml
2SA1534A: http://www.alldatasheet.com/datasheet-pdf/pdf/12706/PANASONIC/2SA1534.html

This looked near enough to me, so I tried substituting it. Unfortunately the 1K resistor between tr20/tr21 collectors and tr22/23 collectors immediately started getting too hot to touch, so I removed power straight away. I will try again next week after I find a real 2SB560.

Thanks again,
A.

On 22/07/13 11:47 PM, Adrian Corston wrote:

On 22/07/13 4:39 AM, Peter Mörck wrote:

I would expect it to be around -15.7V or so when the circuit is operating properly. Exactly how much is it?

How much is base voltage of Tr21, Tr22 and Tr23?

Check if D7 is good.

On 22/07/13 6:31 AM, Hugh Vartanian wrote:

Hi,

A little about the operation of this circuit when operating normally. Hopefully there is a hint in there that will get to the bottom of this! Note, no distinction is made between what is obvious and what is less so in the description. Also, the ideas of ;'more' or 'less' voltage or current are a little funny when talking about positive and negative values. Generally the conversation uses absolute values or magnitudes when using such language.:

1. Tr20 is the main pass transistor, across which is the difference between the unregulated voltage at the collector and the regulated side at the emitter. When it's base is pulled negative, current flows through from the emitter turning on the transistor and pulling the (amplified) current through the collector, driving the emitter to a negative voltage. If there is an open circuit in this transistor, there will be no output. (see below)

2. Tr21 functions as another amplifier (a 'darlington' connected transistor pair with Tr 20) to pull base current out of Tr20, with the low currents being pulled by the rest of the circuit. So, pull Tr21 base negative, amplify that to pull current through Tr20 base, and give us the juice for the -15V supply.

3. If one takes Tr22 and Tr23 out of the circuit (disconnect their collectors), then Tr21 turns on with the current through the 1K resistor, which turns on Tr20. In this situation the emitter of Tr 20 will be at the unregulated supply voltage (minus a few tenths of a volt.). (IF the rest of the synth is disconnected from the -15V supply, then one could do this test to check out Tr20 and Tr21, but I don't think we need to try this just yet and -26V cannot get to the rest of the synth circuitry safely!.)

4. We want regulation of the output voltage, so we must have a way of draining off the current from the 1K resistor and not sending it into the pass transistor(s). Tr22 or Tr 23 do this.

5. First off, Tr22 is there to keep the -15V supply off until the +15V supply is turned on. If negative supply is on and the positive supply is off then Tr22 is turned on through the 47K resistor on its base. This pulls the current from the 1K resistor to ground, keeping it from turning on Tr21. When the positive supply comes up, then Tr22 is shut off by the slight positive voltage on the D7/27K resistor junction. If the voltage at the D7/27K/47K/Tr22Base junction is not positive several tenths of a volt when power is up, then something is the matter in the upper circuit, probably an open 27K resistor (if the 1K above it was open, then the positive output wouldn't be working.).

6. Next is the regulation function. The regulator operates to keep the -15V output a mirror of the +15V output. Pin 3 of IC3 is connected to the junction of 2x 10K resistors, one to +15V and one to -15V. IC 3 will drive through Tr23/Tr21/Tr20 until the junction at the 10K resistors is almost exactly 0V, halfway between + and -15V. If that voltage is positive, meaning +15 is bigger than -(-15V), then we need 'more' -15V. The positive voltage on pin 3 is amplified by the op-amp, making pin 1 more positive, turning off Tr23 and letting more current from the 1K go into the Tr21/Tr20 transistors, giving us more -15V. If the voltage on pin 3 is too negative, i.e. -(-15V) is greater than +15V, then the output of IC3 goes negative, which turns on Tr23 more and shuts off Tr21/Tr20 more, giving us less drive and a 'lower' -15V. By this action, the -15V is kept at a mirror of +15V.

7. It looks like the .22 ohm resistor ought to be part of a current limiting circuit, but I don't see that represented in the schematic. Measured as not open, so it is fine.

8. If something was dragging down -15V, Tr20 would be very hot. Somehow we are just not getting juice through Tr20.

9. The +.6V measured on the -15V rail is probably leak through from the + supply through the circuitry in the absence of any -15V drive. If there was -0.6V on the -15V rail, I'd say the upper 10K resistor is open.

10. Interesting voltages to know, once the transistor thing is straightened out, are pin 3 of IC3 (should be nearly 0V), pin 2 of IC3 (also 0V), pin 1 of IC 3 (somewhere slightly negative, maybe a volt or so), Tr23 collector (-16 or 17V, about 1.5V less than -(+15V)). Also the E/B/C voltages on Tr20 and Tr21. Measure these and report them.

11. Interesting that the 1K resistor is only hot when the bad transistor is installed. I would expect it to be hot if there is no -15V output. Otherwise there is an open circuit in here somewhere. Possible that Tr20 is bad???? An open circuit in the collector would do this. If you pull Tr20, there should be a diode measured with the meter from the Emitter to the Base and from the Collector to the Base. If either of these is open circuit (or shorted), in both directions with the meter on 'ohms' or preferably the 'diode' scale, then this transistor is bad. No hot resistor, no output. Same measurements on Tr21, same outcome (cold 1K and no output). Measure these transistors between EB, CB, and might as well CE with the meter on the diode scale, meter probes in both directions, and publish the results.

12. BTW, small Vce sat differences don't matter when substituting transistors for function is this circuit (even most circuits unless they are RF). Most important is that the right transistor type is used ( for bipolar transistors PNP or NPN (FETs more complicated)), the PINOUT is confirmed for the E/B/C connections, voltage margin is OK (60 is fine for this circuit), and the current capability is OK (only an issue here for the Tr20 pass transistor. Everything else is small signal and almost any substitution will work fine to test this circuit. The transistor substitution listed above (2sb590/2sa1534a) should be fine. Check between the pins with the meter as listed above, both transistors, with the diode range, between each pair of pins. Maybe the substitute was good and the resistor got hot because both Tr20 and Tr21 are bad(!).

Looking forward to voltages and transistor readings!

Hugh

Thank you both for your suggestions.

Unfortunately, one thing I did tonight was to accidentally short out pins on Tr9 to the case of Tr5. I was being so careful, so of course the very first time I forgot to check them before applying power they shorted out *sigh*. So after checking and replacing F2 and it now seems to be mostly working as before, with the exception that the -5V rail is now reading almost exactly +5.00V (!?), and the base and emitter of Tr9 which appear to be shorted to the unregulated supply voltage (-26V) on the collector. I will get a replacement for Tr9; I am pretty sure it's toast. Should I replace other components in that area (maybe the 220uF electrolytic or the 10K and 20K resistors and IC2?) I feel pretty stupid, and now have the five off-board power transistors all sealed up in insulated snap-lock bags to avoid it happening again :-(

As far as the -15V circuit is concerned, I took the following readings. I also tried to work out where the fault may be coming from and did some extra voltage measurements along that path, so hopefully this will help with identifying the root cause.

The 1K resistor across the collector and base of Tr21 gets very hot even with the original Tr21 (2SB560) so that was a red herring, sorry. I've put the original transistor back in now.

All component value measurements below were made with the component under test removed from the circuit.

Tr20 base = +0.180V
Tr21 base = -0.280V
Tr22 base (D7/27K junction) = -0.783V [This looks wrong! Focus on that circuit then]

27K measures 27.0K
D7 tests OK with a 0.626V voltage drop - I replaced it anyway, with a new, similarly tested 1N4148
47K (from Tr22base to the unregulated negative supply) measures 46.9K
1K (from Tr16 base to unregulated positive supply) measures 1.00K

[So the components of the voltage divider look OK, something else must be driving the D7/27K junction negative. Is it Tr19/Tr17?]

27K/1K junction (Tr16 base) = +1.326V
Tr17 base = +0.136V
Tr19 base = +0.700V

[That comes all the across from Tr13, Tr14]

Tr13 collector = +0.693V
Tr13 base = +0.054V
Tr14 base = +0.690V
ZD3 should be a 6.8V zener (marked on the PCB but not in the service manual - I didn't test it though)
ZD3 cathode = +7.48V

Hopefully that gives some more insight - should I check more or is that enough to identify the faulty part(s)?

I feel like I can now almost understand what's happening, but I'm not quite there yet. I think Tr14/Tr13/Tr19/Tr17 might be there to stop both the +15V and -15V rails from operating unless the +10V is OK, or something along those lines.

Due to the rapid heating of the 1K resistor I was hesistant to leave power on on for longer than a few minutes at a time, and I noticed that some of the values above were not constant and changed for about the first minute or so after power was applied. For example Tr20 base started at around +0.190V and kept dropping about 2mV/sec. Also the -15V rail reads about -12V for one refresh of my digital multimeter when power is first applied, then it starts increasing fairly rapidly until it reaches the +0.6V mark where it then remains, again taking a little less than a minute to do so.

Hugh, your breakdown of aspects of the circuit's function and the process of debugging what's going wrong is fantastic and I found it incredibly helpful. Thank you so much! I really appreciate it.

Peter, sorry I forgot to measure the base of Tr23. I'll make sure I remember to do it tomorrow night.

Cheers,
A.

On Sat, Jul 20, 2013 at 3:09 PM, Peter Mörck <peter@...> wrote:

>From my limited knowledge I react on the voltage 0.6V because it sounds like a PN-junction voltage drop.
>So I would guess that something, coming from 0V/ground, via a reverse PN-junction, reaches the emitter of Tr20.
>For example, its own base-emitter junction, if the base of Tr20 is 0V. Is it? *no, base is > 0V*

I would expect it to be around -15.7V or so when the circuit is operating properly. Exactly how much is it?

How much is base voltage of Tr21, Tr22 and Tr23?

Check if D7 is good.

BR,

Peter

(Message over 64 KB, truncated)

Thanks Peter - you're right, the 4.7 ohm resistor on Tr9 emitter is open circuit.

According to the schematic and the board screening, it's rated "230mA", rather than by power. Can I apply P=IV with I = 0.230A and V=-5V to get a requirement for ~ 1.15W for that resistor? I've uploaded a photo of it here. I found nothing online for a "micron HW29347" - Micron seem to make much more interesting components now :-) I have some 1W resistors given to me by a retired pipe organ restorer that might do the job, assuming there's some leeway in that current rating... Or maybe something like these would be better?

+15' and -15' look fine at approximately +14.4V and -14.4V respectively.

Yes, PSU is DEFINITELY disconnected from the rest of the synth and has been all throughout this process.

I will get the scope involved and see what it turns up.

Thanks again,
A.


On 28/07/13 2:54 AM, Peter Mörck wrote:

Looks like everything is pretty messed up now after the short then (Almost all rails are way off).

Check with a scope as well for AC in the wrong places (if rectifiers got zapped).

As Hugh wrote, the negative rails are using the positive ones as reference, but since they seem busted now I guess it won't be easy to fix the negative until the positive ones are in order.

Sorry, I'm not skilled enough to help you, but I'd just try to break it down and go from the source, see what is expected and verify the result.

But, how can there be -26.7V on Tr9 emitter while you have +5V on the -5V rail? That means you have 31.7V across the 4.7ohm resistor at the Tr9 emitter, so the resistor should be, if it wasn't already, turned to charcoal :)

6.7 amps, 213W... unlikely. That resistor has got to be open or the PSU would be on fire.

Check the 10k resistor near IC2 pin 3. The other only way you could get any voltage down there is through the 220uF cap next to it.

Anyway, don't bother with that until the positive rails are done.

Did the +15' and -15' (prime) voltages survive the short?

I don't remember if you wrote it before, but I assume you have the PSU disconnected from the rest of the synthesizer :)

Sorry I can't be of more help.

BR,

Peter

----- Original Message -----

From: Adrian Corston

To: vintagesynthrepair@yahoogroups.com

Cc: Hugh Vartanian ; Peter Mörck

Sent: Saturday, July 27, 2013 5:33 PM

Subject: Re: [vintagesynthrepair] CS-70m power supply issue, -15V rail is dead

I should correct a minor mistake I just made, sorry:

I measured +5V on the -5V test point, but -26.7V on Tr9's emitter. The -5V test point is the other side of the 4.7ohm resistor from Tr9's emitter.

If anyone would like me to post a link to the PCB layouts for this circuit then please let me know.

Cheers,
A.


On 28/07/13 12:51 AM, Adrian Corston wrote:

I hate to reply to myself, but...

I did some more work on this tonight, replacing a bunch of components. Since my last email I have replaced:
- All ICs
- Tr9
- All electrolytic caps except for the big three (4700uF x 2, 10000uF x 1)
- All zeners
- All 1S1555 (with 1N4148)
- All 2SC1815 except for one - Tr 6, because I ran out of new 2SC1815
- All 2SA1015

I have previously replaced Tr20 as mentioned below.

Both faulty behaviours remains as described below - in summary:
* +5V on the Tr9 emitter (instead of the expected -5V)

^^^^^^^^^^^^^^^^^ correction: this should have read "+5V on the -5V test point". Sorry about that.

* On power-up Tr20 emitter starts at approx. -12V but rises to +0.6V within 10-15 seconds then remains there (instead of the expected -15V)

This is despite a perfectly reasonable -26V on the collectors of both Tr9 and Tr20.

I re-measured all legs of the five main power transistors:

Tr20 B=+340mV, C=-26.7V, E=+0.85V
Tr15 B=+7.3V, C=+26.7V, E=+17.6V
Tr9 B=-26.7V, C=-26.7V, E=-26.7V
Tr5 B=+26.7V, C=+26.7V, E=+26.1V
Tr1 B=+3.9V, C=13.7V, E=+3.4V

Peter asked for the voltage at Tr23's base; it's +13.5V. Given this comes from the op-amp in IC2 and IC3 pin 2 is +1.9mV and IC3 pin 3 is +9.2V this is what I'd expect (i.e. the positive power rail of the IC3 minus a bit).

I also tested all PCB traces and did not find any breaks (obviously I couldn't easily test for shorts though).

The schematic can be found here.

Unless anyone has any better suggestions, my next plan is to start replacing resistors and ceramic capacitors. After that I'm out of ideas :-(

Thank you everyone for your suggestions so far!

Cheers,
A.


On 22/07/13 11:47 PM, Adrian Corston wrote:

On 22/07/13 4:39 AM, Peter Mörck wrote:

I would expect it to be around -15.7V or so when the circuit is operating properly. Exactly how much is it?

How much is base voltage of Tr21, Tr22 and Tr23?

Check if D7 is good.

On 22/07/13 6:31 AM, Hugh Vartanian wrote:

Hi,

A little about the operation of this circuit when operating normally. Hopefully there is a hint in there that will get to the bottom of this! Note, no distinction is made between what is obvious and what is less so in the description. Also, the ideas of ;'more' or 'less' voltage or current are a little funny when talking about positive and negative values. Generally the conversation uses absolute values or magnitudes when using such language.:

1. Tr20 is the main pass transistor, across which is the difference between the unregulated voltage at the collector and the regulated side at the emitter. When it's base is pulled negative, current flows through from the emitter turning on the transistor and pulling the (amplified) current through the collector, driving the emitter to a negative voltage. If there is an open circuit in this transistor, there will be no output. (see below)

2. Tr21 functions as another amplifier (a 'darlington' connected transistor pair with Tr 20) to pull base current out of Tr20, with the low currents being pulled by the rest of the circuit. So, pull Tr21 base negative, amplify that to pull current through Tr20 base, and give us the juice for the -15V supply.

3. If one takes Tr22 and Tr23 out of the circuit (disconnect their collectors), then Tr21 turns on with the current through the 1K resistor, which turns on Tr20. In this situation the emitter of Tr 20 will be at the unregulated supply voltage (minus a few tenths of a volt.). (IF the rest of the synth is disconnected from the -15V supply, then one could do this test to check out Tr20 and Tr21, but I don't think we need to try this just yet and -26V cannot get to the rest of the synth circuitry safely!.)

4. We want regulation of the output voltage, so we must have a way of draining off the current from the 1K resistor and not sending it into the pass transistor(s). Tr22 or Tr 23 do this.

5. First off, Tr22 is there to keep the -15V supply off until the +15V supply is turned on. If negative supply is on and the positive supply is off then Tr22 is turned on through the 47K resistor on its base. This pulls the current from the 1K resistor to ground, keeping it from turning on Tr21. When the positive supply comes up, then Tr22 is shut off by the slight positive voltage on the D7/27K resistor junction. If the voltage at the D7/27K/47K/Tr22Base junction is not positive several tenths of a volt when power is up, then something is the matter in the upper circuit, probably an open 27K resistor (if the 1K above it was open, then the positive output wouldn't be working.).

6. Next is the regulation function. The regulator operates to keep the -15V output a mirror of the +15V output. Pin 3 of IC3 is connected to the junction of 2x 10K resistors, one to +15V and one to -15V. IC 3 will drive through Tr23/Tr21/Tr20 until the junction at the 10K resistors is almost exactly 0V, halfway between + and -15V. If that voltage is positive, meaning +15 is bigger than -(-15V), then we need 'more' -15V. The positive voltage on pin 3 is amplified by the op-amp, making pin 1 more positive, turning off Tr23 and letting more current from the 1K go into the Tr21/Tr20 transistors, giving us more -15V. If the voltage on pin 3 is too negative, i.e. -(-15V) is greater than +15V, then the output of IC3 goes negative, which turns on Tr23 more and shuts off Tr21/Tr20 more, giving us less drive and a 'lower' -15V. By this action, the -15V is kept at a mirror of +15V.

7. It looks like the .22 ohm resistor ought to be part of a current limiting circuit, but I don't see that represented in the schematic. Measured as not open, so it is fine.

8. If something was dragging down -15V, Tr20 would be very hot. Somehow we are just not getting juice through Tr20.

9. The +.6V measured on the -15V rail is probably leak through from the + supply through the circuitry in the absence of any -15V drive. If there was -0.6V on the -15V rail, I'd say the upper 10K resistor is open.

10. Interesting voltages to know, once the transistor thing is straightened out, are pin 3 of IC3 (should be nearly 0V), pin 2 of IC3 (also 0V), pin 1 of IC 3 (somewhere slightly negative, maybe a volt or so), Tr23 collector (-16 or 17V, about 1.5V less than -(+15V)). Also the E/B/C voltages on Tr20 and Tr21. Measure these and report them.

11. Interesting that the 1K resistor is only hot when the bad transistor is installed. I would expect it to be hot if there is no -15V output. Otherwise there is an open circuit in here somewhere. Possible that Tr20 is bad???? An open circuit in the collector would do this. If you pull Tr20, there should be a diode measured with the meter from the Emitter to the Base and from the Collector to the Base. If either of these is open circuit (or shorted), in both directions with the meter on 'ohms' or preferably the 'diode' scale, then this transistor is bad. No hot resistor, no output. Same measurements on Tr21, same outcome (cold 1K and no output). Measure these transistors between EB, CB, and might as well CE with the meter on the diode scale, meter probes in both directions, and publish the results.

12. BTW, small Vce sat differences don't matter when substituting transistors for function is this circuit (even most circuits unless they are RF). Most important is that the right transistor type is used ( for bipolar transistors PNP or NPN (FETs more complicated)), the PINOUT is confirmed for the E/B/C connections, voltage margin is OK (60 is fine for this circuit), and the current capability is OK (only an issue here for the Tr20 pass transistor. Everything else is small signal and almost any substitution will work fine to test this circuit. The transistor substitution listed above (2sb590/2sa1534a) should be fine. Check between the pins with the meter as listed above, both transistors, with the diode range, between each pair of pins. Maybe the substitute was good and the resistor got hot because both Tr20 and Tr21 are bad(!).

Looking forward to voltages and transistor readings!

Hugh

Thank you both for your suggestions.

Unfortunately, one thing I did tonight was to accidentally short out pins on Tr9 to the case of Tr5. I was being so careful, so of course the very first time I forgot to check them before applying power they shorted out *sigh*. So after checking and replacing F2 and it now seems to be mostly working as before, with the exception that the -5V rail is now reading almost exactly +5.00V (!?), and the base and emitter of Tr9 which appear to be shorted to the unregulated supply voltage (-26V) on the collector. I will get a replacement for Tr9; I am pretty sure it's toast. Should I replace other components in that area (maybe the 220uF electrolytic or the 10K and 20K resistors and IC2?) I feel pretty stupid, and now have the five off-board power transistors all sealed up in insulated snap-lock bags to avoid it happening again :-(

As far as the -15V circuit is concerned, I took the following readings. I also tried to work out where the fault may be coming from and did some extra voltage measurements along that path, so hopefully this will help with identifying the root cause.

The 1K resistor across the collector and base of Tr21 gets very hot even with the original Tr21 (2SB560) so that was a red herring, sorry. I've put the original transistor back in now.

All component value measurements below were made with the component under test removed from the circuit.

Tr20 base = +0.180V
Tr21 base = -0.280V
Tr22 base (D7/27K junction) = -0.783V [This looks wrong! Focus on that circuit then]

27K measures 27.0K
D7 tests OK with a 0.626V voltage drop - I replaced it anyway, with a new, similarly tested 1N4148
47K (from Tr22base to the unregulated negative supply) measures 46.9K
1K (from Tr16 base to unregulated positive supply) measures 1.00K

[So the components of the voltage divider look OK, something else must be driving the D7/27K junction negative. Is it Tr19/Tr17?]

27K/1K junction (Tr16 base) = +1.326V
Tr17 base = +0.136V
Tr19 base = +0.700V

[That comes all the across from Tr13, Tr14]

Tr13 collector = +0.693V
Tr13 base = +0.054V
Tr14 base = +0.690V
ZD3 should be a 6.8V zener (marked on the PCB but not in the service manual - I didn't test it though)
ZD3 cathode = +7.48V

Hopefully that gives some more insight - should I check more or is that enough to identify the faulty part(s)?

I feel like I can now almost understand what's happening, but I'm not quite there yet. I think Tr14/Tr13/Tr19/Tr17 might be there to stop both the +15V and -15V rails from operating unless the +10V is OK, or something along those lines.

Due to the rapid heating of the 1K resistor I was hesistant to leave power on on for longer than a few minutes at a time, and I noticed that some of the values above were not constant and changed for about the first minute or so after power was applied. For example Tr20 base started at around +0.190V and kept dropping about 2mV/sec. Also the -15V rail reads about -12V for one refresh of my digital multimeter when power is first applied, then it starts increasing fairly rapidly until it reaches the +0.6V mark where it then remains, again taking a little less than a minute to do so.

Hugh, your breakdown of aspects of the circuit's function and the process of debugging what's going wrong is fantastic and I found it incredibly helpful. Thank you so much! I really appreciate it.

Peter, sorry I (Message over 64 KB, truncated)

On 22/07/13 11:47 PM, Adrian Corston wrote:

On 22/07/13 4:39 AM, Peter Mörck wrote:

I would expect it to be around -15.7V or so when the circuit is operating properly. Exactly how much is it?

How much is base voltage of Tr21, Tr22 and Tr23?

Check if D7 is good.

On 22/07/13 6:31 AM, Hugh Vartanian wrote:

Hi,

A little about the operation of this circuit when operating normally. Hopefully there is a hint in there that will get to the bottom of this! Note, no distinction is made between what is obvious and what is less so in the description. Also, the ideas of ;'more' or 'less' voltage or current are a little funny when talking about positive and negative values. Generally the conversation uses absolute values or magnitudes when using such language.:

1. Tr20 is the main pass transistor, across which is the difference between the unregulated voltage at the collector and the regulated side at the emitter. When it's base is pulled negative, current flows through from the emitter turning on the transistor and pulling the (amplified) current through the collector, driving the emitter to a negative voltage. If there is an open circuit in this transistor, there will be no output. (see below)

2. Tr21 functions as another amplifier (a 'darlington' connected transistor pair with Tr 20) to pull base current out of Tr20, with the low currents being pulled by the rest of the circuit. So, pull Tr21 base negative, amplify that to pull current through Tr20 base, and give us the juice for the -15V supply.

3. If one takes Tr22 and Tr23 out of the circuit (disconnect their collectors), then Tr21 turns on with the current through the 1K resistor, which turns on Tr20. In this situation the emitter of Tr 20 will be at the unregulated supply voltage (minus a few tenths of a volt.). (IF the rest of the synth is disconnected from the -15V supply, then one could do this test to check out Tr20 and Tr21, but I don't think we need to try this just yet and -26V cannot get to the rest of the synth circuitry safely!.)

4. We want regulation of the output voltage, so we must have a way of draining off the current from the 1K resistor and not sending it into the pass transistor(s). Tr22 or Tr 23 do this.

5. First off, Tr22 is there to keep the -15V supply off until the +15V supply is turned on. If negative supply is on and the positive supply is off then Tr22 is turned on through the 47K resistor on its base. This pulls the current from the 1K resistor to ground, keeping it from turning on Tr21. When the positive supply comes up, then Tr22 is shut off by the slight positive voltage on the D7/27K resistor junction. If the voltage at the D7/27K/47K/Tr22Base junction is not positive several tenths of a volt when power is up, then something is the matter in the upper circuit, probably an open 27K resistor (if the 1K above it was open, then the positive output wouldn't be working.).

6. Next is the regulation function. ;The regulator operates to keep the -15V output a mirror of the +15V output. Pin 3 of IC3 is connected to the junction of 2x 10K resistors, one to +15V and one to -15V. ;IC 3 will drive through Tr23/Tr21/Tr20 until the junction at the 10K resistors is almost exactly 0V, halfway between + and -15V. If that voltage is positive, meaning +15 is bigger than -(-15V), then we need 'more' -15V. The positive voltage on pin 3 is amplified by the op-amp, making pin 1 more positive, turning off Tr23 and letting more current from the 1K go into the Tr21/Tr20 transistors, giving us more -15V. If the voltage on pin 3 is too negative, i.e. -(-15V) is greater than +15V, then the output of IC3 goes negative, which turns on Tr23 more and shuts off Tr21/Tr20 more, giving us less drive and a 'lower' -15V. By this action, the -15V is kept at a mirror of +15V.

7. It looks like the .22 ohm resistor ought to be part of a current limiting circuit, but I don't see that represented in the schematic. Measured as not open, so it is fine.

8. If something was dragging down -15V, Tr20 would be very hot. Somehow we are just not getting juice through Tr20.

9. The +.6V measured on the -15V rail is probably leak through from the + supply through the circuitry in the absence of any -15V drive. If there was -0.6V on the -15V rail, I'd say the upper 10K resistor is open.

10. Interesting voltages to know, once the transistor thing is straightened out, are pin 3 of IC3 (should be nearly 0V), pin 2 of IC3 (also 0V), pin 1 of IC 3 (somewhere slightly negative, maybe a volt or so), Tr23 collector (-16 or 17V, about 1.5V less than -(+15V)). Also the E/B/C voltages on Tr20 and Tr21. Measure these and report them.

11. Interesting that the 1K resistor is only hot when the bad transistor is installed. I would expect it to be hot if there is no -15V output. Otherwise there is an open circuit in here somewhere. Possible that Tr20 is bad???? An open circuit in the collector would do this. If you pull Tr20, there should be a diode measured with the meter from the Emitter to the Base and from the Collector to the Base. If either of these is open circuit (or shorted), in both directions with the meter on 'ohms' or preferably the 'diode' scale, then this transistor is bad. No hot resistor, no output. Same measurements on Tr21, same outcome (cold 1K and no output). Measure these transistors between EB, CB, and might as well CE with the meter on the diode scale, meter probes in both directions, and publish the results.

12. BTW, small Vce sat differences don't matter when substituting transistors for function is this circuit (even most circuits unless they are RF). Most important is that the right transistor type is used ( for bipolar transistors PNP or NPN (FETs more complicated)), the PINOUT is confirmed for the E/B/C connections, voltage margin is OK (60 is fine for this circuit), and the current capability is OK (only an issue here for the Tr20 pass transistor. Everything else is small signal and almost any substitution will work fine to test this circuit. The transistor substitution listed above (2sb590/2sa1534a) should be fine. Check between the pins with the meter as listed above, both transistors, with the diode range, between each pair of pins. Maybe the substitute was good and the resistor got hot because both Tr20 and Tr21 are bad(!).

Looking forward to voltages and transistor readings!

Hugh

Thank you both for your suggestions.

Unfortunately, one thing I did tonight was to accidentally short out pins on Tr9 to the case of Tr5. I was being so careful, so of course the very first time I forgot to check them before applying power they shorted out *sigh*. ; So after checking and replacing F2 and it now seems to be mostly working as before, with the exception that the -5V rail is now reading almost exactly +5.00V (!?), and the base and emitter of Tr9 which appear to be shorted to the unregulated supply voltage (-26V) on the collector. I will get a replacement for Tr9; I am pretty sure it's toast. Should I replace other components in that area (maybe the 220uF electrolytic or the 10K and 20K resistors and IC2?) I feel pretty stupid, and now have the five off-board power transistors all sealed up in insulated snap-lock bags to avoid it happening again :-(

As far as the -15V circuit is concerned, I took the following readings. I also tried to work out where the fault may be coming from and did some extra voltage measurements along that path, so hopefully this will help with identifying the root cause.

The 1K resistor across the collector and base of Tr21 gets very hot even with the original Tr21 (2SB560) so that was a red herring, sorry. I've put the original transistor back in now.

All component value measurements below were made with the component under test removed from the circuit.

Tr20 base = +0.180V
Tr21 base = -0.280V
Tr22 base (D7/27K junction) = -0.783V [This looks wrong! Focus on that circuit then]

27K measures 27.0K
D7 tests OK with a 0.626V voltage drop - I replaced it anyway, with a new, similarly tested 1N4148
47K (from Tr22base to the unregulated negative supply) measures 46.9K
1K (from Tr16 base to unregulated positive supply) measures 1.00K

[So the components of the voltage divider look OK, something else must be driving the D7/27K junction negative. Is it Tr19/Tr17?]

27K/1K junction (Tr16 base) = +1.326V
Tr17 base = +0.136V
Tr19 base = +0.700V

[That comes all the across from Tr13, Tr14]

Tr13 collector = +0.693V
Tr13 base = +0.054V
Tr14 base = +0.690V
ZD3 should be a 6.8V zener (marked on the PCB but not in the service manual - I didn't test it though)
ZD3 cathode = +7.48V

Hopefully that gives some more insight - should I check more or is that enough to identify the faulty part(s)?

I feel like I can now almost understand what's happening, but I'm not quite there yet. I think Tr14/Tr13/Tr19/Tr17 might be there to stop both the +15V and -15V rails from operating unless the +10V is OK, or something along those lines.

Due to the rapid heating of the 1K resistor I was hesistant to leave power on on for longer than a few minutes at a time, and I noticed that some of the values above were not constant and changed for about the first minute or so after power was applied. For example Tr20 base started at around +0.190V and kept dropping about 2mV/sec. Also the -15V rail reads about -12V for one refresh of my digital multimeter when power is first applied, then it starts increasing fairly rapidly until it reaches the +0.6V mark where it then remains, again taking a little less than a minute to do so.

Hugh, your breakdown of aspects of the circuit's function and the process of debugging what's going wrong is fantastic and I found it incredibly helpful. Thank you so much! I really appreciate it.

Peter, sorry I forgot to measure the base of Tr23. I'll make sure I remember to do it tomorrow night.

Cheers,
A.

On Sat, Jul 20, 2013 at 3:09 PM, Peter Mörck <peter@...> wrote:

>From my limited knowledge I react on the voltage 0.6V because it sounds like a PN-junction voltage drop.
>So I would guess that something, coming from 0V/ground, via a reverse PN-junction, reaches the emitter of Tr20.
>For example, its own base-emitter junction, if the base of Tr20 is 0V. Is it? *no, base is > 0V*

I would expect it to be around -15.7V or so when the circuit is operating properly. Exactly how much is it?

How much is base voltage of Tr21, Tr22 and Tr23?

Check if D7 is good.

BR,

Peter

----- Original Message -----

From: Adrian Corston

To: vintagesynthrepair@yahoogroups.com

Sent: Saturday, July 20, 2013 5:31 PM

Subject: Re: [vintagesynthrepair] CS-70m power supply issue, -15V rail is dead

On 20/07/13 5:50 PM, Adrian Corston wrote:

I'll be having another look at the unit tonight and will try out the excellent suggestions received.

Here we go... my responses are in red in the quoted text below.

Change ANY electrolytic capacitors in that -15 circuit. *done*

Diode "check" any diodes ( make sure they conduct one way and NOT the other) *I don't see any in the -15V circuit*

If there are any tantalum capacitors ( usually teardrop shape) replace them too *not relevant - there are none*

The -15' and +15' values are probably right as they're derived from a zener diode + a PN drop, so 15-0.7 and -15+0.7, roughly.
Sounds about right. *OK*

From my limited knowledge I react on the voltage 0.6V because it sounds like a PN-junction voltage drop.
So I would guess that something, coming from 0V/ground, via a reverse PN-junction, reaches the emitter of Tr20.
For example, its own base-emitter junction, if the base of Tr20 is 0V. Is it? *no, base is > 0V*

Also switch it off and check for a short to ground from Tr20 base. *no short found*

If IC3 is socketed you could try replacing that. Who knows what will happen if that's fried... *IC3 replaced (and socketed for next time)*

Also check for shorts between Collector-Emitter on Tr22 and Tr23. *no shorts found*

I'd also check the electrolytic on the emitter of TR20 isn't pulling anything to ground *electrolytic replaced*

Also check for any shorted tantalum caps throughout the synth... *not relevant, rest of synth is disconnected*

5. No voltage is put out for -15V. Other voltage positions operate correctly.
Probably cause: Tr21 - Tr20 - 0.22ohms.1P *tr20 replaced, tr21 replacement pending, 0.22ohm resistor measures OK*

None of the above things that I tried had any effect; the -15V rail is still sitting at around 0.6V.

I have no 2SB560 (or well-documented substitute) handy for tr21, but I did find a 2SA1534A (in another old power supply) which has a similar spec. The 2SA1534A is only 60V rather than 100V, but I assume that won't matter in a -15V circuit (I only measured 26V for Vce in the circuit with the 2SB560 in place). Pinouts are the same for the two parts. However according to my wonderfully-useful transistor comparison book it is not "lo-sat" (low collector-emitter saturation voltage) whereas the 2SB560 is. ; According to data sheets:

2SB560 Vce(sat) = -0.3V typ, -0.8V max
2SA1534A Vce(sat) = -0.2V typ, -0.4V max

Datasheets:
2SB560: http://www.futurlec.com/Transistors/2SB560.shtml
2SA1534A: http://www.alldatasheet.com/datasheet-pdf/pdf/12706/PANASONIC/2SA1534.html

This looked near enough to me, so I tried substituting it. Unfortunately the 1K resistor between tr20/tr21 collectors and tr22/23 collectors immediately started getting too hot to touch, so I removed power straight away. I will try again next week after I find a real 2SB560.

Thanks again,
A.

>
> Hello guys and sorry to jump in like that but I would suggest to change the big caps as a priority and then check the bridge rectifier again for leakage of AC current.
> 
> I had a big problem with a D&R console's psu in which the voltage regulators kept on going Kapput. Turned out that the big Electrolytics were bad and that induced a big stress on the rectifier bridge, which in turn had the consequence of not only letting DC pass as expected, but also AC at the same time !
> 
> I did test for AC leakage on the rectifier after the newly installed voltage regulator worked for a couple of seconds before shorting...
> 
> After new BIG caps, new bridge rectifier and a 2nd new voltage regulator, everything was sorted.
> 
> Good luck !
>

Hello,
Have you checked the .22ohm 1 watt (1P?) resistor?
I just repaired a Yamaha CS40M that had that resistor out (.47 on the CS40M) and that was the cause of the -15 volt failure and the other voltages not meeting full voltage (+10 was +8 and so on).

Once I replaced that all came into full power.

Check those out.

GD

--- In vintagesynthrepair@yahoogroups.com, "cpt.zilog" wrote:
>
> Hello guys and sorry to jump in like that but I would suggest to change the big caps as a priority and then check the bridge rectifier again for leakage of AC current.
>
> I had a big problem with a D&R console's psu in which the voltage regulators kept on going Kapput. Turned out that the big Electrolytics were bad and that induced a big stress on the rectifier bridge, which in turn had the consequence of not only letting DC pass as expected, but also AC at the same time !
>
> I did test for AC leakage on the rectifier after the newly installed voltage regulator worked for a couple of seconds before shorting...
>
> After new BIG caps, new bridge rectifier and a 2nd new voltage regulator, everything was sorted.
>
> Good luck !
>

----- Original Message -----

From: Adrian Corston

To: vintagesynthrepair@yahoogroups.com

Cc: gerryrdahl

Sent: Thursday, August 01, 2013 9:04 AM

Subject: Re: [vintagesynthrepair] Re: CS-70m power supply issue, -15V rail is dead

Hi Gerry,

Thank you for your advice.

I have the +10V and -5V rails working right now - and you were pretty-much spot on with that resistor guess (although for a different voltage rail). Both of the 4.7 ohm 230mA resistors on those rails were open circuit, and once they were replaced I was able to adjust them back to the correct voltages. At the moment I have some temporary non-fusible resistors in place while I wait an order of proper fusible ones to come in.

My remaining issue then is the original problem I first reported, where the +15V test point is measuring +17V (which does not change when VR3 is adjusted) and the -15V test point measures about -12V immediately after power is applied but then this rapidly slides up to approx. +0.8V after about 10-15 seconds.

For now I'm focusing on the +15V circuit.

I measured the 0.22 ohm resistor in the +15V circuit and my multimeter reported 0.5 ohms, which seems reasonable given it's a cheap multimeter and that is a very small value and presumably hard to measure accurately? If it's just a normal 1W 0.22 ohm resistor I can try replacing it, but I really have no idea what the "1P" means - 1W "P"ower maybe?

There is no measurable current flowing on the base of Tr15 and voltage measurements are approx. +26V on the collector, +17V on the base and +16V on the emitter.

The IC3 (2/2) op-amp has +13.22V output on pin 7 no matter what VR3 is set to, despite pin 6 measuring +6.34V. I think this is unexpected since it looks like a voltage follower to me - shouldn't that mean IN- and OUT effectively track each other? I replaced the 0.047uF capacitor but it made no difference. VR3 adjusts the IN+ (pin 5) between +6.66V and +8.72V which is presumably higher than normal because it's part of a voltage divider across 0V-17V, when it would usually be across 0V-15V. If someone could give me some advice about for this part of the circuit, or measurements of what's happening on another functional CS-70m then I'd appreciate it.

Assuming IC3 is actually doing what it's supposed to I guess the next step is to look at Tr18 and Tr16 (and maybe Tr17 and Tr19), however I replaced them all with brand new transistors recently so they shouldn't be the cause of the problem.

Thanks,
A.


On 1/08/13 1:04 PM, gerryrdahl wrote:

Hello,
Have you checked the .22ohm 1 watt (1P?) resistor?
I just repaired a Yamaha CS40M that had that resistor out (.47 on the CS40M) and that was the cause of the -15 volt failure and the other voltages not meeting full voltage (+10 was +8 and so on).

Once I replaced that all came into full power.

Check those out.

GD

--- In vintagesynthrepair@yahoogroups.com, "cpt.zilog" wrote:
>
> Hello guys and sorry to jump in like that but I would suggest to change the big caps as a priority and then check the bridge rectifier again for leakage of AC current.
>
> I had a big problem with a D&R console's psu in which the voltage regulators kept on going Kapput. Turned out that the big Electrolytics were bad and that induced a big stress on the rectifier bridge, which in turn had the consequence of not only letting DC pass as expected, but also AC at the same time !
>
> I did test for AC leakage on the rectifier after the newly installed voltage regulator worked for a couple of seconds before shorting...
>
> After new BIG caps, new bridge rectifier and a 2nd new voltage regulator, everything was sorted.
>
> Good luck !
>

> 
>     --- In vintagesynthrepair@yahoogroups.com, "cpt.zilog" <cpt.zilog@> wrote:
>     >
>     > Hello guys and sorry to jump in like that but I would suggest to change the big caps as a priority and then check the bridge rectifier again for leakage of AC current.
>     > 
>     > I had a big problem with a D&R console's psu in which the voltage regulators kept on going Kapput. Turned out that the big Electrolytics were bad and that induced a big stress on the rectifier bridge, which in turn had the consequence of not only letting DC pass as expected, but also AC at the same time !
>     > 
>     > I did test for AC leakage on the rectifier after the newly installed voltage regulator worked for a couple of seconds before shorting...
>     > 
>     > After new BIG caps, new bridge rectifier and a 2nd new voltage regulator, everything was sorted.
>     > 
>     > Good luck !
>     >
>

From: cpt.zilog

To: vintagesynthrepair@yahoogroups.com

Sent: Thursday, August 01, 2013 5:20 PM

Subject: [vintagesynthrepair] Re: CS-70m power supply issue, -15V rail is dead

Excuse me but I'd like to know if I did say something false or stupid in my previous message ?
Don't want to sound rude, but if my idea is wrong, my ego can handle it as long as someone corrects me (and write it!) ;)
Thanks

>
> --- In vintagesynthrepair@yahoogroups.com, "cpt.zilog" wrote:
> >
> > Hello guys and sorry to jump in like that but I would suggest to change the big caps as a priority and then check the bridge rectifier again for leakage of AC current.
> >
> > I had a big problem with a D&R console's psu in which the voltage regulators kept on going Kapput. Turned out that the big Electrolytics were bad and that induced a big stress on the rectifier bridge, which in turn had the consequence of not only letting DC pass as expected, but also AC at the same time !
> >
> > I did test for AC leakage on the rectifier after the newly installed voltage regulator worked for a couple of seconds before shorting...
> >
> >; After new BIG caps, new bridge rectifier and a 2nd new voltage regulator, everything was sorted.
> >
> > Good luck !
>; >
>

On 2013-08-01 08:20, cpt.zilog wrote:
> Excuse me but I'd like to know if I did say something false or stupid
> in my previous message ?
>  Don't want to sound rude, but if my idea is wrong, my ego can handle
> it as long as someone corrects me (and write it!) ;)
>  Thanks
>
>  >
>  > --- In vintagesynthrepair@yahoogroups.com, "cpt.zilog" 
> <cpt.zilog@> wrote:
>  > >
>  > > Hello guys and sorry to jump in like that but I would suggest to
> change the big caps as a priority and then check the bridge rectifier
> again for leakage of AC current.
>  > >
>  > > I had a big problem with a D&R console's psu in which the
> voltage regulators kept on going Kapput. Turned out that the big
> Electrolytics were bad and that induced a big stress on the rectifier
> bridge, which in turn had the consequence of not only letting DC pass
> as expected, but also AC at the same time !
>  > >
>  > > I did test for AC leakage on the rectifier after the newly
> installed voltage regulator worked for a couple of seconds before
> shorting...
>  > >
>  > > After new BIG caps, new bridge rectifier and a 2nd new voltage
> regulator, everything was sorted.
>  > >
>  > > Good luck !
>  > >
>  >

>
> I'm sorry, not at all!  I just haven't been able to get to my venerable 
> old Tektronics CRO, which is buried at the rear of my shed behind an 
> enormous pile of junk that is too unstable to climb over :-(
> 
> Your advice is excellent; my failure to follow it is the only thing 
> lacking.  Please accept my apology, I appreciate your help.
> 
> Cheers,
> A.
> 
> 
> On 2013-08-01 08:20, cpt.zilog wrote:
> > Excuse me but I'd like to know if I did say something false or stupid
> > in my previous message ?
> >  Don't want to sound rude, but if my idea is wrong, my ego can handle
> > it as long as someone corrects me (and write it!) ;)
> >  Thanks
> >
> >  >
> >  > --- In vintagesynthrepair@yahoogroups.com, "cpt.zilog" 
> > <cpt.zilog@> wrote:
> >  > >
> >  > > Hello guys and sorry to jump in like that but I would suggest to
> > change the big caps as a priority and then check the bridge rectifier
> > again for leakage of AC current.
> >  > >
> >  > > I had a big problem with a D&R console's psu in which the
> > voltage regulators kept on going Kapput. Turned out that the big
> > Electrolytics were bad and that induced a big stress on the rectifier
> > bridge, which in turn had the consequence of not only letting DC pass
> > as expected, but also AC at the same time !
> >  > >
> >  > > I did test for AC leakage on the rectifier after the newly
> > installed voltage regulator worked for a couple of seconds before
> > shorting...
> >  > >
> >  > > After new BIG caps, new bridge rectifier and a 2nd new voltage
> > regulator, everything was sorted.
> >  > >
> >  > > Good luck !
> >  > >
> >  >
>

All right and thanks for your message(s)!

You don't have to use a scope to test for AC leakage. A multimeter can do the job. Set it to AC voltage test and probe the DC output tabs of the rectifier bridge. If you get a significant reading, then you know that the bridge is toasted.

I think it make sense for a PSU this age (and with an obvious problem) to replace all passive components at first such as electrolytics which are essential for stability (all of them, starting with the big regulating ones), and test others like you did for resistors, as an example. Once you're sure about the reliability of your passives, you can tackle your actives (transistors, IC, etc...)

--- In vintagesynthrepair@yahoogroups.com, eidorian@... wrote:
>
> I'm sorry, not at all! I just haven't been able to get to my venerable
> old Tektronics CRO, which is buried at the rear of my shed behind an
> enormous pile of junk that is too unstable to climb over :-(
>
> Your advice is excellent; my failure to follow it is the only thing
> lacking. Please accept my apology, I appreciate your help.
>
> Cheers,
> A.
>
>
> On 2013-08-01 08:20, cpt.zilog wrote:
> > Excuse me but I'd like to know if I did say something false or stupid
> > in my previous message ?
> > Don't want to sound rude, but if my idea is wrong, my ego can handle
> > it as long as someone corrects me (and write it!) ;)
> > Thanks
> >
> > >
> > > --- In vintagesynthrepair@yahoogroups.com, "cpt.zilog"
> > wrote:
> > > >
> > > > Hello guys and sorry to jump in like that but I would suggest to
> > change the big caps as a priority and then check the bridge rectifier
> > again for leakage of AC current.
> > > >
> > > > I had a big problem with a D&R console's psu in which the
> > voltage regulators kept on going Kapput. Turned out that the big
> > Electrolytics were bad and that induced a big stress on the rectifier
> > bridge, which in turn had the consequence of not only letting DC pass
> > as expected, but also AC at the same time !
> > > >
> > > > I did test for AC leakage on the rectifier after the newly
> > installed voltage regulator worked for a couple of seconds before
> > shorting...
> > > >
> > > > After new BIG caps, new bridge rectifier and a 2nd new voltage
> > regulator, everything was sorted.
> > > >
> > > > Good luck !
> > > >
> > >
>

>
> Thanks for the multimeter suggestion, but I decided it was past time to 
> get the CRO (a Tektronics 465M) out anyway so I could have a good look 
> around and be sure.  The shed needs a bit of cleanup now, but to be 
> honest it's probably not that much worse than it was before anyway :-)
> 
> On the -26V and +26V coming from the rectifier bridge I measure about 
> 0.05V ripple each, which seems very reasonable.  On all the test points 
> (+5V, +10V, -5V and the faulty +15V and -15V) I can't see any 
> appreciable ripple at all (even with the scope resolution wound right 
> up).  So it looks like that is not the issue with this supply.
> 
> However, your suggestion is another good thing to check on future 
> repairs, so thank you very much for that!
> 
> Cheers,
> A.
> 
> 
> On 2/08/13 6:01 PM, cpt.zilog wrote:
> >
> > All right and thanks for your message(s)!
> >
> > You don't have to use a scope to test for AC leakage. A multimeter can 
> > do the job. Set it to AC voltage test and probe the DC output tabs of 
> > the rectifier bridge. If you get a significant reading, then you know 
> > that the bridge is toasted.
> >
> > I think it make sense for a PSU this age (and with an obvious problem) 
> > to replace all passive components at first such as electrolytics which 
> > are essential for stability (all of them, starting with the big 
> > regulating ones), and test others like you did for resistors, as an 
> > example. Once you're sure about the reliability of your passives, you 
> > can tackle your actives (transistors, IC, etc...)
> >
> > --- In vintagesynthrepair@yahoogroups.com 
> > <mailto:vintagesynthrepair%40yahoogroups.com>, eidorian@ wrote:
> > >
> > > I'm sorry, not at all! I just haven't been able to get to my venerable
> > > old Tektronics CRO, which is buried at the rear of my shed behind an
> > > enormous pile of junk that is too unstable to climb over :-(
> > >
> > > Your advice is excellent; my failure to follow it is the only thing
> > > lacking. Please accept my apology, I appreciate your help.
> > >
> > > Cheers,
> > > A.
> > >
> > >
> > > On 2013-08-01 08:20, cpt.zilog wrote:
> > > > Excuse me but I'd like to know if I did say something false or stupid
> > > > in my previous message ?
> > > > Don't want to sound rude, but if my idea is wrong, my ego can handle
> > > > it as long as someone corrects me (and write it!) ;)
> > > > Thanks
> > > >
> > > > >
> > > > > --- In vintagesynthrepair@yahoogroups.com 
> > <mailto:vintagesynthrepair%40yahoogroups.com>, "cpt.zilog"
> > > > <cpt.zilog@> wrote:
> > > > > >
> > > > > > Hello guys and sorry to jump in like that but I would suggest to
> > > > change the big caps as a priority and then check the bridge rectifier
> > > > again for leakage of AC current.
> > > > > >
> > > > > > I had a big problem with a D&R console's psu in which the
> > > > voltage regulators kept on going Kapput. Turned out that the big
> > > > Electrolytics were bad and that induced a big stress on the rectifier
> > > > bridge, which in turn had the consequence of not only letting DC pass
> > > > as expected, but also AC at the same time !
> > > > > >
> > > > > > I did test for AC leakage on the rectifier after the newly
> > > > installed voltage regulator worked for a couple of seconds before
> > > > shorting...
> > > > > >
> > > > > > After new BIG caps, new bridge rectifier and a 2nd new voltage
> > > > regulator, everything was sorted.
> > > > > >
> > > > > > Good luck !
> > > > > >
> > > > >
> > >
> >
> >
>

On 2013-08-07 03:42, Hugh Vartanian wrote:
> Hi Adrian, I have been offline for a bit.  Can you re-cap (! ) where
> the circuit is at today?   Is your post of 8/1 the latest?   I 
> confess
> to being a little amused to hear of +15v issues when debugging 
> -15v!   They are surely dependent on each other, as we all know. 
> Anyhow, please confirm the current (! ) situation voltage readings,
> etc. & we'll figure this out.  Where do you live, btw?  My basement
> always has room for one more thing to work on.  Qualified for 
> hoarders
> show a long time ago!  <smile>
>  -Hugh
> On Aug 7, 2013 5:16 AM, "cpt.zilog" <cpt.zilog@...> wrote:
>
>>  
>>
>> > > I will have to recap my cs-70m's power supply (still working
>> > > fortunately), because it's all original and it takes some time 
>> for it
>> > > to warm-up (too long actually), and the volume pot is noisy when 
>> you
>> > > turn it, but it's not a dirty pot because noise almost 
>> disappears when
>> > > the synth is warmed-up.
>> > >
>> >
>> > Interesting - maybe it is some sort of oxidization that is
>> > temperature-dependent? Sounds like you should break out /your/ 
>> scope
>> > and measure the power rails at that point ;-)
>>
>> I don't think so ;-)
>> As I said, it works quite nicely already, why should I bother NOT 
>> changing the capacitors and thus investigating elsewhere ? :-)
>> The CS had the majority of his life in south France where the 
>> climate is nicely dry, helping avoiding storage oxidization.
>>
>> > > I want to install the kenton midi kit that I have for it, but 
>> will do
>> > > once everything -or at least the psu- be properly recapped...
>> > >
>> >
>> > I had a look at that MIDI kit - looks like it will cost me around
>> > AUD$600 to buy, which is a lot, especially factoring in that I 
>> have a
>> > whole pile of gear I'd ideally like to get converter kits for. I'd 
>> be
>> > interested in hearing what others who have one installed have to 
>> say (or
>> > if anyone has one they'd like to sell me for less than that much.)
>>
>> Well I paid that kind of money for it. It is expensive that's for 
>> sure, but there isn't any alternative. Plus it's such a limited 
>> production that the people who buy it, do install it (quite logical 
>> indeed). Kenton used to sell this kit back in their heydays then stop 
>> producing it. They made a new production run thanks to a group buy 
>> initiated buy Analog Sweden. It allowed for me to be able to put my 
>> hands on one of these kits. What they sell on their website should be 
>> what's left of this group buy (me think). So finding one second hand 
>> out in the wild would be very hard if not impossible to say the least.
>>
>> > > You just don't want to open those beasties to often ;)
>> > >
>> >
>> > In general I agree, but I have to say the CS-70m power supply has 
>> been
>> > pretty convenient to work on. The leads from the transformer are 
>> long
>> > enough that I've been able to leave it connected to the "DC" board 
>> with
>> > that board sitting outside the case, nice! I'm not so impressed 
>> with
>> > the exposed incoming AC supply lines on the rear panel switch PCB
>> > though, best keep fingers away from there :-
>> >
>> > Cheers,
>> > A.
>> >
>>
>> Will keep my fingers away....thanks for the tip ;-)
>>
>> Vincent.
>
> 
>
> Links:
> ------
> [1]
> 
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> [4]
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> [7] http://docs.yahoo.com/info/terms/

On 12 Aug, 2013, at 3:47 PM, eidorian@... wrote:

> Hi Hugh,
>
> Thanks for your email and sorry for the slow response.  I've been  
> doing
> too much other non-electronics stuff lately :-(
>
> At the moment the +15V rail is measuring ~ +17V and the -15V rail
> "exponentially" rises from ~ -12V or so to ~ +0.8V when power is  
> applied
> then stays there  (it gets to 0V in about 3-5 seconds, then slowly
> creeps up to +0.8V over the next 30 seconds or so).  The 1K resistor  
> in
> the +15V circuit is very hot to touch.  I've replaced or tested OK
> pretty-much every component on the board now.
>
> I agree that there is probably just one common problem causing the
> issues with both the +15V and -15V rails.  Next step I think is trying
> to isolate what's causing the 1K resistor to carry all the current
> rather than it flowing through the darlington transistor circuit  
> like it
> should.  I'm hoping once that's fixed the +15V will self-heal due to
> some unusual condition being removed.  Sorry if that sounds vague but
> I'm not in front of the schematic at the moment!
>
> Also I'm about to go on holidays for two weeks in a couple of days'
> time, so unfortunately it'll be a while before I get back to this one.
>
> I'm in Australia, so a long way from most people :-)
>
> Cheers,
> A.

On 2013-08-07 03:42, Hugh Vartanian wrote:
> Hi Adrian, I have been offline for a bit.  Can you re-cap (! ) where
> the circuit is at today?   Is your post of 8/1 the latest?   I 
> confess
> to being a little amused to hear of +15v issues when debugging 
> -15v!   They are surely dependent on each other, as we all know. 
> Anyhow, please confirm the current (! ) situation voltage readings,
> etc. & we'll figure this out.  Where do you live, btw?  My basement
> always has room for one more thing to work on.  Qualified for 
> hoarders
> show a long time ago!  <smile>
> -Hugh
> On Aug 7, 2013 5:16 AM, "cpt.zilog" <cpt.zilog@yahoo.fr <mailto:cpt.zilog%40yahoo.fr> > wrote:
>
>>  
>>
>> > > I will have to recap my cs-70m's power supply (still working
>> > > fortunately), because it's all original and it takes some time 
>> for it
>> > > to warm-up (too long actually), and the volume pot is noisy when 
>> you
>> > > turn it, but it's not a dirty pot because noise almost 
>> disappears when
>> > > the synth is warmed-up.
>> > >
>> >
>> > Interesting - maybe it is some sort of oxidization that is
>> > temperature-dependent? Sounds like you should break out /your/ 
>> scope
>> > and measure the power rails at that point ;-)
>>
>> I don't think so ;-)
>> As I said, it works quite nicely already, why should I bother NOT 
>> changing the capacitors and thus investigating elsewhere ? :-)
>> The CS had the majority of his life in south France where the 
>> climate is nicely dry, helping avoiding storage oxidization.
>>
>> > > I want to install the kenton midi kit that I have for it, but 
>> will do
>> > > once everything -or at least the psu- be properly recapped...
>> > >
>> >
>> > I had a look at that MIDI kit - looks like it will cost me around
>> > AUD$600 to buy, which is a lot, especially factoring in that I 
>> have a
>> > whole pile of gear I'd ideally like to get converter kits for. I'd 
>> be
>> > interested in hearing what others who have one installed have to 
>> say (or
>> > if anyone has one they'd like to sell me for less than that much.)
>>
>> Well I paid that kind of money for it. It is expensive that's for 
>> sure, but there isn't any alternative. Plus it's such a limited 
>> production that the people who buy it, do install it (quite logical 
>> indeed). Kenton used to sell this kit back in their heydays then stop 
>> producing it. They made a new production run thanks to a group buy 
>> initiated buy Analog Sweden. It allowed for me to be able to put my 
>> hands on one of these kits. What they sell on their website should be 
>> what's left of this group buy (me think). So finding one second hand 
>> out in the wild would be very hard if not impossible to say the least.
>>
>> > > You just don't want to open those beasties to often ;)
>> > >
>> >
>> > In general I agree, but I have to say the CS-70m power supply has 
>> been
>> > pretty convenient to work on. The leads from the transformer are 
>> long
>> > enough that I've been able to leave it connected to the "DC" board 
>> with
>> > that board sitting outside the case, nice! I'm not so impressed 
>> with
>> > the exposed incoming AC supply lines on the rear panel switch PCB
>> > though, best keep fingers away from there :-
>> >
>> > Cheers,
>> > A.
>> >
>>
>> Will keep my fingers away....thanks for the tip ;-)
>>
>> Vincent.
>
> 
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