Vintage Synth Repair

Try having a look at this
http://www.onsemi.com/pub_link/Collateral/MC7800-D.PDF it is the data sheet
for ON semi's 78xx range may help with the pinouts, also it may be worth
measuring the output voltages using the AC millivolt range if you have one
on your meter as this will give you an idea of the ripple on the supply
line, it may also be an idea to measure the input to the regulators as if
one of the rectifier diodes or a cap has gone open circuit before the
regulator the input voltage may not be high enough.

 

John Rose

 

(www.astrodevelopments.com)

 

From: vintagesynthrepair@yahoogroups.com
[mailto:vintagesynthrepair@yahoogroups.com] On Behalf Of Alan Probandt
Sent: 25 April 2012 00:22
To: vintagesynthrepair@yahoogroups.com
Subject: Re: [vintagesynthrepair] Re: Simmons SDE

 

  

The description sounds like a linear power supply as were common for music
gear in the mid 1980s.  In the 1990s small units (between 500 milliAmps and
1 Amp) began using switching power supplies.  The switchers run cooler and
more efficiently, but are more complicated.

   Generally test power supplies under full load.  In this case with the
rest of the unit attached.  But if the 7805 voltage is stable +5v with the
ribbon cable disconnected then some chip or part is  pulling more current
than it is supposed to.

If there is no sound except a buzzing, check if the buzzing is 60 or 120
Hertz.  If so, then power supply ripple voltage from the AC mains is getting
through to the output.   The output stage is usually an op-amp with plus and
minus voltages.  Check if there is voltage on the power pins of the op-amp.
Try running the chip numbers through google.  You'll get directed to the
data sheet companies but you can also get data sheets for older parts from
the manufacturer web sites.

  If nothing works, try selling it on eBay.  People there do pay real money
for broken equipment.

 

From: xjnx0x <concken@...>
To: vintagesynthrepair@yahoogroups.com 
Sent: Tuesday, April 24, 2012 1:13 PM
Subject: [vintagesynthrepair] Re: Simmons SDE


Thanks for your reply, to clarify the power supply is 120V input and is
built into the unit on its own little PC board with little plastic offset
clips, inside an aluminum sub-chassis which also contains the transformer.
I can also see plenty of large diodes on the board which are probably part
of the bridge rectifier.  

My idea for the next course of action tonight was testing the input and
output legs of each of the ICs to see what they're delivering.  Should this
be done with the rest of the unit connected via the ribbon cable, or
disconnected?  Will the presence/absence of the rest of the unit affect the
operation of the power supply?  

thanks for your help.  I'll look into an electronics club... good idea.  

Christian

--- In vintagesynthrepair@yahoogroups.com, Alan Probandt <alan_probandt@...>
wrote:
>
> First off, there's several types of power supplies.  The type that plugs
into the wall and converts 120VAC (220vac in UK and Europe) to, generally,
+5VDC and +12/-12VDC regulated. Versus those that have a transformer
wall-wart that plugs into the wall and delivers between 9 and 15 Volts AC to
the unit.  If you have the 120VAC version (there's a wall plug coming from
the Simmons) then trace out the high 120VAC paths on the power supply
circuit board.  Don't touch these copper lines with your fingers with the
power on.
>    The high voltage usually goes to a small internal transformer to be
converted to about 12 volts AC.  Then some diodes, bridge rectifiers, and
high value electrolytic capacitors to make plus and minus DC.  Then to the
voltage regulators like the 7805 and 7806.  The 7805 type of regulators are
very common.  They shut down when too much current makes them too hot.
Commonly used in 1980's electronic equipment.  If the 7805 is putting out
less than 4.9 volts then something is drawing too much current.
>   I'm at work and can't continue.  Try bringing it to a local Robotics
Club meeting, or the Electronics club or students group at the local
community college.
> 
> From: xjnx0x <concken@...>
> To: vintagesynthrepair@yahoogroups.com 
> Sent: Tuesday, April 24, 2012 8:58 AM
> Subject: [vintagesynthrepair] Simmons SDE
> 
> HI, I'm new to the list.  
> 
> I have a beloved Simmons SDE that started acting flaky circa 1998.  The
SDE is a 6 voice, 4-op FM percussion synthesizer midi rack unit.  At the
time it would fail to start up when turned on, but I could coax it into
working by flipping it on and off a few times.  Eventually this didn't work
anymore and I retired it.. it sat in the corner of my studio for about 12
years... Recently I decided I missed it terribly, I opened it up and found
some blown caps in the power supply.  I replaced these and got the unit to
turn on again.  It now powers up nicely and all the functions on the front
seem to work just fine, responds to midi, responds to button presses,
responds to knob movements on the display (only through about half of their
range), but there is no sound except for an intermittent buzzing.  
> 
> I took it into a local shop on Saturday, they didn't know what it was and
they didn't want to mess with it, told me to try reloading the patches...
I'm pretty sure that's not the problem because the patches are at least half
unchangeable presets which I assume would be stored in ROM.. If this thing
is to live again, its is up to me to raise it from the dead.  I can't find
schematics for it anywhere.  
> 
> So I looked up the Yamaha FM chips... The datasheet says they need 5v,
minimum 4.8v, but I measured the voltage between pin 1 and 21 and they are
only getting 4.5v.  
> 
> Because of this, I'm suspecting the PS has more problems that need to be
addressed and I think that's the place to start.  I'm hoping, fingers
crossed, that the PS going out and me flipping it on and off like a dumbass,
didn't fry any of the output circuitry.
> 
> There are 3 ICs in the PS, a 7805, a 7806, and another one I can't
remember off the top of my head right now (88xx something?) but it has the
same package as the two voltage regulators.  All three of these were at one
time bonded to a little aluminum tray inside the case for a heat sink.  I
don't remember if there were still bonded when I opened it up, but they are
not bonded now as I had to remove the board to replace the caps.  I've tried
to make sure they are in contact with the aluminium whenever I power it up
but I don't have thermal contact paste and they and the transformer coil do
get pretty hot.  The datasheet says the 7805 can't deliver as much current
when its not heat sink-ed (heat sunk??) I'd assume this is the case for the
others as well.  
> 
> So, how should I go about troubleshooting the PS circuit, without a
schematic or even knowledge of what the voltages are supposed to be?  Any
suggestions for dissipating the heat with something temporary? (I'm sure it
will be taken apart a few more times before its fixed.)  Anything I should
watch out for?  Could the ICs not being in thermal contact have hurt them?
Could the PS going bad have damaged other parts of the machine?  
> 
> I have some experience soldering and building things, but don't mess with
power supplies much because frankly they kinda scare me.  
> 
> 
> 
> 
> 
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