[sdiy] P.S. troubleshooting help

[Scott Gravenhorst]

"Scott" <Scott at scottwick.com> wrote:
>I have tried w/ a dummy load and get the same results. It's not 
>the synth. I must have done something to the ps last night while 
>I was testing it. 
>
>I need to desolder the main transistors before I can look at the 
>traces on the PCB. I'll do that in a few minutes. (The big 
>transistors mounted to the frame are basically what holds the PCB 
>in place!) 
>
>I'm starting to think it's the transistor... On the positive 
>side, the case reads 23v and the pins read 15.. on the negative 
>side the case reads .66v(what should be -15v if it were working) 
>and the pins read -23. (or is it supposed to be backwards, since 
>it's the negative side?) 

I'm just guessing here, but what you've described is, so far, two positive voltage
regulators in which for the positive rail, the negative side is ground and for the
negative rail the positive side is ground.  As such, for the negative rail, I
wouldn't expect to see a positive voltage on the input side of the pass transistor. 
Is that +0.66V or -0.66V? (which looks suspiciously like a diode voltage for what
that might be worth)  If the positive rail has it's input on the pass transistor's
case terminal, then I'd expect the case of the pass transistor on the negative rail
to be it's input as well.  I am also assuming that your volt meter reference is
always the power supply's ground rail.

>-----Original Message-----
>From: synth-diy-bounces at dropmix.xs4all.nl 
>[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Scott 
>Gravenhorst Sent: Thursday, April 02, 2009 2:09 PM To: 
>synth-diy at dropmix.xs4all.nl Subject: Re: [sdiy] P.S. 
>troubleshooting help 
>
>I would start with a dummy load on both + and - sides of the supply and
>see if you can replicate the failure outside of the synth cabinet. 
>Power resistors calculated to draw a certain amount of current, perhaps
>something in the range of 15 ohms for 1 ampere 60 ohms for 0.25 A. 
>Remember to consider power dissipation and provide for that.  Really
>sucks not to have the schematic.  Do you know how much current the
>synth draws?  Are you possibly causing it to shut down due to
>overcurrent?  The part number - does that suggest 3 amperes per rail? 
>Is it possible to trace out the schematic by looking at the PCB?
>
>"Scott" <Scott at scottwick.com> wrote:
>>Can someone help me troubleshoot my power one hcc-15-3-A? I took 
>>it out of my synth cabinet last night to test it. It tested fine. 
>>(checked for ac ripple w/ the scope, checked to see if it was 
>>loosing power when my AC power drops, etc.. was all fine) I put 
>>it back in my synth today.. doesn't work. Take it out and test 
>>it. I get 15 volts on the + and .66 volts on the - side. 
>>
>>The pins of the main power transistor on the + side read 15 
>>volts. On the negative side the pins read 23! (I'm guessing it 
>>should read -15) Then Im not sure what happens to the signal, 
>>because almost everywhere else I test on the - side of the pcb, I 
>>read .66 volts. 
>>
>>I did try replacing the regulator IC (lm723n) (swapped sides, put 
>>negative on pos, pos on neg) and the positive side still works 
>>fine, so that isn't it. (If you aren't familiar w/ them, they 
>>aren't positive and negative regulators.. they are the exact same 
>>chip) 
>>
>>So, having never tried to fix a power supply, and not having the 
>>schematics for this thing... im not sure where to go from here. 
>>
>>let me know if anyone has any ideas. Hopefully that .66 v on the 
>>negative side will give someone more informed than me, a clue as 
>>to what is going on. I also tested the transformer, and it is 
>>outputting exactly what it should on every 2ndary. 
>>
>>thanks a lot!
>>
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>>
>
>-- ScottG
>________________________________________________________________________
>-- Scott Gravenhorst
>-- FPGA MIDI Synthesizer Information: home1.gte.net/res0658s/FPGA_synth/
>-- FatMan: home1.gte.net/res0658s/fatman/
>-- NonFatMan: home1.gte.net/res0658s/electronics/
>-- When the going gets tough, the tough use the command line.
>
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>

-- ScottG
________________________________________________________________________
-- Scott Gravenhorst
-- FPGA MIDI Synthesizer Information: home1.gte.net/res0658s/FPGA_synth/
-- FatMan: home1.gte.net/res0658s/fatman/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- When the going gets tough, the tough use the command line.