Polymoog Power Supply (Moog Board) +5V supply inoperative

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Polymoog Power Supply (Moog Board) +5V supply inoperative

Postby Dr. Phibes » Sun May 13, 2012 7:48 pm

I'm getting annoyed with this thing. Replaced the old electrolytic capacitors, and checked the rectifying diodes. As far as I can tell with DMM test/ process of elimination the relevant pass transistors are o.k. I don't have the capability to test the little control ICs but I've switched them around and the problem always persists on the the 5V supply.

Now, knowing me I fully expect the problem to be a broken trace or a shitty solder joint and I shall check for these things once again, but if anyone has a suggestion or a bit of advice I'll be very grateful.


Here's one of many pics. I haven't seen any grossly deformed components but maybe I'm missing something.

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Re: Polymoog Power Supply (Moog Board) +5V supply inoperativ

Postby ApolloBoy » Sun May 13, 2012 8:53 pm

I don't know much about the Polymoog but I'm willing to bet it might be those ICs.
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Re: Polymoog Power Supply (Moog Board) +5V supply inoperativ

Postby Don T » Mon May 14, 2012 9:22 am

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Last edited by Don T on Mon Feb 04, 2013 8:04 pm, edited 1 time in total.
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Re: Polymoog Power Supply (Moog Board) +5V supply inoperativ

Postby Steve Jones » Mon May 14, 2012 10:13 am

From the service manual: Look for short to ground on the 5V supply within the instrument. On the PSU board check: P3, CR15 thru 18, Q5 thru Q7, IC3, R33, P4 and C9.
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Re: Polymoog Power Supply (Moog Board) +5V supply inoperativ

Postby Dr. Phibes » Mon May 14, 2012 8:19 pm

Thanks for the responses:

Don T wrote:Ok, what kind of trimpots are installed in the the thing? I've honestly never seen any that look like that.


hehe, if you don't know I sure don't; but they are 1kΩ variable resistors that appear to do the job when tested. I even switched two around to see if I had missed something and problem persisted on the +5V supply.

I've learned to never assume anything, therefore: Check for AC voltage across the two green wires coming from the transformer. Then check for DC voltage at the Anode end of CR15 or CR16 (which should be negative), and at the Cathode end of CR18 or CR17 (which should be positive)


Yeah I feared that it may be a problem with the transformer itself. I'm getting an AC voltage of ~25V across the blue and grey pairs and ~10V across the green pair. Is this correct? I can't seem to find any reference in the service manual.

It's quite awkward for me to probe the 4 big rectifying diodes when the unit is connected but I think I'm getting a small negative voltage on the cathode end of CR17 and CR18 contrary to what you said. I beginning to think I should just whip those out - a basic diode test indicated they were ok but I'm running out of components to blame here! ;)

It may be worth replacing CR19, it's a 1N4748A zener diode, 22V, 1W. Zeners are a PITA to test.


Yep this was something I thought of too but I put in a new one and the problem persists.

Take Q7, Q6, and Q5 off the board and test them out of circuit. Be sure to also check them "backwards" for leakage using the diode test function on your DMM.

Let us know how those turn out! Good Luck!


I think they came out alright but I'll check them again.
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Re: Polymoog Power Supply (Moog Board) +5V supply inoperativ

Postby Don T » Mon May 14, 2012 11:57 pm

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Re: Polymoog Power Supply (Moog Board) +5V supply inoperativ

Postby garranimal » Tue May 15, 2012 1:30 am

What kind of problem is it? Voltage too high/low? Is this in disconnected state or under load? This is a case I would audit all currents going in (from AC) and the sum of all currents going out of the power supply. It should lead you to the cause pretty quick.

I had such problems under load with an old stringer. After the current evaluations I discovered the problem to not be on the power supply board at all. A transistor down circuit had blown and was drawing too much current from the PS.
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Re: Polymoog Power Supply (Moog Board) +5V supply inoperativ

Postby Dr. Phibes » Thu May 17, 2012 8:40 pm

Replaced all four of those large rectifying diodes but still no luck. An awkward request I know, but has anyone got an image of the underside of the power supply? I'm wondering if I may have shorted a trace accidentally. Foolishly I only took a picture of the component side of the board before I started fiddling.

There is a diagram at the very end of service manual I know, but it's fairly grainy.
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Re: Polymoog Power Supply (Moog Board) +5V supply inoperativ

Postby Dr. Phibes » Thu May 17, 2012 8:41 pm

garranimal wrote:What kind of problem is it? Voltage too high/low? Is this in disconnected state or under load? This is a case I would audit all currents going in (from AC) and the sum of all currents going out of the power supply. It should lead you to the cause pretty quick.

I had such problems under load with an old stringer. After the current evaluations I discovered the problem to not be on the power supply board at all. A transistor down circuit had blown and was drawing too much current from the PS.


There are minor problems elsewhere in the instrument, but I testing directly off the power supply so the problem must be there or in the transformer..
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