emu modular synth oddity

looking at this for a friend. We already have it running as I had to build a PSU for it.
I have schematics.  but have an odd issue. I have 2 X vcf(voltage controled filters) on his. One is super quiet and sounds great working as it should.  The second VCF works as is should but is hissy and noisy. the VCF consists of a main PCB that has resistors, pots, jacks on it and a sealed daughter card with the filter in it. Swapping filter daughter cards keeps the nois and hiss to the same main PCB. 
On the main PCB all the resistors are good(have been tested), all the pots and connections are good. I measured traces for high resistance  and inspected for  cracked traces and  did not find any issue.  I have re-flowed all the joints.
Any ideas of what else to check?

You say the resistors test good, but perhaps the value is OK but they are still noisy?  Like my tube tester tests that a tube amplifies OK but it may still be noisy.  Especially if they are carbon comp resistors, it may be worth replacing any high value ones.

mjrippe said:

You say the resistors test good, but perhaps the value is OK but they are still noisy?  Like my tube tester tests that a tube amplifies OK but it may still be noisy.  Especially if they are carbon comp resistors, it may be worth replacing any high value ones.

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hmmmmmm. good point.  there is only 10  X 1/4W so would be cheap to do. 8 X 100K, 1 X 1K, 1X 2.7M so I'll do that and see.

pucho812 said:

hmmmmmm. good point.  there is only 10  X 1/4W so would be cheap to do. 8 X 100K, 1 X 1K, 1X 2.7M so I'll do that and see.

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I would certainly suspect the 2M7, maybe the 100K.  But if you do nine, why not all ten?  ;D

mjrippe said:

I would certainly suspect the 2M7, maybe the 100K.  But if you do nine, why not all ten?  ;D

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exactly. there is nothing else on there., some sealed pots and the filter on a daughter board but moving the filter to a known good VCF mainboard and the problem stays with the bad main board. Reflowing solder joints for jacks and such.

And it's back. Noisy 100k resistor...
re flower joints and swapped out resistors .  Boom sounds like the good one.  So now there are 2 good ones.  Thanks

pucho812 said:

And it's back. Noisy 100k resistor...
re flower joints and swapped out resistors .  Boom sounds like the good one.  So know there are 2 good ones.  Thanks

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Awesome!  Any photos of the synth?

I can get some, I don't have any on hand. This one, started off as my buddy purchased it off eBay years ago. It had no cabinet and no power supply.  Many years later he wants to get it working so he asked me to help refer it. It runs off +/-15VDC and 5VDC.  I  got some oem power supplies and got started. Recapped, cleaning, etc.  I am currently doing a cabinet for it and once done I will have photos.  His version is 2 X rows of modules vs the stock photos that have 4 X rows of modules + keyboard. His version is missing the keyboard. but no matter it can still produce sound and we have ways to get in and out of it to run it as if a keyboard was attached. The first power up with the power supplies + recap and  most modules were working. The singular VCF was the only one that was funny.  But now it's 100% functional.  So photos will be soon.

Nice, never seen one in person.

+1
Which emu is it? You mention schematics, were they with the synth or did you find them on the web?

Jarno said:

+1
Which emu is it? You mention schematics, were they with the synth or did you find them on the web?

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I found them on the web. It's the 1972 emu modular.  the one I did the PSU for, fixed the VCF and such is not a full unit, it is missing a 2 x rows of circuits and the keyboard but it has 2 complete rows of of stuff, so like a half version.