[sdiy] Audio bleed-thru on a minimoog...

[Gene Stopp]

And now for today's exciting episode of "what's wrong with the minimoog"...

Poking around with the o-scope has revealed that the -10 rail going into the
VCF/VCA/ouput PCB has the raw VCO mix superimposed on it. Not very big, a
few millivolts maybe, but there nonetheless. I piggybacked a 3300uf 35V cap
across -10 to ground on the VCA itself - absolutely no effect! Maybe I
should try a 0.1uf...

I pulled this card thereby exposing the power supply card behind, and the
-10 sense lines are substantially clean. The sense lines come from the VCO
card, with a trace across the card from the B connector to the -10 input on
the A connector and so back to the power supply. I buzzed it out and it's
zero ohms.

I also did the eraser trick on the edge connector fingers while I was at it.

So - something on the VCO card pulling excess current? I doubt it, it's all
3 VCO's. Or, is it that the pass tranny on the -10 side on the power supply
card has lost some of it's ooomph? Both + and - 10 volt outputs can be
adjusted just fine with the voltage trim trimpots, although the -10 won't go
below 10 volts. Next step - maybe replacing the -10 pass transistor on the
power supply card. According to the schematic it's an MPS-U05, whatever the
heck that is. Time to google search for a cross-reference...

- Gene

-----Original Message-----
From: Grant Richter [mailto:grichter at asapnet.net]
Sent: Monday, August 05, 2002 1:16 PM
To: Gene Stopp; 'synth-diy at dropmix.xs4all.nl'
Subject: Re: [sdiy] Audio bleed-thru on a minimoog...




This rules out misadjusted balance trimpots on the VCA's. I don't think
there's a trimmer that will solve this problem. Just tossing it around in my
head I am thinking that there may be a ground connection that's opened up.
Probably the first thing I'll do is get the scope out and probe all of the
pins along the bottom of the VCF/VCA card, ac coupled, high gain, to see
where the signal is coming from. I'll let you know what I find!!


Sounds like dried out power supply bypass electrolytics. This would be after
the voltage regulators. If the bypassing is bad the oscillators will appear
everywhere through the power supply rails.