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

Gene Stopp gene at ixiacom.com
Wed Aug 7 20:03:44 CEST 2002

God I love troubleshooting. It's a nice break from kids and work... it's
what I use for meditation. My wife and I were slouched on the sofa last
night watching TV (the nightly "veg" ritual to wind down from the day) and I
kept sneaking off into the garage and trying one little thing after another.
I could not control myself.

I hope that this thread is interesting to those out there who are new to
building and troubleshooting. The troubleshooting process is really
something that is not easily quantified or proceduralized. So many times I
have seen attempts to put it on paper, with flow charts and common-sense
guidelines, like:

Is it plugged in?
    No - plug it in
    Yes - is there output?
        No - look here
        Yes - look there

and so on.

But it's really a matter of dreaming up what could be going on based on your
mental picture of how the thing works as a whole. Any and all ideas are
equally valid at first, and then ways are thought up on how to prove or
disprove them. It gets faster with experience, but even after all these
years the first moments of troubleshooting are the same as when you are just
starting out!

The Minimoog is fixed. The solution was De-Ox-It.

I found the problem with a scope. Tool #1 guys. As you may know the Minimoog
really has 2 VCA's inside, one for the envelope shaping of the amplitude,
then a second after the first for voltage control of the final volume of the
instrument. The second one is controlled by the External Volume control jack
which has a switching contact to V+ so that if nothing's plugged in then the
VCA is held on. Well the first VCA was being held on too, by the loudness
contour envelope, which was never discharging, because there was an open in
a jones connector on the discharge signal path through the chassis wiring

The final decay point in the Minimoog envelope generators is established by
diodes to a signal that goes to ground when all keys are lifted. Although
the Minimoog uses S-Triggers for external envelope control, internally there
exists the more traditional high-when-key-pressed gate signal. This signal
is brought out to an edge connector pin, which goes through the wiring
harness into the pitch/mod wheel assembly by the keyboard, and turned around
at the connector back into the two envelope generators at their edge
connector pins. I saw on the scope that the signal was being output properly
into the wiring harness, but not making it back in to the other pins. A
contact-cleaning at the jones connector and reassembly made things right.

Apparently the VCO bleed-through is normal, and squelched when the loudness
VCA closes. Interesting, huh.

So, maybe rule #1 of troubleshooting should be:

It is probably, not always, but probably, something really easy.

- Gene

-----Original Message-----
From: Gene Stopp [mailto:gene at ixiacom.com]
Sent: Tuesday, August 06, 2002 11:44 AM
To: 'synth-diy at dropmix.xs4all.nl'
Subject: RE: [sdiy] Audio bleed-thru on a minimoog...

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.

More information about the Synth-diy mailing list