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

[Gene Stopp]

Hi DIY,
 
Here's one for y'all - a few days ago I took a risk with my financial
situation (and my wife's opinion of my spending habits) and got a minimoog.
It's been very well preserved and is completely stock. It's an intermediate
model with the middle-version VCO card (not the oldest, not the latest),
black plastic faceplate, and smooth white wheels. It's working great and
nicely tuned up. I am working on convincing my wife that minimoogs are cool
and it's OK to collect them - this worked with Mellotrons, she likes those.
I suppose one angle I can try is that minimoogs fit perfectly on top of
M400's. :)
 
There is a minor problem - you can hear the VCO's all the time, very
quietly, on the output. It's not a bad problem, but it ain't supposed to do
that. I've not yet done anything other than pop the back off to do a
physical exam. But as long as I'm stuck here at work unable to get the scope
out, I thought I'd throw this out on the list to see if others have seen
this before.
 
Here's what I've discovered so far:
 
It's coming out of the main output and the headphone output.
It's unfiltered (raw VCO mix)
It does not go away when the mixer switches are all off
All 3 VCO's are bleeding through
 
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!!

- Gene


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