[sdiy] Serious filter problem, please help!

[Antti Pitkämäki]

Hi!

I posted this question to the Analogue Heaven mailing list, and I got a 
reply that I should post this question to the synth-DIY-list. So I'm here 
only to ask this question, sorry about that. Well actually I have just 
started to learn electronics and I just built a preamp, so who knows, maybe 
I'll become an active listmember some day!

Here is the original question that I posted to AH:

"I bought the mysterious non-working Altair synth from Tom Moravansky. My 
tech has now almost fixed it, but there's one problem remaining: The synth's 
resonance knob does nothing, and the resonance seems to be on a low setting 
all the time. Now I know that most list members most likely know nothing 
about this synth, but since it is very heavily based on the Minimoog, I mean 
not only are the features very similar but also the schematics, and since AH 
must be filled with Minimoog-gurus, I ask:

-What could, on a Minimoog, cause its filter resonance to not to work if the 
resonance-potentiometer works, and the filter works otherwise OK? The 
problem must be some transistor but which one? (the Altair's filter 
schematic is similar but it has a chip (an op-amp?) in place of every two 
transistors when compared to the schematics of the Minimoog) "

I would appreciate any help... My tech is experienced in electronics but he 
is not experienced in repairing analog synths at all, and he finds it 
difficult to figure out what is the problem with the filter. He does not 
really know what part of the filter does what, but he did manage to fix one 
non-working oscillator from the synth so there is hope.

In case of anybody is interested, here's a link to the filter schematic of 
the Altair synth:
http://www.ruskeys.net/pasp/altair231/c/altcxem5.jpg
The schematic is written in cyrillic letters in Russian, but I guess it is 
of use if you compare it to the Minimoog's filter schematics.

Thanks,
Antti Pitkämäki

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