[sdiy] Serious filter problem, please help!

[Harry Bissell Jr]

The "ICs" are obviously dual transistors.

Q1  collector = 2
    base = 3
    emitter = 4

Q2  collector = 8
    base = 7
    emitter = 6

You can redraw the schematic with the transistor
symbols and it will be a lot more obvious what is
going
on. It does look a lot like the Minimoog D filter
schematic.

Looks like maybe the VCA is on this schematic as well.

Resonance pot looks like it goes from W! connector
pins 16 and 12. 

If the filter 'works' I'd trace this connection
carefully... maybe a bad pot, wire, or cap C1

H^) harry



--- Antti Pitkämäki <anpitkam at hotmail.com> wrote:

> 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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