[sdiy] Serious filter problem, please help!

[Peter Grenader]

If it helps, I've repaired about 5 or so filters that have crapped out
resonance control.  A couple of A Sys, a Steiner-Parker orginal, a Doepfer
Multimode and Wasp,  and a Cyndustries. I guess that's more than 5.

There's been a number of causes.  The good news, most resonance circuits are
fairly simple feedback loops, so the componentry involved in this portion of
the circuit is minimal.

But trends I've seen are:

1) bad solder joints

2) blown op amp in the feedback loop

3) bad res pots (believe it or not, this happened twice. both with Alpha
pots).

In most filters, the feedback amplifier replacement is a simple act of
changing an IC. An original Steiner Parker filter for instance however is a
bit more complicated because it's all transistor based.  No ICs in the
circuit at all.  It takes a bit more time to hunt the problem down.

Do you have schematic for this filter?  Without it you're shotgunning and
it's going to make it a lot more difficult.

If you'd like me to look at the schemo, send me a post offline.

hope this helps,

- Peter


Antti Pitkämäki 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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