[sdiy] Serious filter problem, please help!

[Magnus Danielson]

From: "Antti Pitkämäki" <anpitkam at hotmail.com>
Subject: [sdiy] Serious filter problem, please help!
Date: Wed, 08 Dec 2004 17:36:06 +0200
Message-ID: <BAY22-F37B7335C0C37BCC28E5D2CCAB60 at phx.gbl>

> 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 don't have the Altair schematics, but I do have the Minimoog service manual.
Assuming they are the same, except for the details of how the transistors is
packaged, then if the output tap of the Emphasis knob has signal, this should
go straigh to the input transistor-pair of the ladder-filter except for a
trimpot labeled Regen Cal. Excercise this pot if you have it.

> 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.

The bottom transistor is part of a current generator with exponential responce
(together with another transistor). The bottom pair of the ladder act as input
pairs of the ladder while all other pairs act as current-sensitive
transconductances. Four transistors act as high-impedance amplifiers and
generates the output signal. This is sent to the emphasis knob where a feedback
path forms for the emphasis control.

After this very basic plot of the filter it should be more apparent.

> 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.

Pin 16 is the output to the emphasis knob and pin 12 is the feedback from the
emphasis knob. R24 is the trimpot that might be a problem. Pin 7 of Y5 should
have the feedback signal.

Russian Mini - cool!

Cheers,
Magnus - got a Moog one...