Odyssey filter repair

[gstopp at fibermux.com]

     I've had two "dead filter" Odysseys. One had a potted VCF (sealed) so 
     I replaced it with the VCF from my stupid piece of crap spare parts 
     Omni, and the other was open to the world so I replaced the LM3900 and 
     it was good as new.
     
     If you have a truly dead VCF and it's potted, I'd give the following 
     advice:
     
     1. Talk to one of those outfits that has replacement modules for ARPs
     
     2. Find a junk Omni/Soloist for $50 or less and steal the VCF
     
     3. Make yer own!
     
     I've made my own modules before, just use vectorboard and match the 
     pin configuration. This does take some analysis of the surrounding 
     motherboard to figure out. (I have a 2600 with a CEM chip for VCO 1 
     and a 3080 for the VCA). You gotta be a little whacko to go this far, 
     but aren't we all to some extent?
     
     I remember fixing a VCO - it was the 4011 CMOS NAND gate. Can't 
     remember if it was "A" series or "B" series CMOS, or if it mattered 
     (Joachim, are you reading this?).
     
     Sliders can be found at surplus shops. Watch out, there's 100K and 1M 
     and it does make a difference in the envelope generators for example. 
     I think I've had to compromise and use linear taper rather than audio, 
     but that never bothers me.
     
     Keep at it! Even if you gut the thing you can still use the keyboard 
     and the case maybe for a DIY project. Most of the hassle with a DIY 
     synth is the enclosure and panel. The Odyssey is a very cool machine, 
     very ergonomic IMHO. The patching method of parallel input attenuators 
     on each module, with a source select switch, is really ingenious and 
     one of the reasons that the Odyssey is equally at home making musical 
     sounds or spaceship noises. I have an old schematic and on it an old 
     electronics buddy of mine wrote these prophetic words:
     
     "The Odyssey is one of the best-designed synthesizers on the market, 
     both from an engineering and a musician's point of view. It merits 
     close study."
     
     - Gene
     gstopp at fibermux.com


______________________________ Reply Separator _________________________________
Subject: Odyssey filter repair
Author:  haines at quick.net (Matt Haines) at ccrelayout
Date:    4/15/96 11:39 AM


I mentioned the Odyssey that was given to me before (perhaps it was on AH). 
It's a wreck, sliders sheared off and no audio present, etc etc. One of the 
things wrong is, the filter seems to be dead. Pin 10 of the filter module 
is the audio output, according to the schematic. When I crank resonance up 
so that it should be self-oscillating, I get nothing on the scope from pin 
10 (or any of the others). In case the slider was busted, I clipped a wire 
around the slider in the appropriate way to simulate 'full on', and still 
nothing.
     
Two questions:
     
1) did I do something wrong to check this?
     
2) I figure I'd better start replacing components on the module. What sorts 
of things are likely to die first? I'd of course rather not have to pull 
everything.
     
If anyone has any tips on how to fix this, I'd be grateful.
     
BTW, VCO 2 is also dead (I *said* it was a wreck!). On the scope I get a 
pulse every 10 or 15 seconds. I did the same routing around pots and it 
doesn't change. What sorts of things go wrong, and how do I go about 
finding them? Thanks.