Vintage Synth Repair

On Wednesday 31 May 2006 01:16 am, Chris Yakopcic wrote:
> Hi, Ive been working on this Omni 2 for a while and Ive come to a point
> where I wanted to ask some questions before I just replace all of the CMOS
> chips
>
> So far I have the bass voice working (just the synth side, string bass and
> cello still dont) and the synth voice kind of works. When the synth voice
> is played, it sounds like it works when using the 4' and 8' octaves, but
> over top of the sound is a medium to high pichted whine added to the
> correct sounding output. The whining sound does not change in pitch as
> different keys are pressed but it is audible upon every keypress. This
> extra sound can be seen on an ocilloscope before the audio goes into the
> VCF and it is also modulated by the VCF.

Find the one key that doesn't have the extra tone on it,  and replace the 22uF 
tantalum cap for that key (and that voice!).  They used 25V parts in there 
originally in a circuit that can possibly have as much as 30V across it...

> Second, the string voice does not work at all. I believe the string voice
> is being generated because I can see it on a scope going into the phaser
> board, but I dont think the voice is working propery. When I monitor the
> phaserboard on pin 5, which I am pretty sure is the audio signal input, and
> turn on either the viola or violin butoon, audio is shown going
> through without pressing any keys, when I do press the keys, the amplitude
> increases and the string AR seems to be working correctly. When I turn off
> the string voices the wave goes back to a flat line. It seems to be working
> correctly with the synth voices, nothing goes through except when the
> phaser switch is turned on, and only when keys are depressed. Nothing ever
> leaves the phaser board through the output pin which is 6.

I can't speak to this one without digging into schematics,  which aren't handy 
at the moment...

> This might be a long description but I wanted to be as detailed as
> possible. If anyone has experience with these kinds of problems any help
> would be appreciated.
>
> Thanks

Good luck with it.

-- 
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space,  a critter that can
be killed but can't be tamed.  --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James 
M Dakin