OK All, More OB-8 Probs...

[WeAreAs1 at aol.com]

Gavin Muir wrote:

<<     Seems that I now have a wonky oscillator. Oscillator 1 of voice 8 is
misbehaving. Beyond a certain frequency, the scaling seems to go, and as you
go up the keyboard it sounds more and more like it's being modulated by a
noise generator or something. Bad cap (hopefully)? Bad 3340 (hopefully not)?
The tuning is bad enough that the auto-tune has started to drop voice 8 in
its routine. The oscillator still seems to be putting out a decent
pulse/saw/tri wave, but it just loses its pitch stability up top. >>

Gavin,

Don't worry, this shouldn't be a very difficult thing to fix.  Pull the 
suspect 3340 out of its socket and exchange it with another one that you're 
pretty sure is working properly.  If the problem moves to a different voice, 
then you'll know it was the 3340.  If not, the problem would probably lie in 
one of the 4051 demultiplexers or one of the opamp buffer/followers that 
follows the demultiplexer.   

Although I won't rule it out completely, but it's most likely not a bad cap 
that's causing this.  The two caps to check would be the 3340's main 
integration cap and the S&H cap that goes between the aforementioned 4051 and 
its associated opamp buffer.  But check the other stuff first - caps in those 
areas rarely fail.

If that stuff doesn't fix it, there may be a problem with the autotune 
hardware.  This also is a very outside possibility, so check the other stuff 
first.  If those things don't pan out, let me know and I'll try to guide you 
around the autotune stuff.

If you don't already have OB8 schematics, you should probably think about 
getting a set.  I think they're out there on the web somewhere (but I don't 
know where).

Good luck,
Michael Bacich