[sdiy] OB-8 voice glitching, warble

[Bob Weigel]

On 4/3/2012 11:20 AM, Eric Frampton wrote:
> What might cause a single OB-8 voice to make a very fast pitch warbling on both oscillators? Both oscillators modulate together. The individual VCO tune knobs on the front panel are still working, but they don't track the full 1V/octave on the keyboarfd.
>
> I've tried replacing the 4051 and TL084 feeding TR3A and TR3B. Tried replacing the 4051 and TL084 feeding CV3. Just for grins, tried replacing the 4051 and TL084's on each side of VCO1F and VCO2F. Also happened to have a spare DAC and tried swapping that on the off-chance that it had failed.  No change in any case.
>
> Also noticed that immediately upon first power up of the day it's in tune, then as it warms up it falls in pitch and goes into this warbly state.
>
> Thoughts?
>
> thanks,
> eric
>
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>    
Hey Eric I need to brush up on them again to get a couple fixed here at 
long last.  This sounds mildly familiar.  So with just one voice doing 
this it eliminates all power supply questions, all modulation 
questions...all dac questions since indeed that would tend to affect all 
of them and not just one.  So it's got to pretty much be something 
(given that you already replaced relevant 4051's and 084's)... the 
keyboard CV?  That one is the only thing in common isn't it?  Have you 
looked for the signal on the lines correlating to the disturbance?  
Looks like it would have to be a resistor going open or something..not 
likely a poly cap I wouldn't think.  What did I have a while back..one 
high reliability looking cap failing in a korg delta.  Someone else 
thanked me for posting that.  They had the same issue in a Delta!  LOL.  
What are the odds of that?    Anyway  if you can find the signal that is 
causing the issue it would sure help.  You might be able to hear it if 
the rates of transition are hitting the audio range by just putting a 
little cap on the end of a 1/4" jack to an amp if you dont' have a scope.

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