OBERHEIM SYNTH

Ok - this is getting tricky.  You say you have a volt meter. Let's 
make sure the power supply is in spec.

Voice 8 is on the upper try, the service manual say this about the 
upper control board (I think this controls voices 5-8):

Verify +5.6V (+/- 200mV) at Pin 16 of IC 145  (this is a 4051 chip)
Verify -10V (+/- 1V)at Pin 7 of IC 149 (this is a 4053 chip)

The "top" of the chip will have a "dot" or a "dimple".  Pin 1 is the 
top pin next to/to the left of the dimple. Pin 16 is the top pin to 
the right of the dimple. Pin 7 is on the same side as Pin 1, just 
count down 7 pins (it is one pin from the bottom). Pin 8 is ground 
and you can use this or some other well ground part of the synth for 
the ground.

Tell me what you see and becareful when you probe.  Stay away from 
the power supply area so you don't get hurt. Only do this if you are 
comfortable doing the test.

Tell me what you get.   
--- In oberheim@yahoogroups.com, "jringmrath" <jringmrath@...> wrote:
>
> I tried your suggestions.  I still only hear the distortion from 
OSC 1.  I also hear it with 
> either saw or pulse waves (yes, the pulse width knob changes the 
sound).  Panning the 
> voice makes no difference.  I did notice that when I first power on 
the synth, I don't hear 
> the distortion, but as it warms up the distortion becomes more and 
more evident.  
> Hmmmmm...
> 
> I really appreciate your help.  Any other suggestions?  I have 
access to a volt meter...
> 
> Jonathan
> 
> 
> --- In oberheim@yahoogroups.com, "nelsonj_sce" <nelsonj_sce@> wrote:
> >
> > Before you said if you changed the voice board you still had the 
same 
> > problem.  If that is the case, the problem is not on your voice 
card 
> > so don't try changing parts on the voice card. 
> > 
> > You say the problem it is only with OSC1.  So if you ONLY have 
OSC1 
> > on you hear distortion, but if you ONLY have OSC2 on there is no 
> > distortion (make sure OSC2 is at full level), correct?  If the 
> > problem really is with just OSC 1, see the next paragraph.  If 
the 
> > problem only happens when both OSCs are on, the output level is 
> > likely too high.  Each voice passes through a passive (i.e. 
resistor 
> > divider with a pan) before the final output stage.  The pan was 
the 
> > most likely cause, but you already tried that.  Try panning the 
voice 
> > hard left and hard right.  Do you have the distortion in both 
> > positions? 
> > 
> > Does the problem happen on both the Saw and the Pulse shapes?  
Does 
> > the Pulse Width change as you spin the PW knob? If the problem is 
> > only with the Pulse Wave I have some ideas. 
> > 
> > Let me know what happens.  
> > 
> > 
> > --- In oberheim@yahoogroups.com, "jringmrath" <jringmrath@> wrote:
> > >
> > > I tried panning the distorting voice card and cleaning the pots-
 it 
> > didn't help.  I do only 
> > > hear it when I have OSC 1 engaged - with just OSC 2 I don't 
hear 
> > the distortion.  The weird 
> > > thing is that I tried replacing the Curtis 3340 for OSC 1 and I 
get 
> > the same distortion, so I 
> > > don't think it is the chip.  Any other thoughts?
> > > 
> > > Jonathan
> > > 
> > > --- In oberheim@yahoogroups.com, "nelsonj_sce" <nelsonj_sce@> 
wrote:
> > > >
> > > > As for the distortion, try panning your bad voice and see if 
that 
> > > > helps the problem.  Also clean the pan pot with some 
component 
> > > > cleaner.   
> > > > 
> > > > As for the "ghost" note...I have to think more about that. 
> > > > 
> > > > --- In oberheim@yahoogroups.com, "jringmrath" <jringmrath@> 
wrote:
> > > > >
> > > > > I have an 8-voice OBXa and have a couple of nagging issues 
I 
> > > > haven't been able to resolve.  
> > > > > 
> > > > > First, I seem to get some slight distortion from voice 
board 
> > #8.  
> > > > If I move the board to 
> > > > > another location, it sounds fine.  If I move a known good 
voice 
> > > > board to the #8 position I 
> > > > > get the same distortion.  It is most obvious on the organ-
type 
> > > > sounds.  I have been using 
> > > > > it as a 7-voice until I can track down the problem.
> > > > > 
> > > > > Second, if I play the chord "G,C and Eb" in the middle 
octave 
> > of 
> > > > the keyboard, it also 
> > > > > sounds the "E" below the G.  Needless to say, this can be 
> > > > annoying!  Must be something 
> > > > > having to do with the keyboard scanning, but I don't know 
what 
> > > > exactly.  The double mode 
> > > > > also doesn't seem to work 100%.  Maybe a related issue?
> > > > > 
> > > > > Any thoughts on what might be causing these and how to fix 
> > them?  I 
> > > > have the schematics, 
> > > > > but I don't really know what I'm looking at since I'm not a 
> > tech.
> > > > > 
> > > > > Thanks for any help or advice!
> > > > > 
> > > > > Jonathan
> > > > >
> > > >
> > >
> >
>