OBERHEIM SYNTH

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

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
>

I had an OBXa and it came close to driving me to the nut house!  I had 
a repairman in Tampa that would go through that whole tuning process 
and when it was finished, it sounded great.........until he hit auto 
tune and everything went to shit.   I finally put it on a bus from 
Florida to San Francisco  where there was a guru that helped it a great 
deal but still no cigar.

He hard wired as much of the connections as possible and this made it 
much more stable.

I talked to several pros that used them on the road and they all said 
the same thing.  "Great sound when they were working!"

I ended up giving it away and miraculously, my headaches went away. <g> 
  I know this is not advice but then..... maybe it is. <g>

Charlie in the swamp

On Mar 7, 2006, at 11:00 AM, 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.
>
> --

Thanks for the reply!  Who was your Oberheim "Guru" in San Francisco?  I live there, so 
maybe I should give them a holler...

Jonathan





--- In oberheim@yahoogroups.com, Charles Massey <cmassey@...> wrote:

>
> I had an OBXa and it came close to driving me to the nut house!  I had 
> a repairman in Tampa that would go through that whole tuning process 
> and when it was finished, it sounded great.........until he hit auto 
> tune and everything went to shit.   I finally put it on a bus from 
> Florida to San Francisco  where there was a guru that helped it a great 
> deal but still no cigar.
> 
> He hard wired as much of the connections as possible and this made it 
> much more stable.
> 
> I talked to several pros that used them on the road and they all said 
> the same thing.  "Great sound when they were working!"
> 
> I ended up giving it away and miraculously, my headaches went away. <g> 
>   I know this is not advice but then..... maybe it is. <g>
> 
> Charlie in the swamp
> 
> On Mar 7, 2006, at 11:00 AM, 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.
> >
> > --
>

I'll give that a try.  It would be great if it were that easy to fix!


--- 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
> >
>

The name "Magic Music" or something like that comes to mind.  Seems 
like it was a shop that specialized in analog gear.  What am I 
saying?????  The only digital stuff was the DX7 and everything else was 
analog.

Charlie

On Mar 10, 2006, at 10:53 PM, jringmrath wrote:

> Thanks for the reply!  Who was your Oberheim "Guru" in San Francisco?  
> I live there, so
> maybe I should give them a holler...
>
> Jonathan
>
>
>
>
>
> --- In oberheim@yahoogroups.com, Charles Massey <cmassey@...> wrote:
>>
>> I had an OBXa and it came close to driving me to the nut house!  I had
>> a repairman in Tampa that would go through that whole tuning process
>> and when it was finished, it sounded great.........until he hit auto
>> tune and everything went to shit.   I finally put it on a bus from
>> Florida to San Francisco  where there was a guru that helped it a 
>> great
>> deal but still no cigar.

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
> >
>

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@...m, "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
> > >
> >
>

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
> > > >
> > >
> >
>

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
> > > > >
> > > >
> > >
> >
>