OBERHEIM SYNTH

I posted this message to VintageSynthRepair earlier, but have since 
discovered this group and thought I join.   Apologies for the cross-post 
if you're subbed to both.

Recently I acquired a "troubled" Oberheim OB-8 (non-MIDI, firmware rev. 
A8).  It's behavior is very erratic, but the one symptom that is 
consistent is that when using the frequency adjustment for OSC 1 or 2 in 
any voice, or the Master Tune control, the pitch change is not smooth, 
but rather extremely glitchy.  What follows are the tests I've performed 
so far, and their results.

*Verify voltage sources at connectors I and J:  In spec
*DAC calibration:  In spec
*Oscillator volts per octave adjustment:  I was able to get the 
adjustments for oscillators 1 & 2 in spec (both test LED's lit), but in 
every case the adjustment was extremely sensitive and it was difficult 
to get both LED's to stop flickering.  When performing the adjustment 
for F-ENV modulation (T102, T202, etc) on voices 1-4 (lower board) only 
test LED 1 was illuminated regardless of trimmer setting; I couldn't get 
LED 2 to light at all.  In addition, if I revisit any of these 
adjustments a few hours or days later, I find that most are once again 
out of calibration.
*Oscillator drift test:  I ran this test several times, and each time it 
passed without issue.
*Auto Tune does not indicate problems with any of the voices.

I've also noticed that at some point one of the bridge rectifiers for 
the power supply must have let go, as one of them has clearly been 
replaced, and there is a bit of damage on the board in that area.  I'm 
tempted to replace the three of them just for good measure, but since 
all my voltages are in spec at I and J, I don't think they are the 
source of my problems.  Or could they be?  At any rate, given this 
information, does anyone have suggestions as to where to go from here?  
Any feedback would be greatly appreciated.

Thanks,

george

At 12:34 PM 6/29/2009, you wrote:

>I posted this message to VintageSynthRepair earlier, but have since 
>discovered this group and thought I join. Apologies for the cross-post 
>if you're subbed to both.
>
>Recently I acquired a "troubled" Oberheim OB-8 (non-MIDI, firmware rev. 
>A8). It's behavior is very erratic, but the one symptom that is 
>consistent is that when using the frequency adjustment for OSC 1 or 2 in 
>any voice, or the Master Tune control, the pitch change is not smooth, 
>but rather extremely glitchy. What follows are the tests I've performed 
>so far, and their results.
>
>*Verify voltage sources at connectors I and J: In spec
>*DAC calibration: In spec
>*Oscillator volts per octave adjustment: I was able to get the 
>adjustments for oscillators 1 & 2 in spec (both test LED's lit), but in 
>every case the adjustment was extremely sensitive and it was difficult 
>to get both LED's to stop flickering. When performing the adjustment 
>for F-ENV modulation (T102, T202, etc) on voices 1-4 (lower board) only 
>test LED 1 was illuminated regardless of trimmer setting; I couldn't get 
>LED 2 to light at all. In addition, if I revisit any of these 
>adjustments a few hours or days later, I find that most are once again 
>out of calibration.
>*Oscillator drift test: I ran this test several times, and each time it 
>passed without issue.
>*Auto Tune does not indicate problems with any of the voices.
>
>I've also noticed that at some point one of the bridge rectifiers for 
>the power supply must have let go, as one of them has clearly been 
>replaced, and there is a bit of damage on the board in that area. I'm 
>tempted to replace the three of them just for good measure, but since 
>all my voltages are in spec at I and J, I don't think they are the 
>source of my problems. Or could they be? At any rate, given this 
>information, does anyone have suggestions as to where to go from here? 
>Any feedback would be greatly appreciated.
>
>Thanks,
>
>george

The flaky control problem sounds like bad (or dirty) pots.  The wiper of the pot could be losing contact with the resistance element, causing wild swings in the control voltage.  You could test this by using a voltmeter or scope and measuring from ground to the center terminal on the pot and watching for unexpected voltage jumps as you move the knob slowly.
It's probably not related to the VPO adjustment -- as I recall, this was a sensitive adjustment in any case.  

I wouldn't replace the bridge rectifiers, unless they are getting extremely hot, in which case, you could replace them with parts designed for higher current.

Good luck,
Paul White