Vintage Synth Repair

--- In vintagesynthrepair@yahoogroups.com, gnw <atonal@...> wrote:
>
> 
> I've recently 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 
> LED2 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.
> 
> 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
>
Checking power supply rails is the 1st thing I'd do too but with that ,if you can , check for power supply ripple.
Older units are much more likely to have weak power supply filter caps.You'd need a scope or and RMS voltmeter to check for the ripple.

If someone reworked some rectifiers try and find out WHY they quit.
Resolder all the comps around the power supply section.
W/o knowing more about the circuit , that's where I'd go.

I currently have 2 Oberheim DSXs in the shop for rework I need to get to SOON

Cheers,

Scott in Vermont