Vintage Synth Repair

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

--- 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

Scott wrote:
> 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.
>
>   
Thanks for the reply.  I've actually pulled the processor board on my 
OB-8 in order to inspect for damage in the area of the bridge rectifier 
that had been replaced, and I'm glad that I did.  I'm amazed that I was 
able to get seemingly normal voltage readings at connectors I and J.  
I've uploaded a few photos for amusement and review.  They can be seen here:

http://groups.yahoo.com/group/vintagesynthrepair/photos/album/1950312521/pic/list?mode=tn&order=ordinal&start=1&dir=asc

There is also quite a bit of what appears to be "rework" that has been 
done to the board . . . Note the black wires running here and there in 
the photos.  Can anybody confirm that this work is not normal, and was 
probably someone's attempt at a repair?  Any further input on this would 
be appreciated.

Thanks,

george

--- In vintagesynthrepair@yahoogroups.com, Terje Winther <terje.winther@...> wrote:
>
> Hi,
> Thanks for the info.
> I have justed serviced an OB-8 that showed the same problems regarding  
> power, and especially the bridge rectifier. All black board area like  
> your photos. The OB-8 I had in didn´t hvae the mods/straps like yours,  
> but it did have some other added cables across several of the boards.  
> Very unstable unit, and the mux chips got a little too hot for my  
> taste. It will probably need another run of service to dig further in  
> at the power problems.
> Good luck with yours!
> 
> Terje
> 
> Den 7. juli. 2009 kl. 00.23 skrev gnw:
> 
> >
> >

>
> >
>
> >
> > 
> 
> Terje Winther
> terje.winther@...
> http://www.wintherstormer.no/
>
If you're noticing excessive heat on ICs , check the power supply rails. I had a ADCOM CD player, excessive heat on DAC chip. replaced chip but found it was the -5 VDC supply running at -13 (YIKES!)
replaced the -5 reg IC and filter caps . All better now.

Also , check what the IC is driving into. It might be another component with a bad load on the previous IC