OBERHEIM SYNTH

Thanks Nick, 

It's looking like it might a combination of a few small things. The pot is certainly suspect especially the resistance near center. 

Cleaned all of the connections this morning, cleaned the pitch pot (again), cleaned the feet of IC8, replaced that last three original elna caps in the synth on the mod/pitch board and 'exercised'/cleaned the pitch trim pots. It is almost 'right'. Might replace the trim pots anyway. Everything calibrated fine but unless I let the paddle pull itself back to zero, it still returns several cents sharp or flat when set for wide bend.  More stable then before and usable just as it sits. Needs a bit more center 'dead band' since the center is still very sensitive. I don't remember if this is normal or if I never noticed this in years past but I recall seeing a dead band resistor mod for this issue on another board when using a modern Honeywell/clarostat replacement pot.

The NOS AB 10K J type pot should be in next week, it should improve things a bit more. Hope this helps out somebody since a problem in this section can throw off an otherwise working Oberheim. 

James






On Friday, November 8, 2013 11:49 AM, Nick Zampiello <newallianceeast@...> wrote:
 
  
at the other end of that wire harness there's probably a confused or dac or cv input point to the cpu.

not familiar enough to say for sure.  definitely reseat the connectors and clean them too at the main board end.

my ob8 has all kinds of funky quirks that are due to the board connectors inside being old.


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On Friday, November 8, 2013 8:08 AM, FL_799 <fl_799@...> wrote:
 
  
Having an odd issue with my OBX-a's pitch bend. When wide pitch selected, the voices seem to get wobbly and some voices fail, a tap on the paddle induces pitch instability. It's also not returning to pitch and is about a 1/2 step off. The center position on the wide setting seems to be very sensitive even after cleaning the pot. Narrow is perfect, no issues and no hint of instability.

Didn't dig in too deep. Just cracked open the J type 10K pot and cleaned it up (DeOxit) and re-soldered all connections on the pitch bend pot. No improvement on the wide setting. I have a NOS J type pot on order as a start.

Is there anything else in the pitch bend circuit that I should look at in addition to replacing the pot. I'll be getting into this over the weekend but I figured somebody might have seen this in the past.

James