[sdiy] Oberheim OB-Xa Servicing

[Stewart Pye]

Hi Tony,

Thanks for the reply. I had another look at the service manual and 
indeed the lower control board overlay is the same as the board in the 
unit I have. As you suggest, I should be able to just measure the keycv 
on the voice board.

The next step in the procedure is:
DAC FULL SCALE ADJUSTMENT
Press High C (C5) Measure the voltage at pin 9 of IC#64 and adjust 
trimmer T9 for 5.000V.

I assume "pin 9" is a typo, as on the schematic IC64 is a TL082 so there 
is no pin 9. It probably should be pin 7 again, so I'd just be measuring 
the keycv again and it should be 5 volts higher than when C0 was 
pressed. That makes more sense...

Regards,
Stewart.



On 4/9/2013 6:49 PM, Tony Kalomiris wrote:
> Hi Stewart,
>
> I took a look at the service manual and indeed there is no IC64 (TL082) on the component overlay diagram but it's there on the schematic of the lower control board.
>
> You can trace this KCV1 to the Voice 1, section Motherboard 'B13' 'C1'.
>
> Tony K
>
> On 2013-04-09, at 2:54 AM, Stewart Pye wrote:
>
>> Howdy,
>>
>> I'm servicing a REV3 OB-Xa.
>> I've gotten to the upper and lower control board calibration section in the REV 3 service manual and am trying to do the first step:
>>
>> DAC OFFSET ADJUSTMENT
>> Press the UNISON switch . Press low C (C0). Measure the voltage at pin 7 of IC#64 and adjust trimmer T6 (near IC#35) for 0.000V ±2mV.
>>
>> I can't find IC64 on either of the control boards. From the schematic in the service manual U64 is part of a sample and hold circuit which also consists of a CA3080, and are fed by a multiplexer U47. I can find U47 on this board but there are only quad op amps and some capacitors nearby, and no CA3080. Is there a later version of the service manual to cover this PCB?
>>
>> Serial number on this unit is 823706.
>>
>> Regards,
>> Stewart.
>>
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at dropmix.xs4all.nl
>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>
>