[sdiy] Help: Oberheim OB-Xa won't bend up

[Tom Wiltshire]

Looking at the service manual (Pg 54) the OB-Xa uses a unusual differential op-amp arrangement with separate op-amps handling the "Bend-up" and "bend-down" portions, before they're then joined back together by a summing amp. It looks like the motivation for this was to allow a centre dead zone around zero.
Either the diodes that limit the op-amps to one polarity or the op-amps themselves could be dead. Testing the output of the 10K Lin Bend pot and comparing it with the output of U8 324 op-amp at pin 7 would give you some clues. If the pot bends up and down, but the op-amp doesn't, you've narrowed the problem down to a small section of circuitry. The outputs of the Bend-down and Bend-up op-amps at U8 pins 8 and 14 would also be useful to measure - do they move when you waggle the lever?

HTH,
Tom

On 7 Feb 2015, at 22:32, tprivitera at nc.rr.com wrote:

> Hi Everyone,
> 
> My OB-Xa just out of the blue will not bend up at all with the pitch bend paddle (neither Osc).  Bends down just fine and returns to center (wide and narrow) and the low and high octave switches work just fine, as do all the rest of the modulation board functions.  All functions of the keyboard itself and the control panel work just fine.
> 
> The pot itself seems to test fine with my meter (I can see resistance moving up and down while moving the paddle) and as I said, it just stopped working today, so it wasn’t like the pot was wearing out.  This is what I’ve done so far after looking at the schematics:
> 
> -	Replaced U13 and U14 on the modulation board (4053’s replaced with new)
> -	Checked all connectors on modulation board and lower control board
> -	Tried to adjust the ‘bend up’ adjustment trimmer on the mod board – no changes at any range of adjustment
> -	Replaced U14 on the lower control board (4053 replaced with new)
> -	Replaced U46 and U50 on lower control board (4051’s replaced with new) – had the hood up to replace these chips for a different reason, not sure they are associated with this issue, but I swapped several new ones in there to confirm
> -	Power supply has been totally rebuilt and calibrated
> 
> I’m kinda at a loss.  Any ideas?  Anyone seen this before?
> 
> Thanks,
> Tim
> 
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