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

[rsdio at audiobanshee.com]

This seems more like some setting in the firmware, rather than an electronic circuit problem.

But, while we're talking about circuits, have you measured the signals while the unit is on? I mean, don't electrocute yourself, but if the pot is not sending out a changing voltage when it's in circuit, then you know where the problem is. If the pot is sending a changing voltage when you bend up, then you should be able to trace that voltage as it flows through the circuit until you find the point where it stops. I get the impression that you're getting absolutely no change when you bend up - not even a slight pitch increase. If so, then surely something is stopping positive voltages from making their way from the pot to the VCOs.

Brian


On Feb 7, 2015, at 2:32 PM, <tprivitera at nc.rr.com> wrote:
> 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
>