[sdiy] Help with an OB-X issue

[Michael E Caloroso]

> 2 of my OB-X voice cards have an issue with patches that have high resonance
> (ex: Tom Sawyer sweep) . The envelopes abruptly “shut off” in mid-sweep with
> a thump.  Almost like the VCA Sustain cuts off.  This is not something I’ve
> been able to “calibrate out” and I’ve tried swapping 3310’s and also swapped
> the cards around and confirmed it’s a card issue (problem follows the card,
> not the slot).  If I lower the control panel resonance setting enough, I can
> get it stop cutting out.  Filter on all other patches sounds normal, just an
> issue with hi-res stuff.  Any ideas or experience with a similar issue?

Verify the diodes in the feedback path of the filter, they limit the
signal during high resonance.  If either/both diodes are defective
then the OTA and/or opamps will latchup which may be what you are
hearing.

> Also, the noise quit working.  The LEDs light with I hit the Half/Full
> buttons, but no noise on any of the cards.  Anyone have experience with the
> MM5837 chips from China on eBay?  Can seem to easily locate an NOS version.

Check the 4051 and its select lines first.  First generation CMOS from
RCA have a bad history and the OBX is loaded with them.

MC