I have fixed this issue on multiple Oberheims.  The deadband mod is not a good solution.  The real problem is mechanical and oftentimes is two fold.  The first issue is that the sealed pot used has got carbon powder built up inside on both sides of the wiper.  The pot needs to be opened and cleaned or simply replaced.  This will clean up most of the slop, however, the spring loaded pitch bender mechanism has two metal arms inside that are attached to a spring.   These arms are supposed to hold the lever perfectly snug in the center position, but many times, one arm is slightly bigger than the other and needs to be filed down slightly where the arm hits the detent.  You need to shave the arm very slightly so the detent can't wiggle between the two spring loaded arms.  When you get it right, there will be no slop and no need for the deadband circuit (which doesn't actually work properly anyway).  This has solved the problem in 100% of the oberheims I have fixed that suffered from the pitch bender not returning to exact center.  This takes me about 3 or 4 hours to do, and I have done it many times.  I would expect a first timer to take a couple of hpurs more.  Definitely not a quick and easy fix, but worth it when done right.  

-Rob Currier

www.NewEnglandAnalog.com

207-703-2350

---In   oberheim@yahoogroups.com , <robhukin@...> wrote:

Hi All,

I have an OBXa in for repair that has that mod. It is two diodes (1N914 or similar) in parallel (opposing polarities) connected to the pot wiper. The wire that was connected to the pot wiper is then connected to other end of the diodes and this junction also has a 100k resistor to GND.

regards,

rob.

On 12 Nov 2013, at 12:27, 65 Lotus < Lotus@... > wrote:



>>    I recall seeing a dead band resistor mod for this issue on another board when using a modern Honeywell/clarostat replacement pot.

 

That was me. I'll see if I can find my notes over the next day or so.