OB-8 pitch bend question, and Moog pitchbend

[Peter Blackett]

Hi,
I seem to have a problem with the pitch bend on an ob 8 synth.
the control doesn't always return to zero after it has been used, so
affecting the tuning of the synth.I know there was a mod on some moog
equipment [consisting of some added diodes ] to create a deadzone around
the center of the control.
Anyone done this on an Oberheim ?
[BTW I've set up the presetson the pitchbend panel  as the service
manual instructions ].
regards Peter 
South London England