OBERHEIM SYNTH

I resolved my pitch bend issue. It was just a glitchy pitch bend pot and cleaning the pot would not clear the dead-band sensitivity.  Sourced a NOS AB J type 10K and everything is perfect as taper can be slightly different on the modern replacements. I agree that the pot shaft collar and return spring mechanism needs to be setup tight but enough clearance to run as friction free as possible.  Dead band and tuning on both narrow and wide is spot on.

James





On Friday, November 22, 2013 7:09 AM, 65 Lotus <Lotus@...> wrote:
 
  
 
And, it helps to tug on the pot shaft slightly when 
tightening in order to preload the wiper against the carbon trace. This got rid 
of the last little bit of "return to zero" slop in the new-manufacture 
replacement pot on my OB-Xa.
----- Original Message ----- 
>From: robiecurrier@... 
>To: oberheim@yahoogroups.com 
>Sent: Thursday, November 21, 2013 5:01  PM
>Subject: RE: Re: [oberheim] Oberheim  OBX-a pitch bend issue
>
>
>The slop is really slight and it usually takes me a few tries to get it  perfect. Did you replace the pot or clean it?  If it isn't a new one,  there can be a spot where the carbon is worn away, which would point toward  just replacing the pot.  If the pot is a replacement, the mechanism that  attaches to the pot shaft has to be tightened at the exact right spot, at  least that's how it is on the OBX.  On the OB8, I'd need to look at the  schematic again to know for sure.  It sounds like the issue you are  experiencing will take some experimentation to get right.  Before you get  into that, it would be prudent to replace U2 (1458), D1, and D4 just to rule  them out as causes.  If these don't fix it, then it's probably wise to  check out the mechanicals.  
>-Rob
>
>
>---In oberheim@yahoogroups.com, 
  <robhukin@...> wrote:
>
>
>Hi Rob and All, 
>
>
>Indeed, I did end up removing the (extra) deadband mod. The existing  deadband circuit works fine as long as U8 is trimmed for 0V. The problem is  that any offset at the NARROW switch (U14) - noticeable by a slight change in  pitch when it is engaged - is trimmed by the CENTRE LEVER trimmer, which  inevitably moves the bender out of the deadband range. I didn’t find a way out  of this problem, but it seemed I was the only one concerned (not the client).  I had stripped, cleaned and relubed the pitch bend pot but I shall also pay  attention the the spring mechanism next time - although there was no  noticeable slop…
>
>
>rob.
>
>
>On 19 Nov 2013, at 17:16, robiecurrier@... wrote:
>
>
>>>
>>>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.
>>>>>
>>>>>
>>>
>>>
>>>
>