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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
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.