OB-XA upper voices runaway portamento

[meener777]

My OB-XA is an early version and came with XA-F0/1 firmware, tuned fine and worked normally inc portamento. When I installed XA-GA0/1 the upper portamento went funny and so I went back to F firmware. Now that Ive installed Encore MIDI the portamento issue is back (I assume because they emulate G firmware). Basically when I boot up and enter manual mode with portamento at zero everything is fine. However portamento gradually climbs until its at maximum, on the upper voice board only. This makes the upper voice unusable and Ive got to fix it but Im totally at a loss.

Troubleshooting so far (manual mode):
- Verified +/- 15v on inputs to pot T7 which controls upper portamento, output voltage looks good
- Checked resistors are reading okay in place on the board
- Upper portamento ransistors measured .659ish using diode mode in place on the board, looks good
- Voltage on UPORTCV coming out of A44 pin 14 compared to LPORTCV coming out of A45 pin 8 (both TL084s) and these values don’t match, i.e. when portamento knob is fully down LPORTCV is 0 but UPORTCV slowly climbs up eventually hitting 5Vs and making portamento super slow.
- Swapped A44 and A45 to no effect so the TL084’s seem good

Addtl info:
Serial: 814881
Lower board version: 1496 D
Upper board version: 1504 F

My boards are RIGHT before this ECO which seems to add digital portamento and removes entirely the portamento circuits I checked above: https://www.midisoft.de/Oberheim_OB-Xa_ ... eco120.jpg

Im really confused about that because the ECO says it was fixed in version D firmware but my synth had version F firmware and worked fine but goes nuts with version G or Encore. I would have thought that F would have contained any changes that D did. Anyway since the later revisions appear to completely remove the portamento circuit I wonder if disabling this circuit somehow is the fix. Also wondering if that is true, why I dont see this issue on the lower voice board.

Any help or ideas at all would be much appreciated.

[meener777]

After sleeping on it, this am I took another look at ECO 120 which details the board changes for digital portamento. It looks like they simply removed the circuitry that does portamento. That circuit feeds UPC signal to a 3080 opamp which looks like it originally handled the portamento.

It looks to me like simply removing the outputs from UPC into the 3080s should fix this. I tried it out by grounding the output of transistor that outputs UPC and it fixes my problem!

Would anyone care to comment on whether or not this is a reasonable fix? I wonder why its only needed on the upper board in my case, it seems like both UPC and LPC should be removed/grounded out on my boards with the digital portamento firmware.

[meener777]

Just for anyone reading this, I was told on another forum that the runaway portamento is a known issue and autotune corrects the issue temporarily. I tested this and it works for the most part, however is pretty annoying to deal with as the portamento always comes back. FWIW far as I can tell grounding the output pins on the transistors in the old analog portamento circuit fully fixes the issue and I've put some clips in place for my own unit for the time being so I dont have to deal with the issue while retaining the ability to easily revert.