\o/ \o/ Got the control voltage issue sorted out! \o/ \o/
Just to recap things from my earlier post, when triggering notes from the keyboard the CV output at pin 7 of opamp A1 was not driven higher than 4,5V. After swapping Q8 this was corrected (A1 pin 7 going to approx. 13V), but when measured from the junction of R26 and Q8 gate (keyboard CV out, interconnect pin 8) the voltage was still not driven past 5V. So, when playing the highest note on the keyboard (7V), there was a voltage drop of about 8 volts over R26.. Definitely some current leaking, but where?!
All this time, I had had the interconnection board removed/disconnected so there was no load on the CV out pin. Still, when measured against ground this junction showed a resistance of 1kOhms; a thing, that should’ve pointed me direction of what to check next. Unfortunately I just wasn’t thinking far enough at this point.
Board C was the least problematic board of the three.. Only A2 seemed to be leaking VCA control voltage to the audio output no matter how the common voltage rejection (CVR) was calibrated, so I gave it the same socket replacement treatment as the CA3080 on Board B. Other than that, I just replaced the parts on the audio path and the board was sorted out! These replaced parts were R1-R3, R5, R13, R16-R22, R28, R65, C1, C3, C5, C6 and C9 – C12.
Now then, about that CV issue on board A..
This board has most of the sound generating sub-circuits on it, hence the longest list of parts to be swapped. These are R51 – R63, R66 – R75, R77 – R89, R92 – R98, R102 – R104, R108, C6 – C9 and C12 – C19.
The broken parts were Osc 1 square output (bad Z1), LFO output (bad A1), capacitor C4 (measured 2nF instead of 10nF) and LFO modulation for the S/H circuit (bad A3). I also swapped A2a and Z2 while I was at it.. Although these were ok, given the high amount of failed ICs on this synth, this seemed like a reasonable preventive maintenance action. I didn’t have a proper replacement for A3 (CA3080 OTA, canned version) so I glued a DIP-socket to the solder side of the board with a small dab of silicone and connected the required pins with a bit of flat cable cut to exact length.
You do? Good for you, as I seemingly don’t :(((((((
Seems I was a bit hasty with announcing the CV fixed the other day. Sure, the output of A1 is fine now, but I’m still not reading more than a bit over 5V on the keyboard CV bus elsewhere on the Odyssey. Oh well.. Fix it later then, been eye-balling board A to the extent of becoming completely blind.
Finally on to the parts swap and electronics repairs with my precioussssssssss! On board A, the parts that got swapped were R36-R46, C3, C9, C10, C12-C15 and C18. I didn’t have any suitable replacement for C16 at hand so I just left it as-is. In case you never memorized the locations of these parts, why not take a look at p27 of this service manual.. ;)
Broken items on board A were the up2oct calibration trimmer T4 (measured 4Mohms instead of 250R), FET Q8 of the CV output FET-input opamp buffer (not driving CV out higher than 4,5V) and pink noise generator opamp A4 (no output). There was also a bad solder connection on R32 (2nd voice output buffer opamp feedback resistor), this caused the 2nd voice CV output to drift at random.