Update on a Successful Debug of the DOD585-A, I Was Wrong
Quick update on my last post which you read here. Mostly the update is that I was wrong!
I was all wrong about the leakage current thing. I replaced the JFET and the diode and no change which got me back on tracing the whole circuit. Turns out I had neglected to check the amplitude of the delayed signal directly on the far side of the DC blocking cap on the output of the compandor. When I did that I saw a huge amplitude drop which indicated that the electrolytic blocking cap was bad. Replacing that and the whole thing works as expected.
Honestly, I should've started there. In theory I know that the first thing to fail in old gear like this are the electrolytics. What can I say, I don't do repairs all that often and I love to overcomplicate things.
Thinking harder about it, I think my whole circuit analysis of what that JFET is doing was possibly wrong and now I legit am unsure what it's doing. The gate goes between -100mV when the pedal's off and +100mV when it's on, and the source voltage is steadily at about 6V, meaning the VGS is always negative enough to hold JFET off, I think.
Gonna spend more time thinking about this one. Love to barge into a circuit I don't understand.