After a quick attempt to fix my Midiverb II again (and failing) I pulled the JX3P out again to have an attempt at fixing the dodgy octave.
Thinking that it was possibly the keyboard decoder, I replaced IC45, which took a fair amount of time to do it cleanly with the desoldering vacuum.
While I’ve got the board out…
not sure I really like the look of these crinkly paths on the back. Anyway onwards.
Changing IC45 made zero difference, so it wasn’t that. And anyway, I could see the signal appearing on the connector on the keyboard. It took me a while to work out what was going on, and it definitely made it easier yanking the keys out.
This is part of the dodgy section of the keyboard, and it looks a lot like my unhappy Poly61M from the same time. The rubbery circular things are pressed down by the keys – inside the dome is a carbon disc that presses down onto the board and completes the circuit.
One side of the switch connects to the top of the diode below it, with the other half connecting to the decoder IC45 via the keyboard connector at the far side. Pressing the switch should make the signal from IC45 appear on the other side of the switch (the top of the diode).
I can’t believe you’ve got this far, but aaaaanyway, the signal wasn’t coming up on the diodes for the broken section.
The soldered track on the far right here should have given the game away
…essentially the track had failed for some reason. Scratching out the soldermask from either side of the break and soldering a wire across it sorted it. It wasn’t actually super-obvious where the break was from just looking at the track, it took me a couple of test scratches to work it out.
There was also one intermittent key which just needed the rubber switches refitting, and that was sorted too.
I still have lots of screws missing (in the JX, that is…), and the transformer hums like an electricity substation, but at least it all works now.