So...new IC 23 installed. Trace looks like it should now at pin 6, IC 18, the INH input, and I can see the outputs reacting as they should on all but two parameters.
However, the symptoms remain pretty much the same.
Which leads me to the two outputs that are not working correctly. Pin 1 on IC 18 has the exact same voltage as the +5 power rail input into the IC (pin 16, +4.98V). This voltage shows up on pins 1, 2, and 3 of IC 10 also. This is supposed to be the ATTACK control. My guess is that IC 18 has been corroded, but it seems I should replace IC 10 also. 4558s are cheap.
And pin 1 on IC 14 does not show any response from the MG Delay control. I see the signal at it's output on IC 19 (pin 1) and it's input to IC 14 (pin 2), but all I see on pin 1 is a constant +14.51 V. Can't quite figure out where that is coming from, but perhaps I don't understand the circuit. I see a wavering voltage on pin 3, which I imagine is correct due to the Delay function, and there is a negative voltage on the far side of D12, as it appears there should be.
Again, 4558s are cheap. Should I just replace all 3 ICs? Looks like Digi-key still has some 16 DIP 14051 ICs.
One other thing I notice on the working control outputs (muxes?). Most of them get to a certain voltage (somewhere around 70% of their potential), and then fall back to zero with an audible click, which also seems to change all settings. This can be seen on the trace at TP2 also. It seems like this indicates a short somewhere once a certain voltage is reached, but there are two or three noticeable exceptions (MG SP, RESO, and REL) which do not do this.
All of which makes me wonder if I should just be starting with a new circuit board, as maybe this one is just too corroded. but I would like to try to continue fixing this one, and any help is appreciated.
Thanks.