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Looking at the board, the round pad is connected to the ground plane, so that is the -ve terminal of C10, which I think means you have it correct

If your DM doesn't have a continuity test you can do the same thing with a low ohms range. somewhere between 0 and a few ohms reading would indicate continuity. I would definitely test the switch before trying to remove it. Test that the lugs that are supposed to connect in each position do...

The schematic shows lug 1 of the volume pot going to ground. The PCB had the square pad going to ground, so lug seems to be the square pad. The pinouts of 5457s and J201s are the same, so that shouldn't be a problem. It does sound like a ground issue. With power off and one multimeter...

OK, usual Q's. What are the voltages on your op amp pins? Have you got an audio probe?

That is a reputable source. They almost certainly are what they say they are

The traces are so hard to see on a black board (probably deliberately I suspect). Anyway, just to change topics, where did you get your J201s? Most of the TO92 ones currently available are fake. At least one connects directly to 9V. A fake one could potentially muck around the voltages maybe?

OK, I have one of these boards unpopulated. It looks as if the trace for pins 1 and 8 of IC100 comes from one end of R18. Check and see if one end of R18 has 9V, and also check for continuity from that end of R18 to Pins 1 and 8 of IC100

That link from jeds peds looks like the sort of thing. The thing is that J201s in the TO92 casing have been obsolete and out of production for some time now. The surface mount version is still in active production. Nobody bothers to fake the surface mount because it is so easy to get...

The voltages you measured earlier on your ICs are helpful, but not very. We really need voltages for every pin on the ICs. From what you posted we can see that they are getting a bias voltage OK, but we can't see the actual power. For dual op amps that would be on pins 4 and 8, but just give...

ic100 voltages look all wrong. pin 1 and pin 8 are tied together in the schematic so should be the same and should be around 9v. pin 5 should be around -9v and powers the other ics. without the correct voltages here nothing else can work. interestingly, the power pins on the other ics are...

Hopefully that will fix it. If not, another thing to consider is that these switches can be damaged by heat pretty easily. Leaving the iron on a lug too long can break it beyond repair. Doing too many lugs in quick succession without a break to cool can also do it. If changing the jack...

First thing is going to be to make sure that the BBD bias is set correctly. You can't guarantee to get any wet signal without doing that.

So been there done that

If you are concerned, I would measure the resistance with light on it, and then covered (with a finger maybe) and compare the results with one (or more) of the others. If the results are consistent it's probably fine.

No knobs just adds to the mojo

The circuit for the bypass LEDs is a simple on/off switch. You shouldn't be getting any light when bypassed, so that is a sure sign it's not quite right.

Before catching fire they get this really nice red glow happening

I lost the 15V zener that was regulating power to the EQ board in my bass amp once. In that case a very very hot signal had fried all the op amps on the board and they had failed short circuit, so there was way too much current being drawn for a split second until the main fuse blew. Too much...

The pots we can see look as if they are insulated on the back with electrical tape. If that is the case that may well be your problem if all the other pots are done the same way. That tape is very prone to being pierced by the cut ends of soldered leads. That would also explain things getting...

With these, the trick is usually to look from the end. One side will be straight up and down (or close to). The other side will be a double wedge. The datasheet will show which side is which. In your photo, the straight side is on the right and the "wedged" side is on the left Edit...