Here is the complete schematic
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Back in post #2 you noted the cut leads on U12B. This is important. Pins 8 and 13 will
interfere with the FET switching. make sure they are cut or bent out of the socket.

Also I would remove or clip R50 and R51

I'm going the other way, thinking it is the FETs. The thump when switching he mentioned happens when the gate voltage is leaking into the drain-source. Though I don't see why any
garden variety N-CH FET wouldn't work in this circuit.

The FET on the left bypasses the effect, the one on the right monos the two amps.
Still don't know where the output of U12B is going.

I'm imagining they were trying to make a kind of stereo chorus effect but didn't want to
use an analog delay chip. Then when it didn't sound like a chorus they made up the story about the "harmonic divider"

What are the voltages when Stereo/Mono is switched with the fets removed?
The datasheet says -1V to -5V is the pinch-off voltage.

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Here are a few notes. I breadboarded part of the circuit.
2N4093 is available at Mouser and Digi-Key though not cheap.
-if that's the problem.

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Very interesting and complicated schematic, will take some time to understand it.
Do you have the -7,5V on U12 pin 1? Check that the speed jack tip and ring contacts
are shorting when unplugged. There should be some square wave frequency
at U3 pin 6 and U9A pins 3 and 4.

Please post photos of the whole foil side and the whole component side.
The close-ups are too close to understand anything.

Here is the schematic

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The 100K resistor along with the resistor from the input jack sets the input impedance.
If anything I might make it larger like 220K or 330K. It does not change the gain.
Gain of the first stage is set by the feedback resistor divided by the resistor from
the - input to common.
In this case common is the +12V. The gain of the first stage is 1+47K/2.2K = 22.4
You could change the 100K to 1M but you would want a FET op-amp  like TL071 for the first stage

Here is what I get with my pencil-and-paper style

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The 5534 often needs a small compensation capacitor between pins 5 and 8, usually 22pF.
Without that it might be oscillating thus drawing more current.

I can't see all of it very well but here's a rough schematic

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It is in post #3

Here is the Tremolo-Reverb board
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Here is the input schematic. Please post another photo of the tremolo-reverb board.
I can't see the circled area very well.
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