Here is the preamp schematic. The Tone switch is a gyrator notch filter. I can't see the value of the silver capacitor on the tone board but if
I assume it is 1000PF the frequencies are: 1 - Flat, 2 - 230Hz, 3 - 1kHz, 4 - 3.4kHz. If you can read the value, let me know.
I couldn't draw the reverb section because the foil side is hidden by the footswitch jack. If you can move it and post a photo I can do it.
If you want to lift the power board and post a photo of the foil side I can do that too.

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Must be some kind of bad connection if it works sometimes.
Master and slave boards have 4 fuses each. Could one be blown or intermittent?
I always check/re-solder all pins of those white connectors, a common problem.
The circuit is quite complicated and the schematic is a mess too.
Hopefully a good visual check will find the problem.

Here is the rest of the schematic except for the meters. 
If you're not hearing relays clicking while turning
the Voltage knobs, try tapping the circuit boards in various places.
There might be a bad solder connection somewhere. 

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I see from the schematic the relays switch transformer secondary taps for the unregulated DC on C23 and C22. Bad relay contacts seem more likely. Does the output voltage jump if
you tap the relays with a screwdriver handle?


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Could be an open main filter capacitor. Check for ripple if you have a scope, otherwise
check DC or AC voltage across each cap.

The schematic for the 6303DS is here, maybe similar
https://www.radiomuseum.org/r/topward_el_dc_power_supply_6303ds.html

A bit of an ordeal to download but at least it's available.

Here's the preamp

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I tried many methods early on: wax crayons, stick tapes directly to the copper foil,
but quickly found that the photo sensitive resist method was the best. I would make my
layout on Mylar, then contact print it to photographic film to make a negative,
which freed up the donut pads for future layouts, me being that frugal. Then I would
make contact prints of the negative to return to positives. Now I use a scanner and
laser printer for that step.

Of course it makes more sense to use a PCB program, generate Gerber files and send them
off to Korea or wherever for better boards than I could ever make, but I find the computer
method a big ordeal and annoying. And I have a good supply of photoresist in stock.

The Londoner L100a in this thread has ICs.

https://www.ssguitar.com/index.php?topic=4315.0

The schematic in post #8 might be for the earlier non-A version.

Can you post photos of the circuit boards, both sides?

I researched a little and found this type of glue is called Pressure Sensitive Adhesive (PSA) It comes in spray cans too. 3M, Gorilla etc. Not hard to find. I'll try it out.
I could stick the donut pads upside down to a post-it note sticky area and then spray.

The most important property of the glue on the pads and tape is that it's
re-stickable since often things need to be re-posistioned.

It's like the glue on masking tape. I don't know where to get that or how you would
prevent it from peeling off the pad. Maybe there's a primer

Here's a messy first draft. There may be mistakes. *added some minor corrections

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Can you post a photo of the component side by itself? The composite photo is hard to sort out.

I've designed hundreds of PCBs over the years with donut pads and tape. Prefer it to computer design because you can lay the actual components on the layout to see how much room there is. I would still like to do it that way but all my pads and tapes dried out years ago. No stickum.

Here it is from the Guitar Amplifier Handbook

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Here's a slightly better one from my paper files but only the main schematic.You cannot view this attachment.