I've done a lot of repair in my day. I don't have what you need, but at the same time, my experience has taught me that 90% of "electronic" problems are mechanical in nature, such as switches, connectors, potentiometers, and so forth. Once you get into the guts of the amplifier circuit, it gets hard because that circuit is a feedback loop, so the signal can't be easily traced from one end to the other.

In the evolution of my home audio equipment, the amplifiers are all Class-D IC's that are easier to replace at a board level.

Prophylactically replacing things that might be broken can lead to worse problems, especially since a lot of older consumer electronics had relatively delicate single sided circuit boards that were easily damaged by desoldering.

came to post this, been my experience to a t. always start in places where things move, get plugged in, switches, look for exposed/funky looking traces/boards/connections. almost always an easy win once you crack the thing out (which can be a struggle)

sometimes watching youtubers like bigclivedotcom cover failures of cheap electronics can show spectacular exceptions though. for some more realistic failures of common consumer hardware like laptops and cellphones (and general rework wizardry) Electronicsrepairschool is pretty hard to beat

https://www.youtube.com/c/Bigclive

https://www.youtube.com/c/Electronicsrepairschool

Bigclive is the best. Sometimes my mind uses his voice when I’m checking out a circuit board. “What to we have here? A silver metal tube, probably the clock crystal for this kitchen timer. Ah ha, it says 32.768 on the board, so it certainly is.”

At least 90% is mechanical. The list mentioned above plus corroded traces on circuit boards and broken external connectors (RCA jacks, etc.).

For older components, beyond switches/pots the only other things I've ever had to replace are electrolytic capacitors.

Well, if it's the bass alone and in the left channel, that strongly suggests a problem in the wiring of the left speaker. A continuity tester is your friend.

If you get a crackling noise that changes when you change the volume, the volume potentiometer either needs to be cleaned or replaced. Similarly if the volume knob has ranges where the sound drops out entirely.

If one channel doesn't work at all, you trace the signal path starting at the source.

If nothing works, look for a fuse (and look for something that might have caused a short).

> A continuity test is your friend.

Also a spray can of freeze (e.g. CRC Freeze Spray) is useful, and a jeweller's loupe (x20 magnification or so). If your phone takes good macro video, that can be really useful to look at several times (in the absence of a stereoscopic microscope).

Contacts are the first thing to suspect with any fault. Check inside the plugs on the cables.

For intermitttently disappearing and reappearing bass, though: probably the contacts are OK. Inspect (and if possible, re-make) solder joints in this part of the circuit - you may have a hidden dry joint somewhere. Then old electrolytic capacitors, especially tantalum ones, are sources of trouble.

Of course if the amp has been in a house that's been hit by lightning, or where an arc welder has been used regularly, then the possibilities widen.