Antique "Acoustic" amp needs attention ASAP

[schmitt trigger]

The schematic diagram shows it was drawn in 1971.
It is very old, but it is not antique.

This amplifier uses transistor sockets for the TO-3 output transistors. For an amp like this which has been so neglected, those contacts may be intermittent.
First things first, remove the transistors and clean the pins and the sockets with Deoxit.

I've not personally used it, but many people in audio forums swear by it.

DeoxIT D-Series page with shop - CAIG

DeoxIT® D-Series contact cleaner dissolves oxides and sulfides that form on metal surfaces. This restores the contact’s integrity and leaves a thin microscopic layer that protects the metal. Special additives improve conductivity and prevent dissolved oxides from re-attaching, keeping them in...

caig.com

While you are at it, also clean the spade lugs in the large capacitor.

 

[unclejed613]

the big round things are inductors for the equalizer. it sounds like the amp has thermal problems. wait until the amp goes out, and starting with the amplifier board (first picture in post #8 where you pointed out the diodes) spray each transistor with freeze mist (if you can't get freeze mist anywhere, take a can of "canned air" and turn it upside down, the canned air propellant is the same stuff as freeze mist) one at a time (spray the transistor and wait 10 seconds) and see if you get your sound back.