The Mini Metrology Lab - Conrad Hoffman
His "Electronic Mess" section has many pages on repairing and restoring old instruments and equipment.
An Excerpt from his pages is a Reading Pleasure due to his rich experience and enticing language.
Troubleshooting Audio Electronics
I ain't gonna lie. You won't be a master troubleshooter after reading a couple pages here. Hopefully, what I can do is give you the ability to form a logical plan of attack.He also has a Design and Testing Services called Deep Run Design.
Troubleshooting is done mostly with the mind, not with the soldering iron. We all have problems that get the better of us, at least for a while, but approaching the situation as an interesting puzzle, rather than an unpleasant task to be suffered through, will make the work go quickly.
Success will come from a 3-prong approach, a combination of understanding the circuit, having the means to test it and being in the right frame of mind to correctly interpret the test data. Never underestimate that third component.