Diodes from 1988 are easily checked. One diode scale of DVM look for 450-750 volt drop forwards, 9999 or ---- backwards. Backwards may be lower due to parallel resistances.
You don't have any 3 amp speaker back EMF diodes, so the antisaturation diodes on the LTP (differential input transistors) could be 1n4148 world's cheapest diode. The zener diodes feeding the input stage obviously have to be zener diodes rated 12 v but will measure the same. There is a wattage rating on zeners, the 5 W ones are $.35 and the freight from the parts house is $9 so don't calculate to save a dime on 1.3 w or 3 w ones.
I would suggest measuring resistances from PCB trace to PCB trace to find a bad solder joint causing your volume inbalance. Measuring lead to lead you miss that. You may also measure capacitances below 10 uf that way if you have a capacitance meter or function of the DVM.
Bottom line, this is a stupidly complicated circuit with no idle bias control or adjustment. I'd buy some other board suitable for +-24 and salvage your output transistors. Use new heat sink pads & grease. I don't study 20 W amps so I don't have a list of candidates handy, except the 25 W AX6 derivative at the end of this thread:
Retro Amp 50W Single Supply - Page 22 - diyAudio
but a LM3875 board for an IC from e-bay would work. Also LM3886 and tda something IC amps. Those import boards always come without the heat sink to save money and push the listing to the top, so be prepared to buy a heat sink & grease. also the rail filter caps usually. You'll need a drill, drill motor, safety glasses to drill the heat sink. The e-caps ebay boards come with will be **** expect them to last a year, maybe.
You'll need some 1" or 1.5" long screws and nuts plus standoffs to hold the board up off the chassis. If you buy actual threaded standoffs you can get away with 3/8" long screws. I use the long ones and 1/4" air tubing for standoffs, much cheaper after you've bought a $10 100' roll of 1/4 air tubing.`