The 120Hz hum in both pre and power stages suggests a problem in the regulated supply. There is a glue used by yamaha and
others that becomes corrosive and conductive, I'm not sure if that glue was used in the CA-1000/in the pic provided. I think not.
Ideally an oscilloscope is needed to test the regulated supply, I'm in the process of "building" an audio probe which might be
useful for hum, give me a few days on this.
I assume you have a cheap (not true rms) multimeter like me. What you could try is measure the "AC" ripple before and after
the regulation transistors by,
- Black probe connected to chassis GND, measure AC voltage at R706 as shown, Then measure at the +B solder blob on the relay board
- RED probe connected to chassis, measure AC voltage at R705 as shown, Then measure at the -B solder blob on the relay board
Non-True rms multimeters will interpret the AC ripple on the +/-60Vdc as a large AC voltage, maybe 80-100VAC.
The point to the test is to confirm the VAC is significantly reduced at the +/-B test points. Important that red probe
is grounded for the -B test. Next stage would be transistor replacements TR701,2,3,4 but need some measurement
to identify/confirm.