Switch off the Cornet disconnect output interconnects and measure resistance on the output RCAs (left and right) (between center and shell).
I must explain what I want. Sorry, but yesterday I saw the answer very late and cannot make enough attention to it.
I checked the center pin right RCA output by touching it with a screwdriver and there was no audible hum
Between the point and the interconnect there is only RCA. If you insist that the interconnect is OK (by the way - touching the center pin of the right channel of the interconnect from the Cornet side should produce the hum!), then there are only two possibilities:
1. Bad RCA connector.
2. Something shorted the central pin to the ground.
I think the first possibility is unlikely, so asking you to measure resistance I'm trying to reveal that it is the second that is the source of the problem.
I didn't stop as you said but went to the second step you suggested; touching pin 8. this step gave me audible hum from left speaker
I hope (to prevent my brain from bursting) that you just chose the wrong side of "pin 8".
Try to touch all the 9 pins one by one - at least 2 of them should hum on the right channel output.
voltage at output RCA is 0 at left channel and quite variable at right channel, going from 0.6 to 3 mV (measuring DC or do I have to measure AC?)
Yes, it should be DC. I asked you to measure the voltage at R119 to ground too - have you done than?