Swap tubes no change, short input no change.
The power supply is shared between channels, so a bad capacitor is ruled out. If you have swapped
all tubes between channels, a bad tube is ruled out also. Start looking for bad connections or other bad parts.
Perhaps a ground connection has corroded.
I’ve noticed in several MC240s that the 12k 2W plate resistors don’t age well and can be way off. The signal path is push-pull throughout, which normally rejects supply ripple, but If a component in one side of one channel is way out, it could cause an imbalance in that channel, reducing ripple rejection. You may *also* have a weak supply filter cap, with the good channel still valiantly rejecting ripple.
I’ve noticed in several MC240s that the 12k 2W plate resistors don’t age well and can be way off. The signal path is push-pull throughout, which normally rejects supply ripple, but If a component in one side of one channel is way out, it could cause an imbalance in that channel, reducing ripple rejection. You may *also* have a weak supply filter cap, with the good channel still valiantly rejecting ripple.
That's quite a good diagnosis, and certainly shows you have some education/experience in the matter.
Made my day.
Hum. C23 is part of the HV supply so both channels should have had hum. Odd.
One channel may be wired or grounded differently from the other, and see more ripple (if it's there).
I hope he replaced both C23 and C24.
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