Mcintosh MC240 Hum on one channel, need help

[bigpandahk]

Hum sound from left channel, could anyone advise likely cause and faulty component(s), thanks

 

[Osvaldo de Banfield]

I would start by interchanging tubes between channels, to see if the fault is also interchanged.

 

[rayma]

First check for bad cables, and loose fitting plugs. Then short the inputs and see if the hum is still there.

 

[6A3sUMMER]

Wow!
A systems man, thinks outside the box.

 

[bigpandahk]

Swap tubes no change, short input no change.

 

[rayma]

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.

 

[kodabmx]

Or a grid leak opened up?

 

[lens42]

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.

 

[wiseoldtech]

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.
🙂

 

[bigpandahk]

Thanks all for your advice, need some time to diagnose each part.

 

[bigpandahk]

Problem fixed and a faulty capacitor (C23) was replaced.

 

[Brice]

Hum. C23 is part of the HV supply so both channels should have had hum. Odd.

 

[rayma]

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.

 

[bigpandahk]

It matches Lens42's diagnosis. The four 12k 2W plate resistors were measured as 12K, 13.2K 13.2K and 12.9K. Seems the channel with 12K was as little way out, plus the weak C23(not dead), ripple heard from that channel.