From Marcus....

MF XA-50 amplifier, heating up. Replaced trimmer potentiometer and set bias.
Amplifier failed during testing when driven into clipping momentarily without a speaker load. No semiconductor amplifier should fail under these
conditions. Replaced all failed devices and any others which may have been stressed. Set bias to the same level as the other amplifier supplied. Amplifier working well and can handle clipping fine. Tested distortion and it is the same as the other amp. The bias on these amplifiers are not set to the point of lowest distortion as they would probably overheat.Operated on the workbench playing music for a whole day while monitoring power draw. Amp remained stable.

So hopefully that's it for now! Half again from what I paid for them! Thought it worth getting it repaired though.

Mm ok, sounds like one of the transistors was partly faulty, making it run hot. Did he say what the actual bias setting was?

AMR-iFi R&D

From his email...

I don't know the bias setting as I was using the power draw on the mains to match the bias to the other amp.

Good idea without disassembling the other amp. :-)

AMR-iFi R&D

but was the other amp running to spec ?

😄

Well that’s the question...if it was running with slight warmth on the heatsink it should have been roughly in spec. The bias point for class AB isn’t very specific, but for class B it’s better to be finely adjusted, and depends on the speaker impedance. I suspect the MF is a class AB design with not much region of class A out of the class AB.
😉😴

AMR-iFi R&D