Hmmm, opened it up and it seems I did not change the main filter caps. But they test allright on my ESR meter. Also I can't see them failing at the same time with the same issue. Bert

Odd. Mine still runs on its original main caps too. No hum anyway? I’d suspect a switch problem.

Have you checked the thermal compound and mica insulators? Sometimes, the thermal compound is fine after decades, other times it has dried out and shouldn't be trusted. Mica insulators can split between layers. These will look like there are air bubbles in the mica. Mica is a good thermal conductor, air is a good thermal insulator. Disassembling, inspecting and replacing the thermal compound with modern, higher performance compound doesn't take that much time. Even if it wasn't needed, it can provide peace of mind. Addition:  The above comments are primarily targeted to the power supply components. These might affect both channels simultaneously. Also, if you have a FLIR camera, it is really useful for debugging potential thermal issues. Glitch

Check for cracked solder joints, particularly where connectors and the piggy-back boards are soldered in. Does it run warm on the cooling fins? If so check the idle current settings. Martin

Hi all,

• When I serviced the machine all output transistors were replenished with new washers and thermal compound
• There is no hum at all and the filter caps register as fine (outside of circuit)
• Sorry, no FLIR camera but nothing is getting really hot on the board aside from the transistor with the giant 'star' heatsink (which I would expect to get hot). But it doesn't seem to be failing and a failure here would be apocalypse on the power supply
• There is one (1) component which I think is an old thermistor. It is in the middle of the heatsink. On inspection I saw that it's very long legs were touching. That is not good. But I don't know how to test the bugger.

I will indeed check the soldering again. I don't see any cracked solder but i've experienced tired soldering on almost all B&O machines. Problem is that this is very hard to see. Doing a complete reflow of the main board is a total pain to say the least. Bert

Bert You dont say if the falling volume is occurring on all inputs i.e. FM tuner/phono/tape ? Craig

And incidentally one way of testing the thermistor is to blow a hairdryer over it and the fault relay should  activate and shut the amp down. Craig

Is the thermistor R40? If so, the service manual calls it out as "50 0hm 30% PTC". You can test it by removing it from the circuit, hooking it to an ohm meter and observe the resistance as you heat it up with something like a hair dryer. I would also make notes of all of the voltages from the power supply and see how they change with time and temperature. From the schematic, it appears that many of the voltages are referenced from the +15v supply. If this dropped down, many of the other signals would follow. Glitch  

One way of determining if the 15v rail is falling off is to monitor the power ON lamp as this is fed from that supply. Craig