Hi Everyone,

I have a somewhat strange problem with my BM8000. A few months ago I exchanged all capacitors and trimmers in the output stages after I fried the TIPs in the right channel trying to adjust the no-load current with the original corroded trimmer (after I bought it it ran pretty hot on that channel). After the repair and replacement of the TIPs everything worked great. The temperature of the heat sinks was about 37C under normal operation.

A few weeks after the repair, the temperature of the right channel began to increase (~50C). I opened the BM and checked the voltage across R237/236 and found about 100mV (instead of the prescribed 18 mV for minimum no-load current). I adjusted the trimmer slightly and got it back down to 18 mV. Everything fine again...low temperature etc...

A few weeks later, same scenario...about 100mV, and a hot heat sink. I adjusted again to 18mV and checked the DC voltages around the TIPs...all seem normal. The supply voltages are also normal...

Something seems to go south, but I do not see what it might be! Or can it be that the trimmer "self-adjusts" due to temperature fluctuations?

Advice would be greatly appreciated!

Rudy

Did you only replace the TIPs when it was fried ?
Ideally, you should replace all components in the affected circuit, it's DC coupled so
a fault in one stage will affect the next and previous but less will often do.
That means all semiconductors, all resistors in the idle current circuit and check the
constant current generator.
Any component that looks or measures bad should be checked (or simply replaced).
I doubt a trimmer fault but of course bad trimmers can be found.
My bet is on a semiconductor or resistor going higher but I'm afraid I cannot come much nearer than that.

Good that you check and feel every now and then.

Martin

Hi Martin,

Thanks much for your kind reply! I only replaced the TIPs and the trimmers when the fry happened. When it heated up again, I replaced all electrolyte caps...but as we know now this did not have any impact...I will order the other semiconductors and the resistors in the vicinity (the two R237/236 were indeed a bit blackened back then...;-), and give it a go next weekend...Luckily, I have now a working BM6000 to fill in for his bigger friend while he is convalescing (thanks again for the perfectly fitting volume belt!).

Rudy