[Norman Tracy]

Al, I agree with richidoo. Given your temperature monintoring showed "installed only the Mains board from the good speaker into the humming speaker the transformer temp on it did not go above 31C and there was no hum" I would say your original problem is very likely local to the Mains board with overheated and discolored transformer. You are being careful and methodical (I hope, working with live 110v AC circuits!) so I would guess this is your plan, but I'll say it anyway. if it were me next step is install the replacement Mains board you've ordered and monintor its temperature once power is applied.

[richidoo]

Yes, and whenever powering up used components or parts always be prepared for anything. Keep your hand on the plug or power switch while looking for smoke on the “new” mains board, and listening for odd physical sounds. Safety glasses would be prudent.

[Al Dente]

Thanks guys for the replies and advice, I will be careful. I now feel more confident just replacing the Mains board.

I debated whether to start investing money in these old speakers or to just go for something new and inexpensive being that they are mainly my PC speakers. But they do sound great and their odd shape allows them to fit nicely at the ends of my bookcases, I would hate to see them go to waste.

I should have the replacement board in five days and hope my speaker will be singing instead of humming.

[Al Dente]

Received the "new" used board sooner than expected. Installed, powered on and left in standby for an hour without fireworks. The transformer temp did not go over 31C, same as the one from my good speaker, I'll keep checking the temp. I plugged in the audio and it sounds great. We'll see how long it lasts. They say "time is a healer", unfortunately not in the electronics world.

As I'm sitting here monitoring the temp, I am realizing that the large driver magnet is very close to the offending transformer when the enclosure is assembled. It looks like they wind up being a half inch apart. Could a large driver magnet being so close affect the longevity of the transformer?

Thanks all again for your interest and advice. Meanwhile I came up with a new plan for these speakers which I will address in another thread.

[richidoo]

Received the "new" used board sooner than expected. Installed, powered on and left in standby for an hour without fireworks. The transformer temp did not go over 31C, same as the one from my good speaker, I'll keep checking the temp. I plugged in the audio and it sounds great. We'll see how long it lasts. They say "time is a healer", unfortunately not in the electronics world.

Great! 

As I'm sitting here monitoring the temp, I am realizing that the large driver magnet is very close to the offending transformer when the enclosure is assembled. It looks like they wind up being a half inch apart. Could a large driver magnet being so close affect the longevity of the transformer?

Nope

[Al Dente]

You didn't think I was going to get off that easy did you?

I played the speaker a few hours each day for three days with the "new" used board with no problems and high fived myself. Woke up this morning, the speaker was giving me the finger and humming. No soup for me.

I quickly opened it up and found that temps inside were around 31C-35C. The transformer that was overheating on the old board was 31C. There is a large transformer that was 38C. These temps to me don't seem very hot but there was no audio signal going in. It seems that the hum is causing the speaker to turn on from standby, or something else is causing the speaker to turn on resulting in the hum. The chicken or the egg.

The transformer that was overheating on the "old" board was a potential problem but apparently not related to the dreaded hum.

There is no method to its madness, I opened it up laying on its back I couldn't get it to hum. Stood it up, after a couple of minutes it turned on and hummed. Laid it down, it still hummed. Stood it up again, after a few minutes it stopped humming then later it started humming again. Is it possible that by repositioning the speaker, bad caps can work intermittently if they are failing?

I rolled the dice and got snake eyes, what's my next move? Should I start replacing caps, what do you guys think?

[Al Dente]

I replaced all 22 electrolytic caps on the Power Supply and Amplifier board. The speaker played for 5hrs, was on standby overnight, played for another 5hrs with no problems. Then on standby with no audio signal it switched itself on and started humming as before. Every time I put it on standby, without audio signal, after 5 or 10 seconds it switches on and hums. With an audio signal it continues to hum.

I already spent almost $200 on a used Mains board and new caps on the Power Supply and Amplifier board. By swapping with the good speaker I know the problem is somewhere on the Power Supply and Amplifier board. Having it diagnosed and repaired will probably cost more than the speaker is worth. I could buy a pair of new speakers for a few hundred bucks and not have to worry about their age.

Any suggestions before I give up on them?

[richidoo]

Uh oh
You need a good analog audio tech with oscilloscope.
Have you called B&O about sending for repair?

Measure resistance across the woofer and teeetrr on the problem speaker to make sure voice coils are OK.

[Al Dente]

Thanks for the suggestion. I checked resistance on woofer and tweeter, both were at 6 ohms. The manual says "The loudspeaker units can be checked by an ohm-meter.
OK values for tweeter and woofer are approx. 6 ohms"

There are no reliable repair shops locally. I haven't decided yet if I want to pack it up, pay for shipping both ways and repair.