[Al Dente]

I have a pair of Beolab 4000 MK1 speakers that I have been using for 11 years. Now one speaker started to go on and off rapidly without any audio signal while in the standby mode. Then shortly afterwards it stayed on (without audio signal) with a loud hum. Now every time I switch it to standby mode, the green led goes on and the speaker makes a loud continuous hum. The hum does not change whether the audio cable is plugged in or not. I swapped it with the other good speaker using the power cord from the good speaker which is plugged in to a different outlet. The hum still continued regardless of the location.

Has anyone had a similar experience or a guess to what the cause and solution may be? Would greatly appreciate all comments, Thank you.

[richidoo]

Could be the amplifier power supply capacitors have failed due to age, that can cause hum.
Or could be something on the input/preamp board is bonkers.

Bang Olufsen support

[Al Dente]

I have been away for several days and the humming speaker has been powered off that whole time. I switched it to standby and played some music at moderate level, everything was normal, no humming. After playing for an hour the humming started and continues as before.

Apparently the humming is triggered when the speaker warms up (it doesn't get warm on the outside). Does this give you any further clue to what is the cause?

I'm going to open it up see if there are any obvious signs of a problem. Any suggestions of what to focus on? I can clean all connectors and do some basic soldering and replace caps but don't know of any other diagnosis I can do.

I use these as front speakers for my PC with a Velodyne HGS-12 sub all playing through a Lexicon MC-8 DC with Lexicon RT-10 for discs.

[bladesmith]

I think you might have a hum in your amp once it warms up.. 🤔

[richidoo]

Look for swollen capacitors or fluid leaking out of capacitors or burning around them. Replace all of them if you see any signs of failure. They fail from age.
You prob need a tech to probe the voltages to find the cause of the hum. Its probly related to the power supply.

[Al Dente]

Just to clarify, the humming is coming through the driver. The amplifier or power supply itself is not humming. I don't know if that makes any difference in your thoughts.

Just before opening the housing I switched it to standby, it immediately went on and started to hum. Note- when in standby the humming automatically switches it on as if it was an audio signal.

I opened the housing, switched to standby, it went on and hummed for about fifteen seconds, stopped and then went back to standby. I switched standby off and on several times but could not get it to hum again.

I looked at all the boards and caps and could not see any obvious problems. I cleaned all connectors. I poked around while it was powered and didn't hear any noises through the driver.

I thought somehow I fixed the problem but after playing normally for about two hours the hum is back.

Any final thoughts, I think I have done as much as I can. Is it worth fixing, are parts available, who can I get to do it in Southwest Florida or anywhere else?

[Norman Tracy]

Al, the symptons and age of your 4000s point to a trip to the repair shop. In the early 2000s when I worked for a local HiFi shop the repair tech said B&O was his favoriate brand to work on because they were designed for service. Something of a lost art in 2022. A couple of resources are:

https://www.metzgervintageaudio.com/repairs

https://www.beoworld.org/prod_details.asp?pid=370

Good luck keeping your 4000s in service.

[Digi-G]

Loudspeakers typically don't (can't?) introduce hum.

It's probably related to your amp / power supply.

[Norman Tracy]

Loudspeakers typically don't (can't?) introduce hum.

It's probably related to your amp / power supply.

Recall the Beolab 4000 is a powered speaker with amp / power supply built into each speaker. So they can and in Al's case do hummmm when the in built electronics need attention.

[bladesmith]

Both speakers are having humming issues..?
Or just one..?

[Al Dente]

Each speaker is individually powered and amped. Only one speaker hums, with or without the audio cable plugged in.

It puzzles me that merely opening the housing and powering it on while open, without any tinkering, made the humming stop for a couple hours  It seems that whatever is causing the hum can be affected by just handling the speaker. Do bad caps sometimes work temporarily when they are jarred?

[bladesmith]

Can you switched power cables btwn the units..
Or are they hard wired...?
(Sometimes power cords go bad...)

[richidoo]

Just to clarify, the humming is coming through the driver. The amplifier or power supply itself is not humming. I don't know if that makes any difference in your thoughts.

A transformer can make physical humming sound but thats not the problem here. What’s causing this hum is AC current on the DC power supply output. The power supply capacitors are supposed to smooth out the rectified AC so when the caps fail the smoothing stops. This cap failure  is normal and expected occurance with vintage audiongear because the electrolytic caps contain a liquid electrolyte which dries out over time.

Sometimes the caps show symptoms of failure like leaking, burning, swelling, but they often fail without any external signs. When we buy vintage equipment of 20 yo or more it is recommended to replace all the electrolytic caps, especially if there is malfunction, and especially if there are power supply symtoms, the most common is humming.

Any good electronics tech can replace the caps. It’s not rocket surgery.

I would recommend using very high quality replacement caps. Like Panasonic or Nichicon bought from reputable seller like Mouser.com or digikey.com, newark.com, etc. Replace the caps with same value capacitance, equal or greater voltage rating, 80C or greater temperature rating and equal or smaller physical size. Your tech will know all this and may even have the parts on hand.

Easiest route may be to call B&O and ask about sending it to them for repair. Cost, turnaround time, warranty on repair, etc. That may be the best route to take if this piece is sentimentally important to you. There are some hack repair guys out there but B&O repair will be OK.
Good luck

edit: the humming could also be caused by a failed transistor or other parts in the circuit besides power supply caps. this requires a knowledgable amplifier tech to probe the circuit with voltmeter to find where AC is leaking into the output signal.

[Al Dente]

"I swapped it with the other good speaker using the power cord from the good speaker which is plugged in to a different outlet. The hum still continued regardless of the location."

Thanks to all of you who took interest in helping me with my problem, all good advice. Since I have the other good speaker I will try swapping the power supply then systematically swap each board to narrow down the problem. I may attempt to replace some caps, I've done it before on other items. I have the service manual with schematics, just need to learn how to read them. Last resort is to send it out.

I will keep you informed of progress if any.

[Al Dente]

A transformer on the Mains Switch Board in the humming speaker is apparently overheating. After an hour in standby, without audio signal, the outside of the transformer reached 70C. Although at that temp it still did not begin to hum. You can see scorching on the transformer case. The transformer temp in the good speaker only reached 31C in the same amount of time, same conditions and has no scorching.

I installed the Mains Switch Board from the good speaker into the bad speaker. It did not overheat meaning that the other components in the speaker were not causing the overheating.

Apparently the cause of the overheating is isolated to the Mains Switch Board. Could something on that board be causing the transformer to overheat or is the transformer just going bad?

[richidoo]

The burning in the transformer probably corresponds to melted insulation on the windings with intermittentl shorts causing the humming and on/offing. It’s not a stable situation, it wont continue to “work ok for an hour”

Generally any repair will replace burnt parts first, esecially a mains transformer, since it is a fire hazard when it blooms into full short. The fuse should pop then.

The rest of the board looks ok.

Its a custom transformer so doubtful its still available, but you never know. If they have enough repair requests they can order more. Worth asking. But they wont send it to you for DIY repair  😁

At least now you can tell them its burnt power transformer, if you can talk to a tech.
Maybe find a used part? They probably all used that transformer and amp in that period and product series.

[Al Dente]

Since I have isolated the problem to this board, is it likely that something else on this board could have caused the transformer to overheat or was it just a failure within the transformer itself? I know it's a tough question but what is your gut feeling?

I did take your advice, rolled the dice and took the easiest way out. I just ordered a used board on online. The picture doesn't show any scorching. I will monitor the temp and hope there are no other problems with the used board. Hopefully I will be humming instead of the speaker.

Thanks for all comments and advice, I will reply with the outcome

[richidoo]

You make a good point about cause of the tranny fail.

Tranny actually feeds two power supplies. One small one on the tranny board and another on the Power Supply and Amplifier board. If a  rectifier diode is failed it might cause overheating tranny. I'm not qualified to surmise much beyond that. Worth checking the diodes. You can lookup how to do it on utoob, but I think you have to lift one end, but I could be wrong, I only did it once. You could just remove the diodes with the intention of upgrading them anyway then test them when removed. Diodes are pennies. Well maybe not nowadays haha

Good job finding a replacement board. I hope it solves the problem!

[Al Dente]

Am I correct in thinking the Power Supply and Amplifier board is not causing the overheating? When I 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.

Is it likely that some very subtle problem on the Power Supply and Amplifier board could be degrading the transformer so slowly that it would not be evident until after a much longer time? Obviously I won't find out until it is too late.

[richidoo]

Am I correct in thinking the Power Supply and Amplifier board is not causing the overheating? When I 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.

Yes

Is it likely that some very subtle problem on the Power Supply and Amplifier board could be degrading the transformer so slowly that it would not be evident until after a much longer time? Obviously I won't find out until it is too late.

No

Good job, Al!