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Beolab 4000 Mk1 - humming/buzzing

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AlfJ
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AlfJ posted on Mon, Nov 29 2021 9:32 PM

Hi all
Hope you can help. I searched the forum but found nothing useful.

I´ll try to cut it short:

System setup

Beolab 4000 Mk1 type 6636
Connected to Beosound 3000 + Chromecast audio
Worked well for a long time. Have not modified anyhing for years.

symptoms
Suddenly while listening I hear some scratching from the left speaker, like an old potentiometer when you change the volume. Nobody was near the equipment.
After this the speaker started humming. But no more scratching.
Kind of 50Hz buzz, but not clean 50Hz. There are some higher frequencies on top. Hard to describe in words.
Increasing the volume will increase the buzz. Turning off the 3000 turns sets the speaker in sleep mode and it goes silent
Speaker is still playing music, but with that humming/buzzing noise in the background. Almost hidden by dense pop/rock, but very audible with more silent music.

Tried this with no success:
Connected to other amplifier (Overture 2300)
Switched left and right. Humming stays in same speaker
Switched cables
Turned power cord 180 degrees
Switched to other power outlet
Opened the speaker. No visbile damage. No blackened compnents, no expanded capacitors that I have sen in other broken devices.
White foam inside in good condtion, so no mk1 "black foam rot".

Any DIY suggestions?
I can handle a soldering iron, and have alot of patience, but I  cannot start replacing components at random. I would guess the sound comes from the power supply and gets amplifed. Some filter component (capacitor?) that died of high age?

Spare parts available anywhere? Need to know what card to replace before ordering anything.

Any suggestions for a place to have it repaired?
Preferably near Stockholm, but anywhere in the EU would work. Freight is reasonably cheap.

Might be cheaper to find a used pair... These are in mint condition on the outside, so I could do with a less good looking pair to donate some spares.

 

Any input appreciated.

Alf

 

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Ravsted
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Foam rot is a very, very frequent reason for BL4000 MK1 issues. You'll find many articles on beoworld.org.

Buying another pair of MK1's won't help a lot. They will have same problem sooner or later.

I cleaned mine / replaced the foam. However some components had to be changed. 

AlfJ
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AlfJ replied on Wed, Dec 1 2021 7:36 PM

As far as I know only speakers with black foam suffer from this nasty rot.
Mine have white foam inside. The white foam seem to be in good condition, so I think we can eliminate foam rot in this case.

If I look for a replacement/donator I will stay away from the ones with black foam.

AlfJ
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AlfJ replied on Tue, Dec 7 2021 8:00 PM

Update:
issue solved.

Did all the testing again as described in the initial post.
Problem was NOT the speaker. It was caused by the Beosound 3000 amplifier.
When applying pressure on the back cover (from the wall mount) it started humming. Off the wall mounti was  dead silent.
Easy fix: remove back cover. Make sure the big PCB at the back was properly fitted. Getting the PCB too close to the back cover is what caused the humming. No idea how it got out of its place.  It´s been on the wall for several years.
But never mind... The system works again :-)

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