How to fix idle noise on 50 year old Bang and Olufsen

Alaza · 30th May 2011, 07:35 PM

Hello there,

I'm trying to put some life back in to my 40-50 year old Bang and Olufsen Beolab 5000 stereo amp. Basically I've changed most of the caps in the output stage (not the ones in the feedback yet though), and I've been cleaner the potentiometers with contact cleaner.

I still get quite a high humming noise in the speakers when having the volume at 50% and upwards. Is it dried out caps, or should I be looking somewhere else?

I know it's a bit vague, but hope you can point me in some direction.

Thanks

Tore

djoffe · 30th May 2011, 10:53 PM

sounds like power supply filter caps have lost their capacity, leading to much hum on the power supply. Have you checked those?

Alaza · 31st May 2011, 06:42 AM

That could definitely be the problem. But how is it that it influences the output of the amp, if the caps in the power supply is dried it? There is three caps, two 50uF and one 80uF.

I'm not the a power supply expert, so I'm not entirely sure of their design purpose, but I've uploaded the schematic. The quality is really bad, but I can't find anything better at the moment.

harrydg · 31st May 2011, 09:48 AM

Quote:

Originally Posted by Alaza

That could definitely be the problem. But how is it that it influences the output of the amp, if the caps in the power supply is dried it? There is three caps, two 50uF and one 80uF.

I'm not the a power supply expert, so I'm not entirely sure of their design purpose, but I've uploaded the schematic. The quality is really bad, but I can't find anything better at the moment.

i'd start with the 2500uF cap next to the 4 diodes after the transformer (dammit, terminology is failing me at the moment :s)

Alaza · 31st May 2011, 02:18 PM

I already changed that, along with two 3000uF that are connected on the output. I have added the output stage too. Why would you put caps on the output like that?

Alaza · 31st May 2011, 09:09 PM

his047 · 19th April 2012, 10:52 AM

Quote:

Originally Posted by Alaza

I already changed that, along with two 3000uF that are connected on the output. I have added the output stage too. Why would you put caps on the output like that?

Hi Alaza,

I would guess that the output caps are there for protection: many years ago when I taught English to Danes in Copenhagen one of my students was an Old-Style electronics engineer. He swore blind that this kind of circuit could not work!

On a different note, I have just bought a set Beolab/Beomaster 5000 and I am looking for Circuit diagrams. Is there any chance you could send me those (off-list)
All the best
Frederik
(A Dane in Scotland)

djk · 22nd April 2012, 07:06 PM

If the hum increases with volume it is coming from the pre-amp, either that or you are using a turntable?

pdul · 22nd April 2012, 08:07 PM

[QUOTE][/QUOTalready changed that, along with two 3000uF that are connected on the output. I have added the output stage too. Why would you put caps on the output like that?

With single rail ps you will have half of the voltage standing on the output. So to save your speaker from dead by frying. It was normal in many amps back then

30th May 2011, 07:35 PM	#1
Alaza diyAudio Member Join Date: May 2011	How to fix idle noise on 50 year old Bang and Olufsen Hello there, I'm trying to put some life back in to my 40-50 year old Bang and Olufsen Beolab 5000 stereo amp. Basically I've changed most of the caps in the output stage (not the ones in the feedback yet though), and I've been cleaner the potentiometers with contact cleaner. I still get quite a high humming noise in the speakers when having the volume at 50% and upwards. Is it dried out caps, or should I be looking somewhere else? I know it's a bit vague, but hope you can point me in some direction. Thanks Tore

22nd April 2012, 07:06 PM	#8
djk diyAudio Member Join Date: Feb 2001 Location: USA	If the hum increases with volume it is coming from the pre-amp, either that or you are using a turntable? __________________ Candidates for the Darwin Award should not read this author.

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30th May 2011, 10:53 PM	#2
djoffe diyAudio Member Join Date: Oct 2008	sounds like power supply filter caps have lost their capacity, leading to much hum on the power supply. Have you checked those?

31st May 2011, 06:42 AM	#3
Alaza diyAudio Member Join Date: May 2011	That could definitely be the problem. But how is it that it influences the output of the amp, if the caps in the power supply is dried it? There is three caps, two 50uF and one 80uF. I'm not the a power supply expert, so I'm not entirely sure of their design purpose, but I've uploaded the schematic. The quality is really bad, but I can't find anything better at the moment.

31st May 2011, 02:18 PM	#5
Alaza diyAudio Member Join Date: May 2011	I already changed that, along with two 3000uF that are connected on the output. I have added the output stage too. Why would you put caps on the output like that?

31st May 2011, 09:09 PM	#6
Alaza diyAudio Member Join Date: May 2011

22nd April 2012, 08:07 PM	#9
pdul diyAudio Member Join Date: Jan 2009	[QUOTE][/QUOTalready changed that, along with two 3000uF that are connected on the output. I have added the output stage too. Why would you put caps on the output like that? With single rail ps you will have half of the voltage standing on the output. So to save your speaker from dead by frying. It was normal in many amps back then

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