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Taking on the BM8000

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krais
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krais posted on Sat, Dec 28 2019 11:56 AM

Hi all, 

I've recently started restoring an old BM 8000 (type 1901) that I bought second hand years ago. The seller told me that the BM would not turn on (repeatedly clicking relays). I did not try to power the unit up to avoid blowing the output stages so not sure what the actual symptoms are.

After not being able to find a B&O technician in the Netherlands that is willing to take on a BM8000 due to the labor involved I decided to have a go at it myself. I know, I know, these are complicated amplifiers, not really stuff for an inexperienced hobbyist with little knowledge of electronics... I just cannot resist the challenge so decided to give it a try with the help of information available online (beolover website). 

Any help along the way would be appreciated. I love this design so would be awesome to get it working again (or at least take care of the laborious tasks such as recapping and rebuilding the LED displays).


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krais
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sonavor:

Is the hum you are hearing the 50Hz line voltage frequency? If so then it sounds like there is a grounding issue somewhere. Check that the shielding grounds on the source input box are all connected. From there check the grounds to the chassis ground points.

-sonavor

 

Sorry for the delayed reply. Finally had some time this weekend to start troubleshooting the speaker hum.

I'm not sure it is 50Hz line frequency, sounds more like a higher pitched noise to me. Tried to do a measurement with my oscilloscope but difficult to draw any conclusions.

I did check the shielding grounds on the source input box and chassis ground points (the ones at the source input box, near the transformer housing and on the front plate next to the tuning wheel) and found no issues. Also, measured resistance between ground points on various boards but could not find anything out of the ordinary. 

So to recap the symptoms:

 

  • Sharp pitched hum on the right speaker only (there is almost no hum on the left channel and the noise present on that channel has a different frequency). Audible up to 1-2 meters distance from the speaker. 
  • The humming sound does not become louder when adjusting volume
  • Hum is present regardless of selected input. It starts when switching on from standby and remains audible when switching back to standby mode until the reservoir caps have been discharged.
  • Present even if no input cables are connected.
  • Practically inaudible when listening through headphones.
  • Hum is present on both speaker terminals 1 and 2 and with different speaker cables.
  • When reversing the left and right inputs going from the preamp to the tone control board, the hum stays present in the right channel.

 

I have to say I'm quickly running out of ideas so any tips would be very welcome.


sonavor
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sonavor replied on Mon, Sep 14 2020 11:55 PM

Try disconnecting the the audio signal cables from the left and right output amplifier boards and see if the noise is still present.  That will tell you if the noise is coming from the output amplifier circuitry.  That won't necessarily mean the output amplifiers are the source of the hum but should eliminate the preamplifier and tone control sections as sourced of the problem.

On the flip side, if the noise goes away with those connector disconnected then the problem must be coming from one of the stages prior. If that is the case then I think the next step would be to put the audio connectors back on the output amplifier boards, then disconnect the connections between the preamplifier board and the tone control board.

I haven't seen this before but I wonder if there could be noise coming from the Beomaster transformers.  We see it in the older Beomaster 4000 transformers.  But, as I said, I haven't heard of the problem in the Beomaster 8000 before.

Offhand I can't think of anything else.

-sonavor

 

sonavor
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sonavor replied on Tue, Sep 15 2020 12:01 AM

Also, you might post some internal photos of your Beomaster.  Specifically where the black audio ground wires have connections. I often find it good when more sets of eyes are on the problem.

-sonavor

krais
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krais replied on Tue, Sep 15 2020 9:20 PM

Thanks Sonavar!

However, the BM is disassembled once again. I'm in the process of replacing the board connectors from PC9 to PC8 but ordered the wrong parts... Once the new parts are in I'll put the boards back together and post some pictures. 

krais
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sonavor:
Try disconnecting the the audio signal cables from the left and right output amplifier boards and see if the noise is still present.  That will tell you if the noise is coming from the output amplifier circuitry.

The noise disappears when disconnecting the right output amplifier. I guess that's good news as this would probably eliminate the transformers as the source.

When disconnecting the signal cable from preamplifier board to the tone control board, the noise remains present. So it looks like there is a problem with the tone control board PC4 or a grounding connection issue through that board. 

Here are some pictures... 

 


krais
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Preamp connections + ground point


krais
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Tone control board closeup


krais
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Microprocessor board / ground point


sonavor
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From your investigation it sounds like the problem gets introduced in the Filter & Tone Control board.

Connector 4P24 of that board sends the audio signals and ground on to the left and right channel output amplifiers.
That connector is on the left side of the board next to the FM signal strength meter.

Unfortunately that cable is a more complex cable in that it has a single connector to the Filter & Tone Control board but from there it splits into separate left and right channel cables to the respective output boards. In addition, the cable assembly has black ground wires to ground locations on the output amplifier boards and to the grounding point of the BM8000 chassis. I would suspect that harness and the connections involved with that harness.

-sonavor

krais
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krais replied on Sun, Sep 20 2020 9:33 PM

sonavor:
In addition, the cable assembly has black ground wires to ground locations on the output amplifier boards and to the grounding point of the BM8000 chassis. I would suspect that harness and the connections involved with that harness.

I did a bunch continuity checks on the various connections from 4P24 (with other cables disconnected in order to eliminate connections through other boards). However, it definitely looks as though those signal and chassis ground connections are good.

Wondering if a noisy component on PC4 could be the problem... I may yet again remove the tone control board, power with external lab power supplies and trace the signal path with an oscilloscope to see if I can discover something.  

 

Dillen
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Dillen replied on Mon, Sep 21 2020 6:28 AM

A noisy TL072 OpAmp could do this.

Martin

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