BeoLab 8000: high frequency noise disturbs switching to OFF

[KolfMAKER]

I have an issue with one BeoLab 8000, serial# 1086xxxx

My use: I play music via the RCA input plug in the foot. (So no PowerLink use)

My issue: The BeoLab does not switch back to stand-by/Off. Instead, it switches repeatedly to Off > On > Off, …

What I noticed (both when listening close to the speaker drivers, at 1 meter distance these two matters can not be heard).

• When I play no music and keep my ear close to the speaker drivers, I can hear high frequency noise (especially at the tweeter of course).
• When I touch the casing or the AMP/PSU metal panel, there is a slight hum increase.

What I have done

• Since this is a speaker with old foam, I had to clean the foam rot. So I have meticulously cleaned all PCB’s (incl PCB05 and 06), components and connectors.
• I have checked for broken solder joints or PCB traces (incl. measuring continuity). Couldn’t find them but re-flowed solder joints on PCB 01/02 just to be sure.
• Measured voltage output of Trafo T2: 2x 20 volts DC.

 

Questions

• What could be wrong?
• What should I do next?

[KolfMAKER]

Update

• On > Off> On > … – When first powering up the BeoLab 8000, the auto stand-by switch works normal and switches from On to Off/stand-by after music stopped. But after some 15 minutes or so, the auto stand-by does not work normal anymore, and switches back and forth between On and Off. (Something to do with warming up?)
• High frequency noise – I found after thoroughly cleaning the track side of PCB01/02, that the high frequency seemed to have gone. But I know noticed that sometimes it is there and then it is gone for a while and returns again.
• Hum – The hum increase when touching the casing or AMP/PSU panel is still there

[espresso_fanatic]

I wish I had a solution here. I have a pair of BeoLab 8000’s as well and one of them exhibits the same issue. It toggles very quickly and has some high frequency noise like you mentioned. I am in the process of repairing the amplifier in hope of finding where the noise is coming from.

[KolfMAKER]

Update 2

Someone gave me the tip to reflow all solder joints on PCB05:

So I did. Again, the first 10 to 15 minutes, the auto stand-by switching worked well. But suddenly after that time, the repeatedly Off > On > Off > On switching started again. So still no resolution found.

Tips are welcome.

[espresso_fanatic]

I recently redid the entire board and found some possibly helpful notes:
– Check C6, 22 uf. When this capacitor gets drained, it seems to switch the speaker into standby mode. This might be bad given its age.
– IC1 could have gone bad but I suspect the capacitor more than IC1. Desoldering SMD components is quite trick as well.

[Author]

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