Check this board and the LED board for broken solder pins/pads. Especially the ground pins... and use a magnifying glass... BL8000 have lots of problems with broken pads. More or less ALL active loudspeaker have this problem. Sound is stress to solder pads, it's mechanical stress and it is just a matter of time and played volume...   ... and for the acid foam. Maybe broken lines in the preamp section...   Login to view image

... or here: this one is very nice. Acid foam on the ABL section. Really nice broken lines...   Login to view image    

@Die_Bogener Thanks! I will make those checks and come with my findings.

Findings First I disconnected and took PCB05 and PCB06 out of the speaker. Checked again for foam rot rests. Did some extra cleaning to be sure. Then I made following checks and did resoldering. Login to view image Checks PCB 05 – Link shift (in outside foot)

• Continuity: measured continuity on all traces and solder points connected > OK.
• Re-soldering: re-soldered all solder points, incl. connectors and component pins > Done.

PCB 06 – Stand-by (in bottom speaker casing)

• Continuity: measured continuity on all traces and solder points connected > OK
• Re-soldering: re-soldered all solder points, incl. connectors and component pins > Done.

P4 Connector (between PCB05 & PCB06)

• Continuity: measured continuity on all 5 pins of the cable > OK.

  Next After this I put the BeoLab back in original configuration: PCB06 screwed back in normal position, PCB05 connected but hanging loose. iPhone connected (as audio source) via external mini-jack to RCA cable, connected to RCA in foot. Result: There is a little hum, but Auto stand-by does switch OFF. Findings:

• When the P3 connector or cable is touched, Auto stand-by switches ON. But when let loose, it also quickly switches OFF.
• When the P4 cable is touched, nothing happens.
• When the P6 connector (audio cable) on PCB05 (Link shift) is touched, Auto stand-by also switches to ON. But also, quickly switches OFF when let go. This also happens when the connector of this same cable on the amplifier side is being touched.

  Question There still is sensitivity in the audio signal path. What is the best next step I could make?

I’ve had similar hum issues on BL 8000s when using the RCA jack, but not when using PL.  In my case the RCA jack began to loosen on the PCB board.  At first the movement is barely perceptible, and the hum is soft.  Over time as you move the speaker or touch the RCA cable, the jack wiggles more and the hum becomes an uncomfortable yowl.  A mechanical issue surely caused by endless plugging/unplugging RCA over 30 years.   I re-soldered the feet of the RCA jack to back to the board, and (fingers crossed) all is well.

Thnx BeoTool! In my case the RCA connector is not soldered to the PCB. It is fixed to the board by a nut, and I have tightened the nut well. So it must be another cause. Any idea?   Login to view image

Solder it. Hum is just some millivolt. Copper, mixed with chromeplated steel and nickel will always make some chemical noise. You can also make a special shortcut plug for testing. Just a shorted Cinch connector. Then you can test wether there is still humming...

@Die_Bogener: As you suggedted, I connected a shorted RCA plug to the RCA connector on PCB05. Result: the hum is gone, Auto stand-by switches to OFF and stays OFF, and when touching the P4 cable, Auto stand-by does not switch on. So this does look good! But now I need to make a next step, because I need the RCA connections, so I cannot keep them shorted.   Here's some additional things I changed, tested and the results.   Checks 2 I took another look at PCB05: used a dentist tool to scratch along the edges of the RCA GND contact on the PCB, and at some other connections/traces on the PCB. Just to be sure, there's no traces that make unwanted contact. Tested after checks 2

1. Input switch set to RCA: When the BeoLab was switched on again, the hum was more or less gone. The Auto stand-by does switch to OFF, but after one second immediately switches ON again, and then repeatedly goes ON - OFF - ON - OFF.
2. Input switch set to PL: Auto stand-by switches OFF, and stays OFF > OK. Touching the PCB, P3 / 4 cables, etc. has no influence on Auto stand-by, it stays OFF > OK.

  Checks 3 Extra checks on the GND and disturbances of the RCA plug on PCB05.   Tests with Checks 3 All checks done with the input selector set to RCA.

1. Connected a wire between GND of the RCA connector outside shell above the PCB, and the GND underneath the PCB. Problem remains > No positive result.
2. Put a metal cap over the RCA plug for shielding. Problem remains >  No positive result.
3. Connected my old iPhone with a 'mini-jack-to-RCA' cable to the RCA socket on PCB05 > No hum, Auto stand-by switches to OFF and stays OFF.
4. In situation 3. I also touched the cable of P4 on PCB05. When touching it, Auto stand-by switches to ON. When releasing the P4 cable, Auto stand-by switches OFF again and remains OFF.

  What bothers me is, that the Auto stand-by actually works well, but ... Touching the P4 cable makes the Auto stand-by switch ON again. Therefor, it is not yet working reliably.   Questions

• What do you conclude from this?
• What next step do you suggest?

   

Here's another update. I made several other checks. (Check 1) PCB 05 exchanged - I took the 'PCB 05 Link shift board' from another BeoLab 8000 and put it in the 'trouble BeoLab 8000'. (1) Result - Nothing changed. So, when nothing is connected to the RCA input on PCB 05: still a loud Hum. When an iPhone is connected to the RCA input, the hum is gone, Auto stand-by goes to OFF, but it also goes back ON again when I touch the 5-pin cable P4 on PCB 05. So I conclude that the problem is not in PCB 05.   Observation: I noticed that when I touched the 3-pin JST cable from PCB 06 to the Amp, the Auto stand-by also switches to on. Same as when touching the 5-pin cable on P4 of PCB 05.   (Check 2) 3-pin JST cable changed - I hand-made another 3-pin JST cable to check if the original cable is the issue. This is to check if there is something wrong with the original cable between PCB 06 and the Amp. (2) Result - When powering on, a slight hum/noise can be heard. Though it seems to be a higher pitch than before. Unfortunately, this hum/noise seems a bit to strong as the Auto stand-by does not switch OFF.   Observation: In the situation with the hand-made 3-pin JST cable, I noticed that when touching this cable or the 5-pin cable on P4 of PCB 05, nothing happens. Though, the BeoLab does not switch OFF.