Beolab 8000 – Red light, automatic power on failing

[zim2411]

Hi all, I just picked up a pair of 8000’s on eBay, both MK1, 120v for the US. I’m using them with a Bluesound Node 2i, so just using the phono inputs.

One of them arrived with a corroded trace for the relay, and after a quick solder job with a wire to bypass the trace it has been working flawlessly with the LED switching to green and relay firing appropriately to come out of/go into standby.

The other worked at first, but the LED always. stayed red even though it was playing music. But then it randomly stopped working after a few hours. I touched up a lot of solder joints and removed as much foam residue as I could with some acetone to break it down and got it working again, just to have it fail shortly after. I couldn’t find any obviously corroded traces on this one. Interestingly the foam in the upper section of the cabinet was different and in good condition vs the rest of the gooey foam so I think someone at some point partially serviced this.

I’ve followed a few threads so far to try and diagnose it, namely these two:

• https://forum.beoworld.org/forums/topic/beolab-8000-on-line-rca-input-always-red-led/
• https://forum.beoworld.org/forums/topic/beolab-8000-dead-no-red-led-measurements-attached/

but both of these threads end without a resolution it seems.

Some things I’ve tried:

• I tried supplying a 5V DC signal to the Powerlink port on pins 4 and 2 but that did not successfully turn the speaker on. I’m not sure if I did this right though, I don’t have a real B&O product to try it with and was just shoving wires in the Powerlink port
• I have an oscilloscope + multimeter, and have successfully traced that IC1 appears to be working and the signal reaches at least the collector pin of TR17. Probing the collector pin and comparing it against ground (plug 2 pin 2) I can see the waveform changing as I play music/a test signal going from flat to a square wave when playing. Beyond that though I’m having trouble understanding the circuit and what else could be wrong.
• I have tried hot wiring plug 8 pin 1 to plug 2 pin 2 temporarily, and that successfully turns the relay on and LED green, and the speaker plays back fine so this all seems down to the auto start circuitry.

Two questions:

1. What else should I probe/test? I asked ChatGPT and uploaded the schematics to it but it’s asking me to test transistors and caps that seem way outside the auto start circuity so I don’t think it’s really parsed the documents correctly.
2. By hot wiring plug 8 pin 1 to plug 2 pin 2, am I bypassing any thermal protections the speaker has in place? I’m about 80% certain the answer is yes — but if the answer is no I may just go this route and connect the speaker to a 12v-trigger’ed outlet connected to the Node 2i to keep things simple.

Thanks!

 

[zim2411]

I figured I’d post an update if anyone comes across this. I pulled the working amp out and put it side by side with the broken board and was making good progress probing them with the oscilloscope and writing down the differences I could find, but accidentally shorted something and fried the working amplifier.

At this point I gave up trying to get the original amps working and ordered two Dayton Audio KABD-250 amplifier boards and accompanying power supplies. These are 2 channel boards with DSP, so with some measurements and programming I basically recreated the active crossovers. I 3d modeled some brackets to mount the PSU and amp to the original heatsink, and managed to reuse the original RCA jack as well for input. These look basically completely stock from the outside, though I didn’t bother wiring up the LED. The amp does have a slight 60 hz hum but this may be just some noise induced by running the RCA cables next to power, but it’s also not loud enough to overpower music. I paired it with a KEF KC62 sub and a MiniDSP DDRC-24 for Dirac Live and altogether it’s sounding really fantastic now.

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