BeoLab 8000: On Line/RCA input, always red LED ?

25 replies
Keith Saunders
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Keith Saunders
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Posted November 30, 2024 at 16:22 GMT
#1667418
Thank you for your latest information.

Remembering that we are only measuring DC voltages meaning it can only show up general irregularities, not analogue signal..

The voltage at the base of TR14 is in line with what I would expect. This supports the likelihood that the fault is prior to R136 with further support from the fact that you accidently used you meter in diode/continuity mode which triggered TR16 and switched the unit on.

I am assuming that you previously check the +15 and -15 volts on pins 4 & 8 of IC1? That means between a ground point and pin4 and the same with pin 8

Whilst the problem could still be one of the components around IC1, you should change IC1 as the next step..

Location: Hampshire, England

KolfMAKER
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KolfMAKER
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Posted November 30, 2024 at 16:36 GMT
#1667419
Thnx @Keith !

I just checked legs 4 & 8 of IC1 = +/- 28.8 volts

That's quite different than the +/- 15 volts it should be ...

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Keith Saunders
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Keith Saunders
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Posted December 1, 2024 at 09:22 GMT
#1667423
That's because you have put your multi meter between pins 4 & 8 when you should have measured between ground point and pin 4 and again between ground point and pin 8.

Note my statement in the previous post which said "That means between a ground point and pin4 and the same with pin 8"

Location: Hampshire, England

KolfMAKER
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KolfMAKER
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Posted December 1, 2024 at 10:33 GMT
Edited December 1, 2024 at 10:34 GMT
#1667425
Thanks for your patience with me Keith. I am still learning, but I get this one.

Here's the new measurements with: the negative lead of the multimeter on Pin 2 of P2 for ground, and the positive lead to Leg 4 and Leg 8 of IC1.

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Keith Saunders
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Keith Saunders
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Posted December 1, 2024 at 12:47 GMT
#1667429
Whilst the Plus and minus power connections are a little low, it should still work..

You should change IC1 assuming you have the desoldering equipment available to do the job

Location: Hampshire, England

KolfMAKER
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KolfMAKER
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Posted December 1, 2024 at 16:43 GMT
#1667430
Thnx Keith.

Swapping IC1 is on my list for tomorrow.

To be continued.

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KolfMAKER
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KolfMAKER
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Posted December 2, 2024 at 10:55 GMT
Edited December 2, 2024 at 11:02 GMT
#1667440
Update after replacing IC1. Before replacing IC1 with a new one, I checked R136 which I had removed a few steps back. It still measured 10K ohm, so I put it back to position. Then I removed IC1 and replaced it with a new one. And of course I measured the voltage on leg 4 and 8: -14.3v and +14.2v. So exactly the same as with the old IC1. Just to be sure I also checked putting an audio source to the RCA/Line In; as expected, no switching on.   Question
  • What would be the next step to check?
  • If damaged copper traces (by foam rot) could still be a cause, in which part of the PCB do you expect those damages should be?

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Keith Saunders
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Keith Saunders
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Posted December 3, 2024 at 11:10 GMT
#1667456
I was afraid that would be your answer because operational amplifiers are normally very reliable.

With regards to damaged copper track caused by foam rot, yes that certainly could be the cause. Because the damage often cannot be seen, only a continuity test between each of the components in the circuit around IC1 and all the components around it will answer the question for sure. Also one of the capacitors could also be at fault or at least well out of tolerance.

Without an oscilloscope your work is now testing components typically by replacing.

Location: Hampshire, England

KolfMAKER
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KolfMAKER
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Posted December 3, 2024 at 11:36 GMT
Edited December 3, 2024 at 11:36 GMT
#1667457
Thanks again Keith! I hoped for a positive result with IC1, but I took into account that this would not be the trouble component. Nevertheless, we have come so far (at least in excluding trouble causes), so I will not give up.

Next step
Like you suggest, I will continue with continuity testing. To make sure I am looking at the  right components to check ...

Question
Could you pleasde indicate to me, which components I should check, with the reference of the scheme (below) of the suspect area you posted before in this thread? (Take it as the way it takes the least time for you; either the components to check, or the ones I shouldn't)

 



 

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GL
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Glitch
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Posted December 3, 2024 at 15:05 GMT
Edited December 4, 2024 at 13:28 GMT
#1667459
KolfMAKER:  Have you tried the "bad" PCB02 in the "good" speaker? Another thing that might provide clues is to do more mapping of the voltages at key points between the "good" and "bad" speakers. Do you have the equipment to "inject" a signal into the circuit. For example, remove C82 or R136, apply a voltage, and see the reaction of the downstream circuits.

Otherwise, I don't have anything to add to the good advice that you have been getting from Keith.

Disclosure: I haven't repaired any BL8000's myself, so please give the opinions of those that have more weight.

Glitch

 
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