Capacitor that determines switch-off-time in BEOLAB 8000 (Phono-input-operation)

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Martin Posted: Wed, Dec 9 2020 6:33 AM

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Dear fellow enthusiasts,

one of my restored BEOLAB 8000 has a slight issue. It switches a bit too fast (5 seconds) from green to red after switching off music. After studying of the circuit diagram I was wondering if C6, C83, or C87 are responsible for that functionality.

What is your experience?

Greetings,

MARtin

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tournedos replied on Wed, Dec 9 2020 12:09 PM

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My experience on this issue is strictly from the comfy chair, but I understand the circuitry like this:

- C87 is in the tweeter protection circuitry, not related to this

- C6 is the time constant for power-on muting (to prevent any "pops"), not related to this; it is controlled by Powerlink on/off

- IC2 is a dual-stage low pass filter and D82 rectifies its output into a voltage level; this is obviously the signal we need to look at

I believe C83 is the correct suspect - but it is not an electrolytic, so I don't think it is likely to change like this over time. I would concentrate on possible problems with disintegrating damping foam attacking solder joints and circuit traces (very common issue with BL8000). You say they are restored, but if a spot was missed when cleaning the PCBs this might still be going on. Some of the resistors, like R141 (10 M) are very high value so even tiny leakage currents might affect the operation.

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Martin replied on Thu, Dec 10 2020 2:06 PM

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Good thought on that 10M resistor!

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Martin replied on Mon, Dec 14 2020 9:29 AM

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Remark:

By looking at the R141 (10M) which was o.k. I nearby found a small splash of soldering tin under the stereo microscope.... The problem was again sitting in front of the pcb...

Removing the splash of soldering tin and cleaning the pcb wiht CH3(CHOH)CH3 fixed the problem.

Thanks again for the inspiration.