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My BeoMaster 700K makes quite some noise when the power is being switched off.
First I thought it was a kind of 'power draining' through the big capacitor that's in the circuit near the power supply and rectifier. This one is still the original one, and is a 50v, 1000uF capacitor. So I replaced that one with a new one, also 50v and 1000uF. But the noise is still there when shutting off the BeoMaster 700K.
I have attached an audio file, this is a recording of the noise I am talking about.
Also I made a visual check of all capacitors on the main PCB and the smaller PCB on top. Visually I did not see capacitors that show signs of leakage or different shape.
QuestionWho recognises this noise and knows what the source is?Any advise or suggestion is welcome.
Sounds like ambient noise. Perhaps from a nearby energy saving bulb?
Martin
Thanks Martin.
I didn't think about that, but it is a good suggestion.
So I took the BeoMaster 700K around my house, powered it on and off on different wall-plugs, with lights on and also with all lights turned off.The result is no difference in any of these tests, the noise when switching off remains the same.
What else could this be? Capacitors on the amplifier?If so, is there a way to test the capacitors while soldered on the PCB? (I have a regular multi-meter)
I had another look at capacitors in the BeoMaster 700K. And I found a large grey Wicon 800uF, 25v type KPI capacitor (see picture). This one is mounted with a curling metal holder to the backside of the BeoMaster 700K. It has got two leads on the bottom which both are connected with wires to the electronics. On the top there is a small metal pin, this one is not connected.
I looked the Wicon up in the schematics (this is a BeoMaster 700K type 2240), and I found it is directly connected to the speaker output. So maybe this is a suspect causing the noise when turning the BeoMaster 700K off, and candidate for replacement.
Questions
You can replace it with a good quality general purpose electrolytic capacitor.Don't use a low-ESR type. A little series resistance here will safeguard your speaker and not influence sound reproduction at all.
Increasing the capacity to 1000uF won't hurt, but to keep a healthy expected lifespan try to stay close to the 25V voltage rating.If you cannot find a 25V component, you could choose a 33V.
Having said that, I don't think it has anything to do with the noise you hear at power down.What you hear is the result of the amplifier (- and speaker output!) still acting on and reflecting what is going on upstream.I would look at the power supply filter caps. The large one at the rectifier and the one (80uF if memory serves) on the amplifier board. - Though neither is guaranteed to be the culprit.Next - all other electrolytic capacitors. - And you still may not find the reason.A scope could perhaps help - or again, perhaps not, even if you can "see" the noise.The noise you hear may come across as strange to younger ears but is not a sign of anything seriously bad or wrong in my opinion.It's a series of finely calculated and tuned electronic circuits no longer supplied their calculated supply voltage so no longer able to do their finely calculated and expected jobs - in other words, they are failing to operate properly due to lack of power - and they do so in an unpredictable order.
Having the amplifier section (sort-of) running as the "last man standing and reporting" will allow you to listen in on how all the upstream circuits, filters, oscillators and whatnots stop operating. It could easily end up being a "message somewhat unrecognizable".
MartinOld enough to have experienced valve sets in daily use everywhere.
Thanks very much again Martin!
I hear what you say. It is age which comes with ... but as long as it functions.
Concerning the capacitor mounted on the inner back side: I think I will just replace it and try the result. Renewing it also can't be bad.
The large capacitor at the PSU and rectifier (1000uF/50v): I have already exchanged this one to a newer one. There was no difference to the noise after replacing it. The newer one has a totally different size (smaller), but I was able to design and 3D print a bracket to keep it in steady position underneath the curled iron clamp (see picture).
QuestionYou mention a 2nd capacitor on the amplifier board, maybe 80uF. I couldn't find one with that value. I do see four other capacitors in the scheme that are close to the PSU and rectifier: 125uF/16v (2x), 50uF/40v and 4uF/40v. Do you mean one of these?
(I will add a scan of the scheme that came with this BeoMaster 700K in the next post)
Added to the above post: BeoMaster 700K type 2240 schematics PDF.
125uF could be the one.