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Another BM1900

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Aad Jansse
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Aad Jansse posted on Mon, Mar 30 2020 4:14 PM

After having brought back to life of my BM1900-2 and my BM2400 with the very much appreciated help of Beoworld members, I dare to start another thread featuring another BM1900.

The problem: when powered up the standby ( red LED ) is activated, but there I stop:

Selecting one of the sources, FM’s, TAPE or PHONO, the 1900 produces a loud hum iinto the speakers. I do not dare to enjoy this longer than a split second, so my question is:

where to start the analysis of the problem.

Aad

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Aad Jansse
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I appreciate your concern as to recapping, but a bit stubborn that I am, i would like to point out that  very recently I brought back to a pleasant listening  a BM2400 and a BM1900-2, both without recapping, see my posts on other threads .

Nevertheless I will as a stated before keep your and Dillon's advice in mind

Aad.

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Dillen
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Dillen replied on Mon, Mar 30 2020 6:36 PM

A scope to see if the hum comes from the amplifiers supply or inputs.

Martin

Aad Jansse
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Before I am using a scope I want to let you know that  when the sensitive controls are touched the relay is clicking. Does this eliminate part of the search area?

Aad 

Aad Jansse
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So far I have done nothing yet, because I would like to have some more detailed advice: the hum scares me off, I am not experienced enough . I am very sorry to admit that. 

I want to add pointing out that the problem still occurs when PCB 4 is disconnected from PCB 2 does this narrow the search to what you mention as ''amplifier"s supply. 

Aad

manfy
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manfy replied on Fri, Apr 3 2020 1:20 PM

You say that this is another BM1900. So, if you want meaningful help, you have to give us the status of this specific new system.
What have you done to it so far? Is it recapped? Light bulbs checked/changed? Any mods from its original state (such as incandescent to LED mod that you did on the other system, etc etc)?

Aad Jansse
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This BM1900 was standing a couple of months  on a shelf before I acquired it, no visible traces of bad caps.

The only thing I did is what you see on the picture, including original double checked light bulbs. I did not (yet) change any capacitors. If it turns out that that is necessary I will first sleep a night on it.

As I mentioned earlier all the lamps light up, relay clicks and hum appears when I touch any of the senitive pins,even with these pcb's detached from the main body, but only as long as I keep my finger there, no selected source is heard. This situation also occurst when all pcb's are in place and connected..

I get the impression that the culprit is in the output stage, do I have a chance  to find it when I only use my multimeter?.It seems a tough job to use a scope if there is no source to be selected/heard.  (In addition I do not own a scope)

Aad


Aad Jansse
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In the first sentence I should have written: "somebody's shelf".

Aad Jansse
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In the first sentence I should have written: "somebody's shelf".

manfy
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manfy replied on Fri, Apr 3 2020 3:48 PM

The BM1900 was made from 1976 to 1980 (says the internet). So that's good 40 years .... and the best e-caps you can buy today are specified with a lifetime of 10,000hrs. Well, that's formal specs and it doesn't mean that e-caps are guaranteed to die after 10 to 20 years or the specified amount of hours ...and yet, if this were my BM1900 I'd recap it before trying to get into any troubleshooting.

But that's just me. I don't have much hands-on experience on B&O, but all I read on this and other BeoForums goes in the direction of recapping - and that does make logical sense.

Aad Jansse
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I appreciate your concern as to recapping, but a bit stubborn that I am, i would like to point out that  very recently I brought back to a pleasant listening  a BM2400 and a BM1900-2, both without recapping, see my posts on other threads .

Nevertheless I will as a stated before keep your and Dillon's advice in mind

Aad.

Aad Jansse
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Verified by Aad Jansse

I appreciate your concern as to recapping, but a bit stubborn that I am, i would like to point out that  very recently I brought back to a pleasant listening  a BM2400 and a BM1900-2, both without recapping, see my posts on other threads .

Nevertheless I will as a stated before keep your and Dillon's advice in mind

Aad.

manfy
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manfy replied on Sat, Apr 4 2020 7:33 AM

Aad Jansse:

...I brought back to a pleasant listening  a BM2400 and a BM1900-2, both without recapping, see my posts on other threads .

Of course, if some units are still fine then there's no need to break them open and start messing around. You'd normally do that when the unit starts failing and once they're opened up and dismantled, you normally fix all the problems that will happen sooner or later in one sitting -- and the e-caps on old B&O's are one such known problem. But well, it's your time and your money...

Your photo above is not very clear but I believe to see some of those red vertical e-caps that Martin always recommends to replace whenever he sees them in a thread.

If you want to find and fix the source of your hum, your best course of action is to break out the oscilloscope and start tracing the signal paths. You don't absolutely need a signal generator for that. The hum is a signal and therefore it can be measured and tracked with a scope.
You said you can borrow that old Russian scope from a friend. Do that! It's a simple single channel analogue scope -- it doesn't get simpler than that! If you know how to handle a multimeter, you can get proficient in its use in one or two hours of playing around with it.

Happy troubleshooting!

Aad Jansse
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The photo was meant to show what elements were detached from the main body while the hum was still there, which indicates that these parts are not the cause of the hum.

Meanwhile I am enjoying the sunshine, which is one of the few positive things during the present hard times.

Tomorow I will be struggling with the scope and multimeter

Thanks for your comments.

Aad

Aad Jansse
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Found this in the manual, could this be a trail to a solution?

I did not obtain the 30 V(+ and -) when touching this FM5 or an other pin:  the relay sounds like a pinball machine, not allowing the 30 V being steady present .

Aad


Aad Jansse
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I do have the 15V supply.

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