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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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sonavor
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I corrected my sentence in the post above. I typed and posted it too fast and didn't realize it didn't say what I wanted it to say :-)

What I was trying to say is that anyone changing those two reservoir capacitors must be careful to get them connected back up correctly. The polarity of the capacitors is very important and someone not familiar with those original capacitors may not notice which terminals are negative and which are positive. 

-sonavor

Aad Jansse
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Taking the 2 caps out and soldering them back I watched carefully the polarity: the familiar hum or bangconfirmed that the polarity was as usual as before, despite the state of the bridge rectifier that I removed from the pcb, tested, found it 100% and soldered back.

i also found 2 lamps burnt on the source selection indicator, replaced them and continued searching.

Before I power on the scope I can also tell that the problem differs depending on which speaker outlet is selected: speakers 1 with a bang, speakers 2 without, however both same reaction of the relay: continuous  clicking and nowhere a 31V to be found; according to the manual I should have 15V on TP15, but my patient shows some 115mV, maybe because there is no source engaged.

Aad

Aad Jansse
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Pardon me my poor english: the right description of the unwanted sound when the relay clicks is not a hum ( +/- a constant level); therefore I used the word “bang”, sort of popping sound.

Aad

manfy
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manfy replied on Fri, Apr 10 2020 1:48 PM

TP15 will only show 15V once you're able to power it on - but now the fault switch is putting the unit immediately back into standby. So, close to 0V is fine on TP15.

Where does that popping sound come from? From the speakers? If so, I hope you're not using any valuable speakers you plan on keeping. It wouldn't be wise to have any valuable speakers connected while we don't know what's wrong. If for some reason the fault switch fails you could easily fry any speakers connected to the system.

Did you replace the 2 caps on the 31V supply with new ones or did the originals measure OK? If so, what capacitance and ESR did those caps show?

Aad Jansse
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"The Never ending story":

these are my speakers.

The caps, the original ones, measures are

07C       5052 uF     ESR    .15 Ω

0C8       4891 uF         "      .12 Ω

C92       2294 uF         "      .11 Ω

 


Aad Jansse
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"The Never ending story":

these are my speakers.

The caps, the original ones, measures are

07C       5052 uF     ESR    .15 Ω

0C8       4891 uF         "      .12 Ω

C92       2294 uF         "      .11 Ω

 

Aad Jansse
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I can use a headphone instead of speakers

manfy
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manfy replied on Sat, Apr 11 2020 6:15 AM

Well, I myself would not connect anything to the speaker jacks at this point.
One of the Darlington transistors could be fried, in which case there could be 31VDC on the speaker output. And 31V/4ohm =  almost 8Ampere, and that would certainly damage the speakers permanently!
So, I would first measure and make sure that the output amp is ok before connecting high-value speakers.

A popping sound in the speakers is a sign of some signal on the output line and that could very well be a DC voltage. That would also explain why the fault switch is switching back to standby immediately.
What I would do now is disconnecting the speaker, connect the multimeter between right speaker output and ground, set the meter to MAX HOLD mode and power on. The meter should show the max. DC value at startup. Then I'd do the same for the left channel. If any of them shows a significant DC value (e.g. 0.6V or bigger), I'd know that the fault switch is doing its job and that I have to investigate components in that output line to see where this DC voltage comes from. 

Aad Jansse:

The caps, the original ones, measures are

07C       5052 uF     ESR    .15 Ω

0C8       4891 uF         "      .12 Ω

C92       2294 uF         "      .11 Ω

The values and ESR are still quite good considering their age. But they're clearly aging.

Aad Jansse
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It  has been quiet here on the BM 1900 front: I had left my home for quite some time to avoid the stress of being “lockeddown” at home, My wife and I moved temporarely to a cottage (B&B) somewhere in my country wihere there is a less dense population, minor risk to pickup a covib-19 contamination, nor conditions to take my workbench with me.

I’ll be back later

Aad

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