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BM2000 fuses blow on amplifier board

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Aad Jansse
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Aad Jansse posted on Sat, Feb 9 2019 3:17 PM

Playing some tapes on my newly "belted"  BC1200 hooked up with the BM2000 the latter fell silent all of a sudden; after getting access to his"belly" I found 2 blown fuses, F102 and also F201 which one is depicted here. The other two fuses could have stand the current for the short moment that I had the power on.

 The question is, no surprise, what is the cause of the blowing and where to begin checking; it seems not so easy since I can not test with power on.

 

As in all my previous threads on this forum: help is much appreciated.

Aad


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W.Frankenberg
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Your Problem seems to be an overcurrent problem  IC 201 and IC202.

 

Do you have a short cut on the speaker output?


Do you have adjust the "no signal current" without connected speaker and in cold state of the device?

The "no signal current" should be not more then a few milliampers e.g. 20 - 30 ma.

Look to your service manual.

If the current is much two high, and you can't regulate it , I think of defekt  Transistors : 

IC 201,IC 202 (if one is defekt, you MUST change both, these are paired  transistors), the transistor TR 208 could be brocken too.


I have found two threads in beoworld that will be interest for you:

https://forum.beoworld.org/forums/p/17449/152247.aspx#152247

https://forum.beoworld.org/forums/p/7633/67597.aspx#67597


I hope I could give you some helpfull tips, and sorry, my english is terrible.


Best regards 

wilfried

Aad Jansse
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Hi Wilfried,

Thank you for your fast reaction.

As mentioned before powering on the BM results in more business for the parts shop: more fuses.

Is it possible to check the no-signal current without the fuses in place?

I can also remove the output IC's for testing although darlingtons behave differen from "normal 

" transistors" when put to a multimeter

Aad

W.Frankenberg
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hi Aad, I also do not intend to make the parts shops millionaires Cool

Unfortunately, no current can be measured in an open circuit.

if the fuses trip without load through the speakers, then with 99% the Darlington transistors are defective.

These Darlingtons can be checked with the multimeter only for short circuit! 

that could be so in your problem case.

wilfried

Aad Jansse
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Darlingtons in place or detached from the pcboard?

I have somewhere transistors, acquired as new, from a donor 2000 that was in working condition before it was scrapped.

I will also check the speaker circuit.

II will keep you posted

Aad

Aad Jansse
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Correction

It = I

Aad Jansse
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Disregard previous message ( : It )

W.Frankenberg
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Hello Aad,
You should measure the Darligton in the developed state, otherwise you know the influences of the whole circuit, which falsifies.


If a good amperemeter is available, you can use it to determine the current through the final transistors by operating the ampere meter at the fuse terminals with the fuse removed., Short circuits only, high currents up to 4 amps can flow.


In my education, we had lamps that were switched into the circuit to detect a short circuit. If by chance a 40 volt, 60-80 watt lamp is available, you can also use it as a short circuit tester (lamp very bright = short circuit)

Good luck for testing and repair

wilfried

Aad Jansse
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Hello Wilfried,

I won'f give up, hence the next questions:

1. What do you mean by "developed state"? IC's separated from the pc board?  Testing with an Ohm meter should show 0 when the darlingtons are OK?

2.I can buy an incandescent lamp of 42V/60W; when you apply  W= i x V, you get for i : approx. 1.5A, which is the current that the fuse (1.6A) can handle. Am I right?

3. Onboard measuring the "no- signal trimpots" I see about 120 Ohms each.

Looking forward to your reaction,

Aad

Aad Jansse
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One more question:

testing without speakers connected?

W.Frankenberg
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hi Aad,
to the first question:

Measurements of the transistors (darlingtons) must not have any contact with the board, otherwise the circuit on the board falsifies the measurement results!an ohmmeter should read "0", as if no load is being measured, by no means 0.00 ohms, that would be a short circuit !

to question 2:
correct, the light bulb would replace the fuse and in the normal case minimal to no light, in a short circuit but it would shine with full brightness!
The calculation is correct, P = U * I   (Ohm's law)
so 40 volts voltage and at least 1.6 amps make ~ 60 W
but this tip is only for test of o short-curcuit is in the output Stages ! don't use  a lamp as fuse in normal or trim operation !!!

the set value of the trim potis depends on the characteristics of the used Darlingtons, which can change with each production, therefore the two Darlingtons are also selected (paired) with approximately the same values.
the value 120 ohms only states that the porti is approximately in the middle position.

suspected of a short-circuit in the output stages, I would first test without load, that is without a loudspeaker,
The quiescent current (no signal current) must be set without loudspeaker and with volume = 0!
I read in the service manual, the quiescent current should be max 24 mA

 

wilfried

Aad Jansse
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Thank you very much, it is clear to me; so I ordered 2 lamps and I can hardly wait for them to be  delivered, this takes 2 days.

Aad

Aad Jansse
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I had no internet past week (modem problems) just installed a new one: I can keep on asking  questions now.

I now found 0 V and not the required +32V and -32V on the amplifier board: that must be the bridge rectifier faulty/blown.

Can I test this thing when it stays connected to the  transformer but disconnected from the pcboard (so no load)?

Aad

W.Frankenberg
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Hi Aad,

if the transformer supplies approx. 46 volts AC,
you should be able to measure between the blue and red wire from the rectifier about 60 volts DC,

otherwise Ehen the transformer Vorlage is 46 volts AC and between the red and blue wire measure 0 volts DC

The rectifier is dead and gone (defective) !Crying

wilfried

Aad Jansse
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Hello Wilfried,

One of the worst things that could have happened with this project is not faulty electronics, but simply a transformer that generates  0 AC.

I did check the fuses and as we say here: the the AC meter's display says an inexorable zero!!

Next quest is trying to find a donor BM2000.

 


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