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Beomaster 4000 model 2406 resurrection

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AntonvZ
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AntonvZ posted on Wed, Feb 24 2021 11:57 AM

I was picking up some Beovox P45's from a 2nd hand dealer when I saw the BM4000 lying in a corner. "How much for that" was answered with "It won't even turn on, take it if you want it".

Of course it went home with me and it is still a work in progress.

  • Both fuses were blown (that could explain why it wouldn't turn on)
  • The main rectifier was dead and the safety cap as well - replaced
  • After that fix got sound on right channel, power light works, signal strength meter works, tuning lights work, stereo light blown
  • Radio works but skips like crazy - will sort that once the left channel is working
  • PCB 8002135 (small one with voltage divider resistors) had a major short, parts of the circuit were burnt off - fixed that
  • Was not getting correct output voltages on PCB 8002135, replaced all divider resistors and now sorted
  • Signs of capacitor leaks on left channel main output PCB - replaced all orange caps on all boards
  • Replaced the main caps
  • Left channel had been worked on previously on main output PCB, ceramic resistors had wrong values, replaced left and right channels
  • Replaced all the trim pots on the main output board
  • Getting 12mv on right channel

 

And this is where I need help.  I have the BM4000 service manual, can solder and have the most basic electronics knowledge. Have multimeter but no fancy oscilloscopes and such.

I am getting 65-70 volt readings all over the left channel towards the output side and cannot figure out where it is all coming from. Getting 70v from the Darlingtons and the small transistor tucked behind the Darlington PCB. 

There are two resistors (100ohm #583 and 330ohm #570) that instantly turn to toast every time I replace them.

Would greatly appreciate any help with looking for likely culprits that are causing the 70v flooding of the board. I know the red/white wire supplies power to the board at 70v but whatever is supposed to drop it down to the 35v stage is not working.

Thanks to everyone that ever posted on the Beomaster 4000, your advice and solutions have helped me get this far.

Best regards

Anton

 

Model:

Main PCB, can see the two transistors that keep burning - (100ohm #583 and 330ohm #570)

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fkatze
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fkatze replied on Thu, Feb 25 2021 9:52 AM

Is there a short in your output transistors?

AntonvZ
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fkatze:

Is there a short in your output transistors?

 

Thanks, used that as a starting point.

First desoldered all the wires connecting board 8002095 to the Darlington board.

Desoldered from 8002095 TR52, TR53, TR55, TR56. After testing them on the NPN/PNP sockets on my DMM realized they were all toast.

Replaced all four of them with closest equivalents I could find and that sorted out most of the voltage readings.

I was still getting mV readings on some of the solder posts in attached picture so after tracing the circuit realized it could only come from the new trimpot I had installed previously (#569)

Took it out and tested and the 250 ohm trimpot was reading 1-48 Kohm.

Replaced that with a new one and now getting near equivalent voltages between the left and right channels as measured on the soldering posts of board 8002095.

I dont want to reconnect to the Darlington board just yet as I suspect this will undo most of what I have managed to fix.

Will post some measurements in the next post.

Regards

Anton

AntonvZ
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How to measure Darlington Transistors with a DMM?

Well from what I've read you can't really do that. The Darlingtons MJ3001 and MJ2501 both contain resistors and can apparently only be tested in a circuit to see if they turn on and off as they act like intergrated circuits.

Way beyond my current abilities.

So I did this the hard way and measured everything I could comparing left (suspect) to right (working) channels.

The Darlington board is currently disconnected from the main board, measurements taken at the transistors, power off.

Have included the table below in the hope that someone can confirm the following for me.

 

  • Both MJ3001 and MJ2501 are dead
  • Probably also the reason why TR54 exploded after I replaced that previously (spectacular - plastic flying everywhere)
  • Do not reconnect the left channel of the main board to the Darlington board until this is resolved as it will probably undo everything I have fixed thus far

 

Are there any suggested equivalents that I can use - I know I am not going to get originals in South Africa and with all the Covid blah blah anything I order internationally will only get here in 5 years when they have cleared the backlogs.

Best regards

Anton

 

 

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