Sign in   |  Join   |  Help
Untitled Page

ARCHIVED FORUM -- March 2012 to February 2022
READ ONLY FORUM

This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022

 

BM8000 - Strange problem

rated by 0 users
This post has 3 Replies | 0 Followers

krais
Top 500 Contributor
The Netherlands
Posts 136
OFFLINE
Silver Member
krais Posted: Sat, Jan 15 2022 11:30 AM

Hey everyone, a while a go my BM8000 that had been working perfectly for about a year all of a sudden started acting up. 

When I turned on the Beomaster, I heard a loud thumping noise when turning the volume wheel (low volume setting, no input audio signal). Much louder than the regular, almost inaudible ticks when changing volume. Also this thumping sound would happen about every 3 volume steps, not on each individual step up/down.

Then, after about 10 seconds of fiddling with the volume wheel, all of a sudden the house circuit breaker engaged and cut power to the electrical group the amplifier was connected to (no bang or magic smoke, just the circuit breaker cutting power).

Initially I was worried that the power amps had blown, but after checking the BM8000 I found that all fuses were still ok. The darlington transistors and quiescent current resistors measure ok as far as I could check and I couldn’t find any apparent damage. I haven’t switched on the Beomaster since, worried that might lead to more serious damage. 

Not sure what to make of this. It doesn't seem likely to me that a high current surge was the cause as all fuses are still intact and the circuit breaker has a 16A limit (no high current devices connected to the electrical group, never had any issues). Any tips on where to begin troubleshooting?  

Spassmaker
Top 500 Contributor
North Germany
Posts 223
OFFLINE
Gold Member

Hi Krais

Was the Beomaster already serviced?

If so often the AC Line capacitator on the relay board is forgotten, could cause a short, had myself one defect while the amp was playing music, a lot of bad smelling smoke and crackling like a firework, but the amp was still playing music.

 

So this cap goes often into heaven and could cause a short that could engage your house breaker circurit.

The fuses of the Beomster come later in the circurit diagram as far as I remember.

 

IF YOU ARE NOT FAMILAR WITH ELECTRONICS  AND HIGH VOLTAGES ASK A 

                         FRIEND OR  SOMEONE WHO KNOWS ABOUT !!!

                                               DANGER 220 VOLT!!!


                                PULL OUT THE MAINS AC PLUG FIRST!!!!

 

I write this because I don´t want to be responsable when you or someone else gets hurt or your house is burning down.

 

 

Turn the amp with it´s back to you and losen the 4 screws of the lid in the middle to get access to the relay board.

Have a closer look to the yellowish square capacitaor... is it broken or exploded?

Let us know what you find.

This must not be the cause of this behaviour but that´s the way I would go first, maybe others would go in another direction ....

 

Kind regards

Christian

krais
Top 500 Contributor
The Netherlands
Posts 136
OFFLINE
Silver Member
krais replied on Sun, Jan 16 2022 11:58 AM

Hi Christian

Thanks for the warning, yes I'm aware of the dangers of high voltage.

I already had replaced the AC line capacitor with a new X2 safety cap when repairing the BM8000 a while back. I double checked and it measures fine, no short (and btw the safety cap is located behind fuse F2 which hasn't blown).

While investigating PC7 I also measured resistance on the primary windings of the two transformers: 1.25 ohm for the large transformer and 56 ohm for the small one. I believe that is consistent with expected value ranges. 

Also, the fuse on the power supply board PC6 is fine as well.

 

 

 

 

 

Spassmaker
Top 500 Contributor
North Germany
Posts 223
OFFLINE
Gold Member

Hi Krais

You are right, the fuses are before the cap, and as the fuses are not blown... it´s very strange that the house circurit breaker has engaged.

Would be nice to have a variable isolating transformator with an amperemeter, so you could see what´s goind on while you crank up the voltage.

You could ceck the rectifieres and the large caps (10000uF) of the main rail voltage + - 55 Volt .

You could pull off the main +- 55 V voltage from both poweramps ( isolate the plugs ! )

I won´t give you more hints to sort out the trouble, because I don´t know how far your skills in ekectronic is and I don´t want that you burn your fingers or even more.

We are talking about an 220 V house circurit breaker, that´s the time for me to stop, hope you understand me right.

So maybe there is someone else who gives you some tips.

Regards

Christian

 

 

Page 1 of 1 (4 items) | RSS