The Workbench

Re: That Got Hot

Eugene — Fri, 16 Jul 2010 05:52:15 GMT

A little Elmers will glue all things back togethrr

and i would love to say it all ended up splendid but it did not, I dialed back the trimmer to 2 turns clockwise to drop the power and plugged the unit in and "poof" the very resistors you speak of went up in smoke.

i havent gottten around to replacing them yet. What i bought were .22 @ 1watt flameproof carbon film dont know if that is correct or not dont see how the added wattage would affect anything but i dont see in the spec if i should be usuing non inductive or some other style instead. I also used standards as far as the 4 resistors on pcb3.

I purchased NTE's only because that is what my local supplier sells. Not brand loyalty. Same with the trimmers. I actually would have preferrd a surface mount cermet (horizontal0.

Bad news - the fix failed. Good news - Power was restored do I know the mains transformer and some other parts are OK.

So i have read some of your threads regarding the BM 5000 and am interested in making the sound better.

As it stands I have a second 5000 but this one has problems with the left channel. When I took possesion of the unit it would power up with sound for about 10 seconds. So i gave both trimmers a 1/8 turn counter and the set the idle current down to 5.05 V. Played beautifully for about 1 day then "poof " the .22 resistors went up in smoke along with a darlington and a few of the smaller resistors in the area as well -and r38 -r41 got hot as well.

So for this particular one a "modified" version is certainly of interest to me.

BTW I repaired a turntable BM3000 (jammed solenoid the had to be broke out and glued back together the repair method coming from a post by you. Still works

OR perhaps i could cut the two boards in half and glue the good parts back together !!! wouldnt that be something

Re: That Got Hot

yachadm — Wed, 14 Jul 2010 13:27:58 GMT

Hey Eugene,

You did a great job just gluing that output BDV back together.

Looks very much like the BM5000 I had in here a few months ago(without the burn marks).

If you're feeling adventurous, (on both channels, of course) - substitute 2SC5171's for the BF857's (TR208), and 2SC1845's (hFe matched) for the drivers (TR201 TR202) - lovely improvement in sound. The Philips BC5xx's are very bland in the output stages, and the abovementioned substitutes warm up the sound beautifully.

Apart from the trimmer (I like the Bourns 3352 and the Piher PTC10 Cermet replacements), those drivers are probably a central cause of the 0mV problem. On the 0mV line (TR206 Emitter), a good number is 10-20mV, but definitely nothing over 50mV for sure.

R224 and R225 may check OK, but when they get hot, they change their resistance characteristics, so I would change those, if the problem persists. Carbon resistors are better in this part of the circuit than Metal film units, because Carbon handles heat better. That's why B&O split this position into 2 resistors in parallel - the circuit is very sensitive here, and heat prevention is critical.

Of course, check D201-206.

Regards

Menahem

Re: That Got Hot

Eugene — Mon, 05 Jul 2010 21:39:24 GMT

another view ....

and the fix .....

i also replaced R38 thru R41 on pcb 3

Re: That Got Hot

Eugene — Mon, 05 Jul 2010 17:34:54 GMT

I suppose ths might be part of the problem as well. I cant see it working very well with a split darlington.

Re: That Got Hot

tournedos — Wed, 30 Jun 2010 06:29:36 GMT

A selection of baked mystery PCBs, perhaps

Might have solved this one myself too - the component designations fit the series, and the BM5000 was the last one to have hand taped PCBs instead of CAD layouts

Re: That Got Hot

Dillen — Wed, 30 Jun 2010 05:17:29 GMT

You are funny guys
Can I win something ?

Martin

Re: That Got Hot

Craig — Tue, 29 Jun 2010 11:51:52 GMT

Peter :

This looks like a wonderful new game! Pictures of bits of circuit board and see if Martin knows what it is! He is scoring 100% so far!

Great idea, we need to find some really obscure ones. Although I reckon martin will still know what they are.

Craig

Re: That Got Hot

burantek — Tue, 29 Jun 2010 09:58:33 GMT

Peter :

This looks like a wonderful new game! Pictures of bits of circuit board and see if Martin knows what it is! He is scoring 100% so far!

I was trying to put the same thought into words... When I saw Martin state that it was a BM5000 , I out loud!

Re: That Got Hot

Peter — Tue, 29 Jun 2010 09:42:21 GMT

This looks like a wonderful new game! Pictures of bits of circuit board and see if Martin knows what it is! He is scoring 100% so far!

Re: That Got Hot

Eugene — Tue, 29 Jun 2010 05:38:14 GMT

Yes it is a BM 5000.

Dont plan on using more than two set of speakers and I generally dont run the volumes much higher than 4:2 typically I run at 3:4 -3:8 during extended plays. so while self oscillation may have helped cook this Beomasters goose. I dont see a repeat in the future.

First I want to restore sound in both channels then I will look into a full recap.

I did an AC volt test off both sides of the transformer and got the same values ( I already forgot them) .

Re: That Got Hot

Dillen — Tue, 29 Jun 2010 00:28:29 GMT

Looks like a Beomaster 5000

Those PREH trimmers are generally bad.
They are used heavily in Beovision LX models where they are also known to cause troubles.
If you have no idle current in one channel, you will have to check from the output stage and back
to the power supply, including the idle current circuit.

The burned R41 on the preamp board (and maybe R38 too - check it) gives an indication that
a heavy DC was present on one of the speaker outputs, which again suggests that the output stage needs checking.
That'll be the output darlingtons, the BF... and the small signal transistors before them.
Check also the emitter resistors and the other components electrically nearby.

The idle current trimmer could cause this - so could self oscillation as we've discussed in a
previous thread here at Beoworld so check all decoupling caps as well, also in the power supply.

Martin

That Got Hot

Eugene — Mon, 28 Jun 2010 22:20:54 GMT

Just got this ome in. So would a failing trimpot at R226 have led to this? I have sound out of the left channel but none out of the right including headphones.

A no load test at TP200-201 showed 0mV DC while I was able to read around 13 mV DC at TP100-101. I suppose besides replacing the trimmer and the obvious resistors i should consider IC200-201 and TR208-209. is the muting switch on PCB3 toast as well ?