Hey Beoworld!

First, I'll introduce myself: I'm Chieljan, 22 years old and I live in the Netherlands. I just entered the wonderful world of vintage Bang & Olufsen audio by buying a beomaster 1200+ matching beovox 1200 speakers. I'm totally in love with it, it is a wonderful machine!

 Problem is, it went broken  a week ago. Volume on the right channel is very low and the tone is very thin (no bass)

I've traced the problem down to be somewhere on the preamplifier PCB, but I can't find the exact component that is failing. The volume pot seems to be OK, bass and treble pots seem to work OK too. For the rest I just don't know where to start looking!

 Anyone who has a suggestion on what might be wrong?

Hope te find some help here!

Hi Chieljan

Most likely cause is a bad capacitor.  Beoworld member dillen can supply a kit with comprehensive instructions.  Whilst you have the BM1200 open you may also consider replacing the bulbs.  Dillen also has the kit for this and it includes an upgrade of the bridge rectifier in the PSU and a modification to the small board that holds the bulbs.

I have just applied the kits to a BM3400 which is the same kit (i think...)

Welcome to BeoWorld

Richard

Hey Richard!

Thanks for your reply! By capacitor, do you mean the electrolytic ones or the capacitors in general? :)

 Chieljan

Chieljan

Yes, the electrolytic ones.  The electrolyte inside the capacitors degrades/evaporates or just leaks out over time.  I have renovated 19 out of my collection of 25 old beomasters/beocenters and 86% of them were fine after a capacitor change and a good clean.

Richard

The Beomaster 1200 is quite different from the Beomaster 3400 and doesn't have the same problems but as Richard states, a capaitor kit is available and recomended. Lamps are available too.

Try swapping the leads to the two large capacitors at the speaker output, near the headphone socket, they often go bad. See if the problem switches to the other channel. You can also switch the inputs to the amplifier stages - that'll be two of the four little plugs (the other two are power so make sure to swap the correct ones).

Also the sliding potentiometers could need stripping down and cleaning.

Welcome to Beoworld !

Martin

Ooops my mistake, I was thinking of the BM2000.

I have so many of these old Beomasters including 2 BM1001s but not a BM1200.

Richard 

Eureka!

I appear to have found the culprit: It's a transistor.

Swapped the yellow and green wire, this made the problem change sides (from left to right speaker)

So, the problem had to be with something on the board with the potentiometers (this board contains the preamplifier, I guess?)

Changed the elco's on this board, did nothing...

Swapped the transistors, from the left side to the right, one by one, and this revealed a BC174 transistor to be the problem!

Now, all I have to do is find a replacement for a 40 years old transistor... :S

I thank you very much for all your replies/help and I'll keep you posted, it's off to the electronics store for me first :)

Victory!

I changed the transistor (which was a BC114, not 174, by the way) for a new one, and my sound is back!

I'm so happy I finally found the problem ^_^ I'm going to sit and listen to my beomaster for the rest of the day.

Thanks again for your replies!

Chieljan

Good job !

Are you sure it's a BC174 ?
I don't remember seeing of them before. Anyway, if it really is a BC174, you can substitute directly with a BC546, which is a more common type.

Martin

Hey Martin,

Made a mistake typing, it was a BC117 indeed. The guy in the electronics shop sold me a BC 549, it works just fine. Probably just like the 546 you mention.

Strange things, these old beomasters... when you have one working it makes you want to go out and buy more and more and more of them...

Chieljan:

Strange things, these old beomasters... when you have one working it makes you want to go out and buy more and more and more of them...

Yes, I have always wondered how none of the other manufacturers managed to do that

(3x 900, a 1000, 2x 2000, a 4400 - and I pretend to have a use for each of them!)

Good job, one more classic B&O saved ! 

The BC549 is the "deluxe" version of the more basic BC546, lower noise etc.

Watch out for that bug, I started with a Beomaster 1900 ... 

Martin