I tried my Beomaster 900K today which I bought as 'untested', and it doesn't work.
When switched on, it powers up and emits a low hum with any button pressed, if you turn up the volume, it develops a slow vibratto.
With no aerial attatched, I can just about hear the BBC on long wave.
Bearing in mind that I am not a techie, I opened it to see if anything was visibly wrong.
After switching on, the AD139 above the power lead gets too hot to hold your finger on, after about a minute. Half a minute later, the bigger transistor below it(SP1446) also gets hot.
A little later, the back panel gets hot near where it says Beomaster 900. Looking behind this spot, there is a flat component like a credit card but thicker. I think it is a rectifier and it says Seimens on it.
I hate to say it, but do you think it's days are numbered?

Andy T.

Jandyt:

I hate to say it, but do you think it's days are numbered?

Of course not! It was built by humans and can be fixed. In any case, to prevent further damage don't turn it on anymore until you (or someone) has had a better look into it.

The output amplifiers have a couple of mass resistors which have a tendency to fail as they get hot and old, and after that the output transistors (AD139) are in danger. These can also fail for simply old age, it could be that the hot one has a short, the amp overloads the power supply and then you have hum.

The one I bought had the other channel nicely "disabled" by someone - balance setting at the other end, the leads from one AD139 to the circuit board hanging in mid-air. I replace that transistor (shorted), the aforementioned resistor (0.38 ohms, I think) which had also shorted, and the receiver was all right again 

-mika

Thanks for the reply Mika.
If a transistor is hot, has it definately failed? Also, would it be easy for me to find out where the suspect resistors are? I have the service manual and the schematics.
I have just polished it up and it looks gorgeous, but I would like it to be more than just an ornament.

Andy T.

No, the hot transistor might still be OK; you'll have to test it. But chances are good it has a short circuit between C and E (desolder the leads at the PCB end and then it's easy to measure). Finding replacements can be a little tricky, but they are still available. I got mine in the old TV repair shop next door

You can see the suspect resistors marked as "0,4" in the schematic, there's one for each of the AD139s. If they are OK, as well as the transistors, then you'll need to dive deeper in there. Look also for bad solder joints and damaged areas in the PCB, the old board material has a nasty habit of turning into charcoal when it heats too much. Just don't keep the machine turned on for too long at a time until you have found the problem; there's definitely a faulty component somewhere, and I'm pretty sure it's in the power amp that has the heating AD139.

-mika
 

Thanks Mika, that's something to be going on with..

Andy T.

Most likely a capacitor problem and definitely worth and possible fixing.

Disconnect from mains and put an ohmmeter across the large capacitor next to the transformer. A reading across the cap number 514 (80uF sitting on the amplifier board) could also interesting. Note that component designators vary with the different versions of 900K's.

Martin

I shall go and buy a new multimeter tomorrow, this one is all over the place (Tandy Radio Shack)

Hope you get it working Andy, maybe another photo thread. Had mine on earlier today whilst on spud peeling duty.

 

Craig

 

 

Well, I went out and bought a decent(ish) meter today. I thought I was thick until I started reading the instructions over my salmon.(mmm)
I now realise I am actually twice as thick as I thought I was. I won't give up though.
Repeated over and over, it said "make sure any high voltage capacitors are discharged first". This leads to the first of many questions----

1) What would you call a high voltage?
2) How can you spot a high voltage cap?
3) Do you discharge it by just shorting it out?

Andy T.

When I have to discharge capacitors at work I just put my multimeter across them. You will then see the voltage on reading on the display start to slowly drop until it reaches almost zero. Not sure but I think you can damage them by shorting them out.

 

Craig

 

In the BM900 there are no high voltage caps. Just a 30V or so and it won't be a problem since it will de-charge rather quickly when powered off and in your case the too high load on the power supply will drain the cap very fast.

Do take care though; at the left side of the chassis is the mains fuses and the transformer primary UN-covered, don't touch anything here !

Martin