Further on my earlier remarks = I've had a look at the Fault Switch etc (disregard remarks about RL1 for the moment).  A protection circuit appears to be activating (the Fault Switch) because of a fault in the amplifier final stages!  You can verify this by putting your multi-meter on pin 19 of the microcomputer chip when you will see the volts go LOW - then the BM goes to St Bi.  So, you need to work on the amplifier stages - as others have suggested, the balance may not be correct or a number of things, including the Darlington transistors (IC103, 104 etc) which work very hard handling large signal currents.  A short cut - you may not want to risk this - would be to disable the protection circuit by isolating the emitter lead of 9TR2 from ground. which will stop pin 19 of the micro going LOW then see what happens! All interesting stuff.  = Brian (Northumberland)

A bit more = If you look at the Service Manual for the BM6000, there is a very good explanation on the Fault Switch.  Is the BM tripping to St Bi after a few seconds or almost instantaneously?  If you do the test that I suggested ie monitor the status of pin19 of 2IC4 and you see it go LOW, you almost certainly have a fault in either or both sections of the amplifier! If this is the case, them I'm sure you will get more guidance on how to investigate further. = Brian (Northumberland)

Well, you guys convinced me to check the amplifier section again. And...*surprise* it was the amplifier that was faulty. 

Sorry for not checking that earlier, but I just couldn't believe that this thing would have stopped working just from not being used for two months...

Anyway, after disconnecting the complete left channel section of the amplifier the BM works perfectly. Sound from the right channel is great - just as it should be.

Seems like I will have to search for some faulty component again...the usual faulty parts (trimmers, electrolytic caps) have been replaced and there is nothing that is obviously burnt or something like that.

By the way, I only now saw that the axial caps used in the amplifier section are described as -10/+50% in the service manual. I replaced them with +/-20% ones, will that make any difference?

ablaumeise:

By the way, I only now saw that the axial caps used in the amplifier section are described as -10/+50% in the service manual. I replaced them with +/-20% ones, will that make any difference?

Hardly - electrolytic caps traditionally have very broad tolerances. If some applications requires an exact capacitance, the designers would not have used electrolytics, or the variations would have been covered by some adjustment.

Have you gone through the solder joints?

Great job!
Now, check the DC offset. The trimmer, as I understand it, has already been replaced, that's essential.
Adjust with no signal (volume at zero), no speakers attached, to minimum DC readout at the
speaker output.  If the Beomaster keeps shutting down, readjust the trimmer slightly around its center
position and find a spot where the Beomaster will stay awake. The adjustment is a bit "liquid", it also
depends on the Beomaster being powered up for apprx 30 sec or more to be just close to
a stable adjustment. Adjust a bit, allow a couple of seconds to settle, read out, adjust again etc..

Of course, there can be a number of other causes, but the above is by far the most common.
If another component is causing DC on the output, enough to trip the safety circuit, you will easily
spot the DC on the speaker output and the Beomaster will probably keep shutting down.
Anything above a couple of millivolts is bad.

And I agree on the capacitors and solder joints.

Martin

I tried to adjust the DC offset, but it seems that the problem is somewhere else....

DC readout is 1,9mV; I can't get it any lower than 1,8mV. Right channel is 2,1mV, so I guess thats more or less correct. Another problem is that the BM shuts down almost immediately - after about three seconds. Not really time for the voltage to settle.

I checked the solder joints and resoldered all connectors and the components that got hot. But none of that improved the problem.

If you are sure that it's 1.9mV (not 1.9V), then the CD offset is not the problem and with that small amount of DC
on the output, there's probably nothing very seriously wrong in that channel.

Did you also replace the bipolar 47uF caps ?
If not, I suggest you do so.
If you don't have them at hand, swap the cap with the one from the other channel, try again and see
which channel causes the shutdown now.

Martin

Well, looks like I killed the complete amplifier now.

I tried swapping some components to find the fault and that didn't really work out well.

There was one thing I noticed about the left channel amplifier last time it worked (approx. three months ago): It sounded great, but had only about 80% of the volume of the right channel and it got extremly hot even after short time.

So I tried swapping the complete heatsink-mounted board (IC 201/205/204/203/202). And now I had exactly the same fault in the right channel: low volume and way to hot. But as I said the left channel is completely dead by now. And so I was afraid whatever was faulty on that board would damage some other components (thats what might have happened to the left channel).

To test if everything else was still working I quickly swapped back the boards. And then it happened: The BM played fine for about five minutes and then IC103 suddenly caught fire (literally!). The BM didn't switch of before as I forgot to screw R2 back to the heatsink. Great. I guess that means the right channel part is damaged as well now.

Does someone have an idea what might be wrong with the amplifier? To me it looks as if one of that five ICs is clearly faulty and damaged some other components as a consequence. But what components?

The trimmers were set to match the amplification etc. of the IC's on the other board...

If I get this right, you swapped the output stage IC's board with another one and
then the trimmer settings were no longer correct.
This damaged the amplifier board (and maybe also the IC board).
Then you mounted back the original IC board to the amplifier module that was now
bad. Then the whole thing burned...

Sorry for me asking this, but are you educated in electronics in any way ?
Don't get me wrong, but DC-coupled amplifiers like this one can be very hard to service and
many repairers, even experienced ones, fear them like the plague for the same reason.
When everything is fine, they are some of the best you can get but one little fault in a driver
or predriver can set off a disaster in the following stages, destroying components as the avalanche rolls.
If you are not familiar with DC coupled amplifiers, I suggest you leave servicing to someone who is.
The risk of damage, personal injury and fire is very real and compared to the cost of having a
Beomaster serviced professionally, not worth risking.

- not that it matters much now, but did you check the bipolar cap, I suggested ?

Martin

Dillen:

Sorry for me asking this, but are you educated in electronics in any way ?

I would say I am, though my knowledge stops about where ICs begin...

But I'm well aware of the risks in this stuff. I knew the trimmers had to be readjusted, but I thought the fault switch would energise before anything like what happend might happen. I still can't believe I forgot to put that damn resistor back onto the heatsink.

I guess the simplest solution now would be to simply replace all components on that board?

Having that thing repaired is not really a option - the next qualified repairshop is 30km away and they would charge twice the price the BM is worth.

Dillen:

- not that it matters much now, but did you check the bipolar cap, I suggested ?

That was the first thing I did, sadly that wasn't the problem.

Hi again = Sorry to learn about your blow-up - you havn't been bloodied until you've caused some smoke (lots?) and been 'bitten' by a super-charged electrolytic!  I stick by my advice about disabling the protection circuit (watching closely for results) - it is the motoring equivalent of doing a controlled skid! However,  I do think you need to be very careful if you are not so experienced - but I am sure you are learning. We all (most, perhaps) travelled the same road - ouch, that hurt!!!

Well, if you think you can do it, why not try a re-build and replace all the silicone?  It would be a long job (enjoyable, perhaps) and worth it in the end. Good luck = Brian (Northumberland).

And finally...the BM works again!

I simply replaced every single IC and Transistor in the amplifier section, adjusted the offset and no-load voltages and now it works! No random shutdowns anymore, no excessive heat, great sound. Paid 10,51€ for the spares, so I think it was definitely worth it.

Thanks to all for the great help and support!

Congratulations and well done! = Brian (Northumberland)