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 wrote:

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 wrote:

- 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)