After 34 years of regular use, my Beomaster 4000 is starting to show signs of malfuncion.  It has never been serviced . The thing is that on radio mode one of the channels sometimes does not work. If I switch the mono system on, it works fine again. This only happens on radio mode. Phone, aux and tape are ok.

Another problem is a very annoying noise, like a high volume thunder, when I move the Bass control or when I turn the system off.

Any ideas?

The Beomaster 4000 is very similar to the Beomaster 3000 (and 3000-2) in these aspects so
if you do a search in the forum, you will find more information but
the missing channel could be a switch problem.
Try playing around a bit with the switches when it fails again.
It could also be a problem with the leads that go from the tuner board to the switch row.
Some Beomasters have stiff leads here that often breaks lose from their solder joints on the tuner board.
The missing channel could also be a capacitor problem but in that case, it would almost always
affect other sources too.
The thunder that can be heard "dying out" after power off is usually a capacitor problem and
has, at least so far, only been noted on Beomaster 4000's.
The bass potentiometer will need a good cleaning. It can be taken apart and cleaned.

If it was never serviced before, it may benefit from a thorough service.
You may be missing a lot.

Martin

Thanks Dillen