Hi
I am afraid that there is no simple answer to your problem. You will definitely need some decent equipment to help investigate the situation and if you haven’t got that – you are stuck. A four point program to fixing the unit might be as follows:
Become a silver member and access the schematics to find out just what you are up against. I have a Beomaster 3300 and know that the service manual that is available here is fairly limited. You might be able to get a better version elsewhere.
Taking the back off the unit is fairly straightforward and at that point, you get a good view of the main circuit board. There you will get a general appreciation of what you are up against.
Since your amplifier and switching section seems to be working normally, I would find the two test points where the radio audio frequency signal leaves the decoder and goes into the switch section. A test with a scope would quickly show whether you have a front end problem or a switching problem.
Thereafter, it is simply a case of delving into the appropriate section to find your problem. Bear in mind that the nearer your problem is to the front end, the more sophisticated the equipment requirements are.
I would seem that in your case, if you really want to get the BM3300 working properly, find a reputable service engineer to help you. It would probably save you a lot of heartache. On the positive side, give us a list of the type of equipment that you have access to and perhaps we can make a plan.
Regards
Geoff