Just purchased a beomaster 2400, not used for 15 years. When I connect the power, after a few minutes it gets very (!!!) warm on the left side (the amp?). It overheats even when not in use, but power ON. I have a Beomaster 1900-2 which doesn't run hot like the 2400. Read some post on overheating, do I have to replace some electrical components? Is this a DIY for a not very skilled electric circuit person.

What is wrong, can I use it?

Don't use it. You probably have a problem with the No Load Current potentiometers in the amplifier-section of your BeoMaster. They need to be replaced and readjusted. And while you are in there, you will probably also find some leaking capacitors. So the BeoMaster needs some TLC!