first .... there is nothing wrong to run an amp without speakers or only one channel. but disconnect speaker cables from the amp terminals to prevent speaker wires accidental short... most new amps has adequate protection , but vintage ones do not survive .
so you got 2 possibilities... one is the other ends of speaker cables touched each other . or just bad luck and amp got fried with no reason whats so ever . now, can you turn on the amp ?preferably not... for one reason ...to help a repair man. when he opens the unit and turn it on hopefully the defective part still smokes and he can easily determine the problem. but if you continue to press power button the part eventually would stop smoking and if there is no easily noticeable burn mark troubleshooting is more difficult .
so you got 2 possibilities... one is the other ends of speaker cables touched each other . or just bad luck and amp got fried with no reason whats so ever . now, can you turn on the amp ?preferably not... for one reason ...to help a repair man. when he opens the unit and turn it on hopefully the defective part still smokes and he can easily determine the problem. but if you continue to press power button the part eventually would stop smoking and if there is no easily noticeable burn mark troubleshooting is more difficult .