To perform a repair on an amplifier, you must first identify the exact problem, such as no power, no sound or bad sound. The first thing you should do is clean the amplifier. Clean off any dust, and then plug in the amplifier to test it. If this fails, unplug all external audio sources from the amplifier, and use only the internal receiver to test whether you've resolved the audio problem. If the above measures fail, the problem is internal.
You have to disconnect the amplifier from a power source and open it so you can view its internal components. The first thing to inspect is the electrical cord. If it is loose or damaged, replace it. If the electrical cord is in good condition but the amplifier still receives no power, locate the internal fuses and test each of them. If there are any faulty fuses, replace them, and test again.
The next thing you need to do is clean the internal components of your amplifier. Use a can of compressed air to blow the dust off them. Alternatively, use a light vacuum cleaner. Test the amplifier again. If these measures don't solve the problem with your amplifier, seek additional assistance from a professional.