Before you can repair a piece of electronic equipment, you first have to find the faulty components on its printed circuit board, or PCB. This can be a challenging task, because different components call for different test procedures. It makes sense to check transistors first, because you can do a quick in-circuit test. Passive components such as resistors and inductors tend to fail less frequently, though even they can break or burn out.
Turn off any power to the circuit before examining the board. Disconnect the power cord from the AC outlet.
Check the circuit board for fuses. If you find one, pull it out with the long-nose pliers and see if it appears to be blown. If you have a glass fuse, look at the filament inside. A blown fuse will have a broken filament. If you have a ceramic fuse, check it with a multimeter. Set the meter's function to continuity and touch the meter's probes to the fuse's metal ends. If the meter beeps, the fuse has continuity and is good.
Examine components on the board for signs of physical damage. You may see burn marks, cracks, broken wires, bulges or crushed parts. Assume that any parts that appear damaged are faulty.
Turn the digital multimeter on and set it to its diode test function.
Identify bipolar (NPN or PNP) transistors on the schematic and find them on the circuit board. Touch the multimeter probes to the collector and emitter pins on each transistor. The meter should read "open" or "high resistance."
Touch the negative probe to the collector and the positive probe to the base of each NPN transistor. You should get a reading of a few hundred millivolts. Move the negative probe to the emitter. You should get a similar reading. Reverse the probes. The meter should now read "infinity," "overload" or "high" resistance. Move the positive probe to the collector. You should get a similar reading.
Connect the positive probe to the collector and the negative probe to the base of each PNP transistor. The meter should read at a few hundred millivolts. Move the positive probe to the emitter. You should get a similar reading. Reverse the probes. The meter should now read "high" resistance. Move the negative probe to the collector. You should get a similar reading.
Replace individual integrated circuits (ICs) with spares of the exact same type, if the ICs are socketed. Test the circuit by plugging the power cord back in and turning the unit on. If it was performing poorly or completely dead before and is now working fine, the integrated circuits were defective.
The integrated circuit test at Step 7 assumes the problem lies only with the IC and no other parts.