Inspect the PCB for any solder joints that look suspect. This includes cracked joints, evidence of shorts or burnt traces, bodged repairs by previous owners, etc..

Example of cracked solder joints.

Evidence of shorting, and botched repairs that may need to be redone.

Re-solder any joints that appear suspect. If a component such as a resistor is obviously burnt, replace it with one of a similar value.

Use a multimeter's continuity tester function to check for shorts among nearby components in suspect areas.