Transistors can be damaged by heat when soldering or by misuse in a circuit. If you suspect that a transistor may be damaged there are two easy ways to test it:
testing a transistor
Testing an NPN transistor
1. Testing with a multimeter
Use a multimeter or a simple tester (battery, resistor and LED) to check each pair of leads for conduction. Set a digital multimeter to diode test and an analogue multimeter to a low resistance range.
Test each pair of leads both ways (six tests in total):
* The base-emitter (BE) junction should behave like a diode and conduct one way only.
* The base-collector (BC) junction should behave like a diode and conduct one way only.
* The collector-emitter (CE) should not conduct either way.
The diagram shows how the junctions behave in an NPN transistor. The diagram was found here..
http://www.kpsec.freeuk.com/images/trtest1.gif
