If, for some reason, you can not get an exact replacement for the transistor that you want to replace, you can refer to one of the transistor substitution guides or book, and try to identify a "near replacement". Beware, however. A substitution guide will sometimes list a replacement for your part, even though the two parts are not very similar. The most commonly used transistor substitution guides is the Philips ECG semiconductors Master Replacement Guide. Selecting the right transistor parameters or specifications for the equipment you repair will avoid callbacks from customers. Callbacks or repeated repairs due to wrong transistor substitution will cost you extra money and a waste of time.
Some important transistor parameters are:
Maximum collector-to-base voltage (VCBO) – The replacement part should have a VCBO rating equal to or higher than the original.
Maximum collector-to-emitter voltage (VCEO) – The replacement part should have a VCEO rating equal to or higher than the original.
Maximum emitter-to-base voltage (VEBO) – The replacement part should have a VEBO rating equal to or greater than the original.
Maximum collector current (IC) – The replacement part should be able to handle as much (or more) collector current as the original.
Power dissipation (PD Watts) – The replacement part should be able to dissipate as much (or more) power as the original part.
Gain (hFE) – The replacement should have a gain equal to or better than the original and it should be as close to the original specification as possible.
Gain-bandwidth (fT) – the replacement should have a transition frequency equal to or faster than the original and it should be as close to the original specification as possible.
Below is a table of application requirement and important specification that you can use for guidelines when comes to transistor substitution.
Application Requirement and Important Specifications
1. General purpose low level amplifier and switching transistors. Spec- hFE, IC (max), fT, VCEO
2. Power switching applications. Spec- VCEO, VCBO, hFE, IC (max), PD (max)
3. Low level IF / RF amplification. Spec- IC (max), fT, VCEO
4. Audio amplification. Spec- IC (max), PD (max), hFE, VCEO
5. High voltage transistors. Spec- VCEO, VCBO, IC (max), fT
Remember, the replacement transistor should not run hot and if problem still persist, try another replacement number or get the original number if possible. Do not overlook that the surrounding components also may caused the transistor to run hot even if original number is used.