It does not matter what you are repairing. There are times you "may" want to check transistor DC gain (hFE). One thing that is missing from the discussion is how conditional hFE can be. I am going to take 2SD718-R as an example to setup test while mitigating the condition that must meet minimum requirement. Going by Toshiba, Yamaha, Pioneer, Panasonic and Mitsubishi training from good old days, it goes like this. The first step in testing a device is to obtain a reference data.
Then interpret the test graph to obtain data. In the case of 2SD718 and all high power transistors you know the Base voltage often is given at least 700mV (most are over 1.0V) or higher on schematics diagrams for power amplifiers. Just sticking a device into a test instrument of any kind without satisfying the condition could result a disappointment such as low gain, or unnecessarily noisy.
You might NOT be able to test the device at a collector current of 1A but from the graph you can see that the hFE value reaches closer to minimum of 60 when collector current is 0.01A (10mA) at 25 degree C. You don't want to use a low collector current as you can not be sure of device temperature which is not a room temperature. Since it is a none linear function, scalability is tricky but quick approximation is possible. Leader LTC905 and Tektronix 7CT1N recommend using 0.1mA or 0.2mA for base current for power transistors. You can always increase or decrease base current according to the trace and wave characteristics. See below.
Then interpret the test graph to obtain data. In the case of 2SD718 and all high power transistors you know the Base voltage often is given at least 700mV (most are over 1.0V) or higher on schematics diagrams for power amplifiers. Just sticking a device into a test instrument of any kind without satisfying the condition could result a disappointment such as low gain, or unnecessarily noisy.
You might NOT be able to test the device at a collector current of 1A but from the graph you can see that the hFE value reaches closer to minimum of 60 when collector current is 0.01A (10mA) at 25 degree C. You don't want to use a low collector current as you can not be sure of device temperature which is not a room temperature. Since it is a none linear function, scalability is tricky but quick approximation is possible. Leader LTC905 and Tektronix 7CT1N recommend using 0.1mA or 0.2mA for base current for power transistors. You can always increase or decrease base current according to the trace and wave characteristics. See below.
This DC current gain graph (right side) is very important: it the device should maintain its gain over the intended application range. The flatter and wider, the better (but wishfull thinking). Usable up to 8A max I'd say, above that the base current explodes (left graph).