Testing transistors

[ironsniper1]

need some advice, i have two vcr's that get no power and both have shorts to ground on the power rail, the first one i removed a transistor and the short went away then i did a continuity test on the transistor it was shorted on all three legs and when i tested for the short on the power supply area it was gone so i am assuming this one is bad? as for the second one same issue short to ground and i removed the transistor but on this one i get no readings at all, can i also assume this one is bad as well? the short to ground did go away when i removed this transistor as well, i just want to make sure my findings are correct

[Andy Chee]

Sounds all reasonable so far.

Look around nearby for some collateral damage that shorted transistors could potentially cause e.g. shorted base/gate resistors

[ironsniper1]

no more shorts that i can find

[Seekonk]

After replacement, test with an incandescent lamp in series with the power. Switchers can have various issues like shorted diodes on the output.  A short disappearing after removing a non shorted component sounds very suspicious as to what you think you are seeing.  Sometimes I even fool myself.

Seeing and believing are often both wrong.  FOW

[ironsniper1]

i am not getting any reading even in diode mode, its like its dead dead

[Sensorcat]

For basic testing of an NPN silicon bipolar transistor with a DMM in diode mode:

Connect the base to + and the emitter to COM: display should be forward voltage drop, approx. 0.5V ... 0.7V
Repeat with collector connected to COM: about the same voltage.
Repeat both measurements with inverted polarity: reading should be overflow, displayed as OL or something similar.

If a transistor shows these results, it might still be damaged. But if it doesn't, it certainly is.

For PNP, the polarity of each measurement has to be inverted.

All this with the transistor connected only to the DMM. In-circuit is a different story.

Please note that OL is not 'not any reading', it's a measurement result. So if you see this and want to get help on what's going on, you help the helper if you write 'I get OL.'

[SteveThackery]

I assume the second transistor failed open circuit?

Rarer than short circuit, but feasible?

[ironsniper1]

well either way i replaced them and no change, so to start i am working on one vcr, it has a short on all of the caps, i removed the caps and tested them and they needed replacing so i did that, the short is only in the area marked with the orange square, there is no short in the blue square