Testing a transistor In-Circuit?

[rodm1]

HI everyone I have an automotive electrical problem. I think it's the dash light dimmer switch that is the problem. If I remove the dimmer switch from the circuit it won't blow the fuse but If I hook the dimmer up the fuse will blow after fixing burnt trace.

Taking the dimmer switch apart I noticed and fixed a burnt trace on the N lead of a  A1012 PNP transistor. If I test it in circuit I get .3 on one side and 598.1 on the other side. Is that a good enough test to weren't removal for further testing?

 

[Kleinstein]

It is difficult to test transistor in circuit. As many, especially larger transistors most often fail with a dead short, there is a chance to find those cases in circuit. So if the emitter - collector connection ready like a short, chances are good the transistor is blown. However transistors might also blow in a different way.

So 0.3 Ohms or 0.3 mV on the diode/continuity test somewhat points to a short.

However just the blow transistor should not cause the fuse to blow. A shorted transistor would more like turn the light on full power or not at all.
So very likely there is something else wrong, like short in the cabling that than causes the transistor and fuse to blow.

[Armadillo]

Replace the shorted transistor and you should be good to go.

OR;

Take out the transistor, use a 12V light bulb jumper wires over the fuse connection [without the fuse], if the bulb don't light up, means confirm without the transistor, there is no short.

But if I will you,  I will just replace the transistor and give it a go.

[madires]

The "N lead" would be the PNP's base. So there seems to be too much base current. And that could be caused by a short circuit in the driver for the 2SA1012. Could you please post pictures of the dimmer's PCB (front and back)?

[Samogon]

Shorted EC on power transistor is one of most probable reasons to blow fuse.