Transistor Fault Checking

Good in your hand and bad on the board. The logic is the board is bad. rite?

Any advice on how come when a particular transistor when soldered gives out different readings as compared to when it has been desoldered and taken out.

When desoldered and taken out from pcb, gives good readings indicating the transistor is good. However when on board, it gives inconsistent readings showing the transistor is bad.

End of day, is the transistor good or bad? Thanks.

Am using analog multimeter under the diode function for checking on transistors.Thanks.

SeventhPrince???

While in-circuit, you are measuring resistances or other current pathways which are in the transistor circuit under test. Out-of-circuit you are measuring the transistor characteristics alone. If it tests good out-of-circuit, it's "good", meaning it isn't shorted, open, etc. That is not, however, necessarily the same thing as saying it's still within design specs. If a transistor has been stressed by some failure elsewhere in the circuit, it can in fact be pushed out of tolerance (e.g., gain suffers). Most often, though, when subjected to such catastrophic stresses they just develop opens or shorts.

In-circuit testing with an ohmmeter can be confusing since other components provide current pathways. In-circuit voltage testing is valid but one must understand a bit about the theory of devices and circuit design. One quick example: for a linear amplifier, the base to emitter voltage should be around 0.6 volts and the collector to emitter voltage should be around half of the supply voltage.

Thanks everyone.

You don't mention a thing about the type of transistor? Nothing about the multimeter used for this?

I check transistor in circuit all the time, you just have to know what is in parallel and in series with the junction that you are reading.

I used to be good with the old Simpson 335 analog meter, one had to figure out that the red/black leads didn't mean negative/positive since the voltage coming out was reversed also you could ony check transistor on the ( X1) scale since that was only scale that was providing enough DC voltage to go over that diode/transitor barrier ( 0.7 Vdc about).

In any case with the new multimeter most have a diode checking position, much easier, just a metter of finding the Base if it isn't maked, it is the one common to both junction and if the test lead is negative, then it is a PNP transistor and vice versa if the common lead is positive.

Anyway a lot of blah blah, what is important is look at the circuit and figure out what is around it that could affect your meter reading. If the resistance is lower than your junction (0.7 VDC) then one has to remove from the circuit in order to be sure. Or else just disolder that lead that you want to test.

Good luck

I am sure that lots of other techies have different ways of doing things

Any advice on how come when a particular transistor when soldered gives out different readings as compared to when it has been desoldered and taken out.

When desoldered and taken out from pcb, gives good readings indicating the transistor is good. However when on board, it gives inconsistent readings showing the transistor is bad.

End of day, is the transistor good or bad? Thanks.

Am using analog multimeter under the diode function for checking on transistors.Thanks.

SeventhPrince???

In circuit testing, diode function test and analog meters are not going to be proof that the part is good although they will find bad parts they will miss high resistance leakage in transistors. Out of circuit tests with a good digital meter is also not positive proof that the part is good but more likely than not if it shows good then it proably is. Whenever you are not sure check a new same type part (if possiable) and see how it reads compaired to the suspected part. Jeff