Electronics > Repair

damaged BJT or diodes? with fine diode test

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I read about this a little bit, but, what are the main ways a transistor can fail, and still read OK on the diode test?

I read the gain of transistors can go down while still reading OK on the diode test. Can anything else go wrong?

David Aurora:
I've definitely been stumped by some odd failures in both things before. And not just passing diode tests, but also passing on basic multifunction testers at low voltage.

I haven't seen much in the way of simple gain changes, but I've seen other stuff. I've seen quite a few noisy/crackling diodes and transistors test perfectly fine on basic test gear but play up in circuit. Or breaking down well below the specified limits but still well above your standard low voltage tester (e.g. 50V rated parts that eat shit at 20V). Leakage can also be a thing that doesn't show up on basic tests (though sometimes can depending on meter specifics and severity of fault).

Hell, last week I had a REALLY weird one- I tested a suspect transistor in circuit and it was OK on diode tests. Wasn't convinced so I pulled it out and checked it on one of those little eBay transistor testers, it passed as a PNP transistor. Only it WASN'T a PNP transistor, it was a (bad) NPN transistor  :o

I usually chuck these things aside to look at properly later/keep as references of odd failures. In most cases it becomes extremely obvious that these parts are faulty when I check them on my Tektronix 576 at healthy test voltages/currents, but yeah, often that's a last resort test after something has passed basic inspection but presents symptoms pointing to a bad device.

I've heard prolonged reverse bias above the breakdown voltage will cause this, but it isn't something I've experienced myself.

m k:
So nicely and not so nicely partially faulty.
Maybe overheat treats it more gently than over current.

Partially exploded traces inside ICs is a regular thing, maybe bubbles also.

I guess all kind of diminished values are possible.

If tin whisker level connection is a power route, it probably burns away.
But adding over time and it's just an add-on.

The voltage used by the meter for the test can also have something to do with it. If you use a voltage around a few volts, the junction may test bad while it test good with a voltage below 1 volt like many meters use.


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