2018 February 22 Tech, Electronics
Transistors are very common in radios, vintage electronics, power supplies, etc. They go bad sometimes.
This is just a quick article on testing them in-circuit.
(Caution: electronics repair can be dangerous. Disclaimer.)
In This ArticleSafety First
Most signal-transistor circuits are low-voltage. However, remember that line-powered electronics have areas where there's high voltage. There can be capacitors that stay energized with dangerous voltage when the radio is unplugged. Once in a while you'll also find an AC transistor radio that's got a hot chassis design.
If you don't know how to work safely with electronics, please practice on battery-powered circuits until you have the skills and knowledge. Safety is entirely up to you (disclaimer, again).
"Base to Emitter" and "Base to Collector" should each test like regular diodes. Conductive in one direction; "infinite" or "open circuit" in the other. With a DMM you'll want to use the Diode Test mode, not the Ohm modes.
"Emitter to Collector" should appear open-circuit in both directions. PNP or NPN-type, same basic result if they're OK.
Transistors usually go short-circuit, unless there's a massive power spike or surge. Then, they can give "OL" readings when they shouldn't. Most often, though, a bad transistor is shorted: you'll find continuity in both reverse and forward polarity.
Do any of the tests look different when the transistor is in-circuit? Let's find out.
Other components and circuit paths can make in-circuit testing kind of unreliable... in theory.
Trying to learn why, starting with a non-functioning radio full of possibly-bad components, is only going to confuse you. The best way to learn, at first, is to use a solderless breadboard to simulate various situations. (Try this link). It's possible to get a no-frills breadboard for a dollar or two, but the better ones have features that are a lot more convenient. For example, non-skid feet, external power-supply connectors, etc. (Also try this link and this one for some good ones.)
Now, on to the actual results you'll get in-circuit.
Two-way continuity from B-C or from B-E could mean a shorted transistor, and it often does... unless there's something else providing a path around it.
Low-ohm circuit paths will indicate a short where maybe there is none. However, in any transistor circuits I can think of offhand, the resistors are a few k-Ohms or more. This is too many ohms to confuse a diode tester, from what I've seen. Now of course the path around could be due to another semiconductor, and that's different.
Any continuity from C-E, in either direction or both, also probably means the transistor is bad. Emitter of an NPN transistor is usually connected to GND. There shouldn't be any direct path that joins emitter to collector, except through the semiconductor junction or high-ohm resistors that go around it. Otherwise, it would be like putting a switch in parallel with a bypass wire: no purpose to that.
Unless it's other semiconductors (which again could cause a false reading), paths around this junction are usually going to be well up in the kilo-Ohms range. That means the diode tester should give reliable readings. If you have an ohmmeter that you know for a fact won't bias semiconductor junctions, you can test the circuit's ohmic resistance around that junction. Look for a meter that's down around 0.2 or 0.3 volt in Ohms mode.
Infinite or OL in both directions (forward and reverse bias) means a bad transistor.
Transistors don't usually fail open, though... except when they take a heavy surge or power spike. And that does happen.
If you're not sure, desolder that transistor and check it again.
One More Idea
Several companies made benchtop equipment for in-circuit testing of transistors.
Some of the ones with three test leads can reliably test transistors in-circuit. Some of these units need re-capping or other repairs, but often you can find a working one. Try this link and this one.
Transistors can be tested in-circuit; sometimes the results are useful. There may be a circuit path around what you're testing, and it could be another semiconductor. Aside from that, there shouldn't be any low-ohm path that joins the emitter and collector pins of a transistor. If you're not sure, desolder one of the leads and check again.
If base-collector or base-emitter show "open-circuit" in both forward and reverse bias, the transistor is definitely bad.
Transistors go bad fairly often. Static electricity or undetected power spikes are a major cause of that. That's another reason why I keep recommending these for AC-powered electronics.
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