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How do I meter transistors??
Old 17th June 2005
  #1
How do I meter transistors??

Such a dumb question. I've just pulled the 8 output transistors out of my power amp. 4 pnp & 4 npn.

Can someone please remind me how to check if they are working correctly or not??

Any help greatly appreciated.
Old 17th June 2005
  #2
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I assume an analog meter vs. a digital, since the voltages used at the test leads for the latter don't work correctly for this sort of test much of the time. Hence, I carry both a beat-to-hell Rat Shack analog meter in my tool kit as well as a newer digi-meter.

Between base and emitter and base and collector, you should see a diode ("medium" ohms with leads one way, infinity the other), while between emitter and collector it should be infinity ohms regardless of which way the leads are connected.

Which lead goes where depends upon the internal battery polarity of the meter, but *typically* black (minus) lead to base, then red to collector or emitter should see the "conducting diode", with reversed leads seeing open circuit, on a NPN transistor. Collector to emitter should be an open circuit regardless of the test lead polarity. Flip the test lead colors for a PNP.

With my Rat Shack meter, I use the Rx1 range when looking for the "diodes", and a higher range (maybe Rx100) when checking emitter to collector.

This test is usually invalid if the transistor is in-circuit since other directly connected parts will provide "sneak paths".

Bri
Old 17th June 2005
  #3
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tmarra's Avatar
Quote:
Originally Posted by brianroth
Which lead goes where depends upon the internal battery polarity of the meter, but *typically* black (minus) lead to base, then red to collector or emitter should see the "conducting diode", with reversed leads seeing open circuit, on a NPN transistor.
You did mean to say PNP, right Brian?

Tony
Old 17th June 2005
  #4
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Tony, nope, that's how the polarity for the "ohms range" is wired on my Rat Shack meter, and one or two others I recall using.

Hence, my "weasel words" regarding polarity, since you need to figure that out ahead of time. The best bet is to test a known-good transistor with a known polarity to verify.

Bri
Old 17th June 2005
  #5
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tmarra's Avatar
Sorry Brian,
After retiring all my old analog meters years ago I just figured everbody's "RED" probe put out positive voltage.

So what Brian and I are saying is that you should know which one of your probes puts out positive voltage in oder to get a transistor or diode to conduct in the proper direction. This way not ONLY can you test the transistor but you can also determine if it is a NPN or PNP.

Tony
Old 17th June 2005
  #6
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Indeed, analog VOMs are all over the map regarding the meter lead polarity on the "Ohms scale". In fact, the manual (I never throw anything away) even makes note of the reversal, and I've encountered it on other analog meters as well.

Testing transistors is about the only reason I keep this beat up old meter in the tool kit.

Bri
Old 18th June 2005
  #7
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Quote:
Originally Posted by thenewyear
Such a dumb question. I've just pulled the 8 output transistors out of my power amp. 4 pnp & 4 npn.

Can someone please remind me how to check if they are working correctly or not??

Any help greatly appreciated.
Hi

I use a Radio Shack 22-330 for quick checks and figuring out which lead is which.

http://www.tubebuilder.com/images/te...hackTester.JPG

I lightened the image so that you can see the front panel LEDs

http://usera.imagecave.com/auroraaud...hackTester.jpg

I use a Heathkit IT-18 for serious gain and leakage measurements.

Old 18th June 2005
  #8
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Yes, as Geoff implies, the VOM is basically only good for finding gross faults, like shorted or open junctions, or perhaps a seriously leaky junction. But, when working in the field, a VOM is better than nothing...heh.

One other thing I should emphasize again...a VOM can give VERY erroneous results when measuring a transistor in-circuit.

Bri
Old 20th June 2005
  #9
Thanks very much for all the info Brian, Tony, Geoff. It has been very helpful.

I have all the transistors out of circuit (getting it disassembled was no easy task). I have metered them all using the diode setting on my digital meter & they all meter as expected according to Brian's advice.

The analog meter I have here only meters voltage and current so I can't use it to check resistance. Bearing in mind my situation is there anything else I should check before I put the transistors back into the amp & look for the fault elsewhere...
Old 20th June 2005
  #10
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nosebleedaudio's Avatar
 

And don't forget the most important aspect of checking a bipolar transistor is under a real load. a border line trans may check OK with a meter but not under a normal load. since you said it was a power amp, well, If it was me I would replaced all and not worrry about it. As with all DC coupled amps the ONE trans you did'nt replace is the faulty one, it can take out all the new ones you just replaced.
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