♩ I'm going slightly mad... ♩ Transistor Craziness!

[Refugee]

Power transistors need to be tested with a much greater IC than one of those little testers can provide.
I have a home made tester that tests the transistor from 2ma to 100ma in steps with the ability to run up to whatever an external bench power supply can offer.
Power transistors need to see 50ma or so for a reliable HFE figure to be obtained.

[julie_m]

I suppose this is for the same reason why you have to set a quiescent current to avoid crossover distortion; the transistors do not behave linearly at low collector currents, and hence they give false readings on a simple tester. (Mine says it uses a collector current of 2.5 mA.)

[Ti Pwun]

All fixed now - it was indeed the two transistors I had suspected. The tester clearly wasn't up to the job for those transistors, which I hadn't realised. Had the replacements not read the other way and had I not become ill mid way through the repair then I wouldn't have become confused.

The tester threw me, though, for sure. The two with low Hfe were the culprits but after connecting one low and one higher to each channel and a fuse blowing, I connected the two higher Hfe transistors to the channel that blew its fuse (after replacing it) and I got audio on that channel. I then swapped them to the other channel and got audio on that channel.

It was only a very brief test but it showed me that those higher Hfe transistors are OK. I haven't done it yet but it sounded very good during that test so I'll dig through my stock and replace the other two.

Not sure what's going on with the replacement MJE2955 transistors having a different pinout than the originals but I have plenty of others I can use so I don't need to do any bodging.

My mistake for coming back to this unwell but alls well that ends well. Definitely a lesson learned with the tester for those transistors. I'll knock up a little tester circuit I can use to test them on my bench power supply, rather than using the Peak tester.

Thanks again for all the input.

[Radio Wrangler]

Those semiconductor testers offer to do an awful lot but they only really work on devices which are in good working order, and only if they're low to medium power types. The thing they don't do is analyse a damaged transistor and tell you what it was before it blew up.

There's a huge variety of discrete semiconductor devices, and testing leaves a lot of ambiguities.... This diode breaks down at 7.48v at lowish current. Is it a) an 8.2v zener tested well below its rated current, b) the base-emitter junction of a bipolar transistor, or c) a low threshold high frequency detector diode d) a 7.5v zener tested close to its rated current?

I suppose the main use of such a tester is in trying to identify known good devices which have been mixed up in the same bag.

The little LCR meter with the SMT tweezers is worth its weight in gold in this age of unmarked chip capacitors.

The ESR meter has paid for itself as well.

David

[Ti Pwun]

I have the LCR40, the ESR60 and the DCA55. All have proven themselves to be very useful but other than small transistors I have only ever used the DCA55 on the MJE2955 where it hasn't shown 'Unknown/Faulty Component' when there is a fault and instead gave the pinouts, allbeit an erroneous reading.

I don't have tweezers for the LCR tester but that would be useful. I have hooked the leads to copper tacks to carry out such tests but it is a faff around.

I built a little tester to quickly check zener diodes so I have that covered, but a circuit to test transistors that could cause problems for the DCA55 will be useful.

[BottleMan]

Originally Posted by Radio Wrangler

Designed by someone who didn't quite make the changeover from Germanium?

I'm working on a Plessey PR155 receiver that consists of modules designed by separate teams (well, it seems like it). The front end transistor is a germanium PNP, the rest are a mixture of plastic and TO-18 silicon NPNs, the power supply uses OCs, there's a 7490 divide by ten and it all works off a -15V supply rail! Sympathetic murmurings would be appreciated.

Graham

[Herald1360]

So sorry to hear of your stressful time with that Plessey contraption. I do hope the men in white coats help with its more maddening problems. I hope your recovery is speedy and you get back to vacuum devices soon.