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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 6th Jun 2017, 10:23 am   #1
TonyDuell
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Default Indentifying transistor pinouts with a DMM

If you have one of those cheap component testers you don't need this, but if not, I have found this to be a useful way of working out the pinouts of a transistor using most digital multimeters. You need the diode test feature and the Hfe transistor test feature. I have seen the latter claimed to be useless (and to be fair it is not a good test of a suspect transistor), but here is a use for it.

Start by using the diode test function, both ways round on each pair of leads (6 tests in all). You should find it tests like 2 diodes with one lead either a 'common cathode' or 'common anode'. That lead is the base, in the first case it's PNP, the second is NPN. This is just the common '2 junctions' test for a transistor, of course.

Now use the Hfe range. You know which the base lead is, connect that to the 'b' hole of the test socket. Try the other 2 leads each way round in the 'e' and "c' sockets. One way will give a much higher reading than the other (for the 2N3906 I've just tried it was around 200 one way, 3 the other). The connection that gives the higher reading is the right one, you have the collector in 'c', etc.
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Old 6th Jun 2017, 10:38 am   #2
paulsherwin
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Default Re: Indentifying transistor pinouts with a DMM

Before I got a component analyser I just used to use trial and error. There are only 4 common arrangements, CBE, EBC, BCE and ECB, each as PNP and NPN. If the transistor doesn't give a sensible hfe reading in one of these positions then it's a CEB/BEC HF transistor like an MPSH10, not a bipolar transistor at all, or faulty. Possible CEB types received further attention.

The Chinese analysers do make life a lot simpler though, despite sometimes getting it wrong.
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Old 7th Jun 2017, 12:17 pm   #3
BulgingCap
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Default Re: Indentifying transistor pinouts with a DMM

We used to test with an Avo8 (on ohms x100 i think) by finding the base lead then connecting the black (+ve on an Avo) and the red lead (-ve on an Avo) to the other two leads. A wet finger between the open base and the black lead would cause a 'reduction' in resistance if the black lead was connected to the c of an NPN. If little change was seen then the black lead was assumed to be connected to the e, and the connections could be reversed to confirm. I have never tried it with a DMM.
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Old 7th Jun 2017, 12:49 pm   #4
paulsherwin
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Default Re: Indentifying transistor pinouts with a DMM

You can do this with a DMM too, though most have built in transistor sockets and an hfe test mode, so it's not really necessary.

Beware of faulty or intermittent contacts in the socket if using a cheap DMM.
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Old 8th Jun 2017, 10:16 pm   #5
Argus25
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Default Re: Indentifying transistor pinouts with a DMM

Quote:
Originally Posted by TonyDuell View Post

Now use the Hfe range. You know which the base lead is, connect that to the 'b' hole of the test socket. Try the other 2 leads each way round in the 'e' and "c' sockets. One way will give a much higher reading than the other (for the 2N3906 I've just tried it was around 200 one way, 3 the other). The connection that gives the higher reading is the right one, you have the collector in 'c', etc.
I have seen a trick used to quickly identify the C from the E with a meter, Though the meter must be a type with an Rx10k range and a 22.5V battery, like the Hioki A-10. It is based on the fact that the typical zener voltage of the B-E is around 7v for a silicon signal transistor, and usually much higher for the B-C. So the B-E conducts with both meter polarities on the B-E connections but only one way on the B-C connection.

However, I've never thought this was a good idea because each time the B-E is zenered, it slightly degrades the transistor's hfe, without altering the transistor's other parameters (the mechanism of this effect has been the subject of some good research papers by Motorola). I have experimented with this effect as a tool to hfe match transistor pairs, its very real.

I have seen it claimed (by Audiophiles) that due to coupling capacitors to bipolar OP amps, when the power is switched on and off, that the input transistors b-e 's get zenered, slowly degrading the OP amp performance. I haven't bought into this notion completely, but I guess its possible.
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Old 8th Jun 2017, 11:24 pm   #6
MrBungle
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Default Re: Indentifying transistor pinouts with a DMM

I have also experimented with beta degradation unintentionally and it is indeed very real. I was attempting to use a 2n3904 as a varactor and zenered it. Next thing I know, it make its way back into the parts box and it wouldn't oscillate in a VXO. Ended up replacing the transistor and hey presto it worked. I did a spot check with a cheap DT830 DMM and the beta was around 40 compared to the average 190-250. Took me a long time to put two and two together, then I tried it again and it was reproducible.
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Old 9th Jun 2017, 2:47 am   #7
Argus25
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Default Re: Indentifying transistor pinouts with a DMM

I think what this means, for testing signal transistors, is to avoid zenering the B-E junctions ever. Also, for vintage germanium signal/RF transistors be wary of how much base current is passed by the test system/meter as they are more fragile and have lower max current ratings than typical silicon type. There was once a recommendation not to test these on a standard meter on Rx1, but it might pay to look up the specs of the actual device.Though with power capable germanium devices, AC128/188, AD149 etc etc, I don't think any multi-meter I know of could harm them.
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Old Yesterday, 10:49 am   #8
stuarth
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Default Re: Indentifying transistor pinouts with a DMM

I'd always thought low ohms ranges were more of a problem than high resistance ranges.

An AVO 8 on the low range is effectively a 1.5V battery with a 20ohm series resistor, giving a short circuit current of 75mA. Some far-eastern meters have a low range with 3V and 20ohm drive, a current of 150mA, and can blow the filament of Dx96 valves!

An AVO 8 on its high resistance range uses a 15V battery, but with a 200kohm resistance, I'd always thought the current (75uA) too small to harm most devices. Does anyone have any information on the current levels which cause degradation when zennering a B-E junction?

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