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Prooer way to troubleshoot / test bipolar Transistors

tonymazz

New Member
Proper Way to test transistors

I am working on ressurrecting an older HeathKit IM-110 VOM and want to test transistors in circuit. These transistors are 2n3393 NpN .... Is it correct, that in order to correctly see if there are any "opens" / shorts I'd have to live one connection off the board by desoldering ?

Can I simply take readings similar to a zener ..i.e. will show some resistance in one direction and infinite ohms in another....for example base to emitter...

Then measure with circuit energized and take a voltage from emitter to collector.. base to ground...etc...

Any way I am a bit confused and appreciate some assistance.....

Thanks

Tony
 
Last edited by a moderator:

killivolt

Well-Known Member
I am working on ressurrecting an older HeathKit IM-110 VOM and want to test transistors in circuit. These transistors are 2n3393 NpN .... Is it correct, that in order to correctly see if there are any "opens" / shorts I'd have to live one connection off the board by desoldering ?

Can I simply take readings similar to a zener ..i.e. will show some resistance in one direction and infinite ohms in another....for example base to emitter...

Then measure with circuit energized and take a voltage from emitter to collector.. base to ground...etc...

Any way I am a bit confused and appreciate some assistance.....

Thanks

Tony
You might want to try also posting in the Repair section of the forums.

kv :)
 

ericgibbs

Well-Known Member
Most Helpful Member
I am working on ressurrecting an older HeathKit IM-110 VOM and want to test transistors in circuit. These transistors are 2n3393 NpN .... Is it correct, that in order to correctly see if there are any "opens" / shorts I'd have to live one connection off the board by desoldering ?

Can I simply take readings similar to a zener ..i.e. will show some resistance in one direction and infinite ohms in another....for example base to emitter...

Then measure with circuit energized and take a voltage from emitter to collector.. base to ground...etc...

Any way I am a bit confused and appreciate some assistance.....

Thanks

Tony
The best way is to remove the transistor from the pcb.

Using an ohm meter the Base to Emitter resistance should be about 1000ohms
with the test leads connected one way and when you reverse the meter connections the Base to Emitter should be a very high resistance.

The same test on the Base to Collector should give simliar readings as the Base to Emitter readings.

Low one way, high the other.

Do you follow OK.?
 

JimB

Super Moderator
Most Helpful Member
OK a few quick and dirty checks.

With power ON, measure the voltage between base-emitter,
for a germanium transistor this will be 0.2 to 0.3 volts,
for a silicon transistor this will be 0.6 to 0.7 volts.
A darlington transistor will show about 1.2volts.

With power OFF, measure the resistance from base to emitter and base to collector. This test is usually best done with the transistor removed from circuit.
In this test the transistor should look like two diodes with a common connection at the base.
When using a digital multimeter set it to the "diode test" function (using the ohms range probably will not work):
A PNP transistor should show a low resistance from base to emitter and base to collector when the -ve probe of the meter is on the base and high resistance when the +ve probe is on the base.
A NPN transistor should show a low resistance from base to emitter and base to collector when the +ve probe of the meter is on the base and high resistance when the -ve probe is on the base.

When using an analogue multimeter set it to the 10/20kΩ range:
A PNP transistor should show a low resistance from base to emitter and base to collector when the +ve probe of the meter is on the base and high resistance when the -ve probe is on the base.
A NPN transistor should show a low resistance from base to emitter and base to collector when the -ve probe of the meter is on the base and high resistance when the +ve probe is on the base.

Caveats trips and traps.
These tests will not necessarliy find all problems, but will identify a transistor which is busted.
Dont forget to check from emitter to collector, the recesses of my memory tell me that I once found a transistor which checked out OK B-E and B-C but was short circuit from C-E.
Digital multimeters and simple analogue multimeters are different, on the ohms and diode test ranges, the polarity of the probes is opposite.
A transistor is not two diodes back to back, it just looks like that when tested with a multimeter.

JimB
 

kchriste

New Member
Forum Supporter
First let me point out that the scatter gun approach of testing individual parts is the hard way to fix anything.

As Eric pointed out, it is best to test the transistor out of circuit. This is because there are other components connected to the transistor which can give you deceiving results. For example, a 100 ohm resistor across the base-emitter junction of the transistor will make it look like the junction reads the same in both directions.
In addition to Eric's advice on out of circuit testing, remember also to ohm meter between the Emitter and Collector. It is possible to have a short between the Emitter and Collector and still have normal readings on the base-emitter and the base-collector junctions.
A better way of "testing out of circuit" is to build a simple beta tester with a couple of resistors, LED and a small power source. You could do this easily on a breadboard.

The best way is to test the entire circuit. To test the circuit, you need the schematic of the device. Then you apply power, look at the symptoms of the failure, go to that section of the circuit and measure the voltages on the transistors to figure out which ones are not working properly. There may be nothing wrong with the transistors at all and it may be another component, such as a capacitor, which is at fault.
 
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mneary

New Member
It's best to start with a schematic and symptoms as has been stated before. Keep in mind that transistors and resistors don't deteriorate with age like switches, connectors, and capacitors.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Keep in mind that transistors and resistors don't deteriorate with age like switches, connectors, and capacitors.
Not entirely true, some resistors do deteriorate with age, mostly high value resistors that tend to drift higher.

But as others have suggested, a circuit would be helpful, and try and fault find to locate the fault, don't just blindly test things.
 

btcg

New Member
Not entirely true, some resistors do deteriorate with age, mostly high value resistors that tend to drift higher.

Another consideration:

Resistors work on "smoke."

If you let all of the smoke out of them, they no longer work.

Always keep the smoke inside the resistor.
 

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