NPN transistor good or bad

[johnfin]

I have a transistor that passed on 2 of my transistor testers BUT...... failed on the old digital VOM test. It failed on the test where you go positive on collector and neg to emitter - should read O/L for NPN, doesnt, it reads .5v. Transistor is 13005D. Any thoughts

[budm]

Re: NPN transistor good or bad

How about pos on E and neg on C?
Any thing connected to the Base while testing?
How about resistance in both direction?

[redwire]

Re: NPN transistor good or bad

MJE13005D Unisonic Taiwan versions have an internal C-E diode, I think that is what you are seeing.

[RepairNewbie47]

Re: NPN transistor good or bad

With the price of transistors these days , my rule is usually "if in doubt change it out" Normally I dont used a transistor tester i just used a multimeter and if is passes there I call it good , I certainly wouldnt use 2 or more different testers because it can muddy the waters as you are seeing . My personal opinion is that there is a diode in it between collector and emitter . This would give you the results you are seeing .

[budm]

Re: NPN transistor good or bad

I have a transistor that passed on 2 of my transistor testers BUT...... failed on the old digital VOM test. It failed on the test where you go positive on collector and neg to emitter - should read O/L for NPN, doesnt, it reads .5v. Transistor is 13005D. Any thoughts

OP said he has his old Digital VOM positive lead to Collector and Negative lead to Emitter, so for internal diode to conduct, his meter may have reverse polarity output Voltage source to make the diode conducts. I wonder what kind of old Digital VOM he is using.
That may have caused wrong results while troubleshooting this thread: https://www.badcaps.net/forum/showthread.php?t=65349

[stj]

Re: NPN transistor good or bad

the letter "D" in the part number is the giveaway, anybody who worked on crt tv's / monitors knows about these type of transistors.
BU508(A / D) being the most common.

[johnfin]

Re: NPN transistor good or bad

yep, the diode was causing the reading. all is good.

[bujar66]

Re: NPN transistor good or bad

You are compliantly right.

[Andrew F. Ali]

Re: NPN transistor good or bad

I have an analogue meter for testing Bi-polar transistors and diodes. With the transistor out of circuit and the meter set at Rx1000 placing the RED lead on the Base and the BLACK lead on the Emitter and then on the Collector if I get a reading near to 0 ohms and then switching the RED lead to the Emitter and Collector with the BLACK lead on the Base and get no reading or infinity I can safely say the transistor is good or bad. I would also make a check across the Emitter and Collector in both directions and if it is good it would read open provide of course there is no diode across the Emitter/collector junction.

[redwire]

Re: NPN transistor good or bad

With three leads, there are six possible combinations to test.

Transistors usually fail open/short on one junction so it's easy to find.
But some transistors are a bit leaky or slightly damaged, and diode test looks fine but ohms reveals a damaged, leaky junction.

I use diode-test, do all six, and then repeat on ohms. So that's 12 readings for a thorough test. Important to do when fixing audio amplifiers.

[RepairNewbie47]

Re: NPN transistor good or bad

With three leads, there are six possible combinations to test.

Transistors usually fail open/short on one junction so it's easy to find.
But some transistors are a bit leaky or slightly damaged, and diode test looks fine but ohms reveals a damaged, leaky junction.

I use diode-test, do all six, and then repeat on ohms. So that's 12 readings for a thorough test. Important to do when fixing audio amplifiers.

Thats why i just tend to shotgun replace them if they are fairly old . As you say they can test good sometimes and still be knackered . If I`m working on audio gear especially I will just go ahead and replace the lot .

[Andrew F. Ali]

Re: NPN transistor good or bad

Thats why i just tend to shotgun replace them if they are fairly old . As you say they can test good sometimes and still be knackered . If I`m working on audio gear especially I will just go ahead and replace the lot .

Agreed. With audio equipment especially. Transistors can become 'noisy' and still show as good on diode and ohms tests.

[redwire]

Re: NPN transistor good or bad

Sony professional broadcast/studio service manuals advised to always replace the transistor ahead of the last bad transistor found.

Even though it tests OK, it can be weak or damaged and will fail. For the old techs that can remember, this was in the 1980's and stopped a lot of repair-warranty returns.

[Andrew F. Ali]

Re: NPN transistor good or bad

sony professional broadcast/studio service manuals advised to always replace the transistor ahead of the last bad transistor found.

even though it tests ok, it can be weak or damaged and will fail. For the old techs that can remember, this was in the 1980's and stopped a lot of repair-warranty returns.

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