How can I check 2sc1971 transistor quickly to be sure that it is damaged or not?

[rumpfy]

Willen,
can I suggest you buy a new cordless phone.
My post #17 was pretty clear that you cannot really test these things. If you have no test gear and its only a cordless phone then take the least cost way and buy a new phone. If you want to get involved with this type of semiconductor, then buy an old 2 way radio (ex taxi etc), and try to get something working that has some value.
I'm only trying to be helpful.
If you look carefully at all the posts, most of them aren't helping you; they are only crosstalk between others discussing your problem but not solving your problem in terms that helps YOU.

 

[audioguru]

Usually a class-C transistor has a resistor from base to emitter to turn it off, not an open base.

Since your transistor conducts all the time like a diode then it is defective.

 

[alec_t]

Try shorting the base to the emitter and then checking if the collector to emitter path is still conducting.

 

[audioguru]

The transistor probably has high leakage from collector to base. Shorting the base to the emitter covers up the problem.

 

[Willen]

Usually a class-C transistor has a resistor from base to emitter to turn it off, not an open base.

Since your transistor conducts all the time like a diode then it is defective.

Did you mean 'Internal base emitter resistor'?

What you mean ''Shorting the base to the emitter covers up the problem"?

 

[Willen]

An old cordless phone where the same value (C1971) transistor was used as an output transistor and it has a 22 ohms Base to Emitter external resistor on its PCB. So I guessed that there is no internal Base to Emitter resistor inside of the transistor (I don't know more )

Look at the attachment, I tested few conditions. What it means?

 

[alec_t]

Looks like there's a collector-base short or, as AG mentioned, that junction has high leakage.

 

[rumpfy]

try this test set up.
Ic MUST be zero
Ib must be zero
IF not then 2sc1971 is us.

 

[mvs sarma]

you better try to salvage from another faulty phone or some scrap if sold on scrap market..
The local markets use any device that had the similar pin out and re print and sell as 1971. Thus
it might not work or if works very low performance. mostly you get fake ones.

 

[Willen]

try this test set up.
Ic MUST be zero
Ib must be zero
IF not then 2sc1971 is us.

Did you mean to say- current throught these points (Ic, Ib, Ie)? OK will do and will say.

 

[rumpfy]

exactly.

 

[Willen]

try this test set up.
Ic MUST be zero
Ib must be zero
IF not then 2sc1971 is us.

Hi again,


Some said transistor might has reverse diode (Anode connected to emitter?) inside transistor but it acts as forward diode (anode connected to collector) so positive supply directly goes to emitter/Gnd throught the diode

I did almost all simple tests-

-- I shorted emitter to base, but still transistor is conducting.

-- I biased the transistor with collector resistor- 1k and variable pot 50k for base. I measured collector voltage but I got 0.14V to 0.70V up-down by changing pot resistance. Then pot burnt out badly. I used fixed base resistor. 5k gave 0.14V, 10k gave 0.20V and 47k, 100k, 4.7M or no resistor gave 0.7V.

-- I made a circuit suggested by rumpfy (at last) but used 1k collector resistor and another was 220R and got

Ic=6.2
Ib=3.6
Ie=2.6

What it mean?

 

[rumpfy]

The purpose of the test circuit was to reverse bias the base emitter junction.
if you are seeing collector current with the circuit I drew, I think you can say the transistor is US.
The problem with these transistors is that they dont behave like normal transistors and I can tell you that normal sort of ohm-meter tests can be almost useless. one of the problems with these things is related to a partial failure of the transistor structure such that it just does not do what it has to do. As an RF amplifier, these things are base driven to almost saturation and then are cut off, and the actual structure of the device is changed permanently by failures in the material due to heat and excess voltage gradients.
these things are expensive and troublesome, and semiconductor designers go through many iterations to get these devices to be robust and fail proof.
I can't really help you more than I have, except to add that I have boxes full of high speed/ high power germanium switching transistors that dont switch properly because they are damaged. Ohm-meter testing doesnt really show up an obvious problem with them.
It is from this background that I offer my help.
I think your 1971 is cactused.