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

[Willen]

I am going to buy few old 2sc1971 rf transistor because i can't find new one. The transistor is used on Cordless Phone Set but set has been damaged now. So it is necessary to check the transistor c1971 to sure that it is in well condition or not. Tell me easy and quick idea please!

 

[DerStrom8]

Hi Willen.

You can always test it using the diode test setting on your multimeter. There should be a voltage drop of approximately 0.7 volts when the positive probe is placed on the base (pin 1) and the negative probe on the emitter (pin 2), but not the other way around. You should get a similar drop when you move the negative probe to the collector (pin 3), but not the other way around. Also, if you connect the positive probe to the collector and negative probe to emitter, or the other way around, you should get an "open circuit". If the transistor is damaged, it will probably read a voltage drop close to zero. That would probably be the easiest method.

Hope this helps!
Regards,
Matt

 

[Willen]

hm....voltage drop 0.7 mean...I can see 0.7 volt on the display of multimeter...Yes!

- Can I check all the transistor and RF transistors like this method? And will got simillar result?
- And if i have a analog multimeter without the function of diode test setting? (Actually I have analog)

 

[DerStrom8]

Only NPN and PNP transistors can be tested using this method, so if the RF transistors are one of those types, then yes--it should work. It's based around the fact that each PN junction in the transistor acts as its own diode. You can measure the drops to test if each diode junction is working properly. Here's a table of the correct connection values resulting from a diode test for both NPN and PNP transistor types:

______________________________________________________________________________
     | |   VBE    |    VEB    |    VBC    |    VCB    |    VCE    |    VEC   |
-----------------------------------------------------------------------------|
-----------------------------------------------------------------------------|
NPN  | |   0.7v   |    inf    |    0.7v   |    inf    |     inf   |    inf   |
-----------------------------------------------------------------------------|
PNP  | |    inf   |   0.7v    |    inf    |   0.7v    |   inf     |    inf   |

*NOTE* "inf" is an open circuit

Hope this helps

Regards,
Matt

P.S. If I have made any mistakes, please let me know. I think I did that correctly, but perhaps someone could check my work. Thanks!

 

[alec_t]

A transistor which fails the 'diode test' above is definitely dead; but one which passes the test may nevertheless be faulty if, for example, it breaks down at a voltage less than it should. That's not something you can check with just a common multimeter.

 

[Willen]

Last time I bought a new C1971 RF transistor. I tested its emitter to collector (emitter black and collector red probe of digital multimeter) using diode test mood; But I got around 600mV (like a diode forward drop). Rest of this unusual reading other testing are usual. Got fake then? Have you heared, found such situation?

 

[audioguru]

Last time I bought a new C1971 RF transistor. I tested its emitter to collector (emitter black and collector red probe of digital multimeter) using diode test mood; But I got around 600mV (like a diode forward drop). Rest of this unusual reading other testing are usual. Got fake then? Have you heared, found such situation?

If the base is not connected to a voltage then the transistor is conducting when it should not conduct so it is bad.

 

[Willen]

Some said about reverse protection diode on Mosfet but may not in transistor. Datasheet of C1971 has not included any internal diodes too. But it acts like a diode- shows no reading when red probe to emitter and black to collector but shows reading around 600 when black to emitter and red to collector.

I think I got fake brand! Oh my few dollars!!

 

[alec_t]

Are you absolutely sure you're measuring emitter-collector and not base-collector or base-emitter? Pin-outs can be ambiguous/unclear on some datasheets.

 

[Willen]

Are you absolutely sure you're measuring emitter-collector and not base-collector or base-emitter? Pin-outs can be ambiguous/unclear on some datasheets.

I am sure it has BEC pinout. All other junction tests are usual But only one test- (black to emitter and red to collector) shows such reading around 620mV drop.

 

[dr pepper]

Using a multimeter, as well as the diode v drop test from base to emmitter as mentioned, also check for a short between collector emmitter, from the threads it looks like there is.

A really good and super quick way to check the device is with a 'scope with a component test, I'm guessing you dont have access to one.

 

[Willen]

Ya, I don't have. If I bias the transostor with 1K Collector R and 10K pot for Base and if got variable voltage at collector, Can I say the transistor is good?

I heared that some RF transistor damage partially so works with voltage test but will not work with VHF, is it?

 

[dr pepper]

The chances are its ok if you do that test, however as you said its not certain, the only proper test is in circuit.

 

[large_ghostman]

Using a multimeter, as well as the diode v drop test from base to emmitter as mentioned, also check for a short between collector emmitter, from the threads it looks like there is.

A really good and super quick way to check the device is with a 'scope with a component test, I'm guessing you dont have access to one.

is that using the " octopus circuit?" or some other way?

 

[dr pepper]

Yep, there are various names for it, mine is a proprietry one, they stopped making them, must be noone repairing stuff anymore.

 

[large_ghostman]

it's on my must make list soon as i find a decent schematic i have a very old analogue scope that i would like to keep but it dosnt get any work so one of them connected to it would be great!

 

[rumpfy]

Hi Willen,
Can I say that these devices are seriously unusual, and I have found in the past, that measuring them like they are a 'normal' transistor, gives funny results. Base emitter junctions can look like a 300 ohm resistor for example.
The TO220 case on this type has the EMITTER as the pin2/mounting pad. Check the data sheet. This will be to isolate the output from the input pin.
As well, these devices are a bit fussy as to how the base emitter circuit is driven. Note that the power gain of the device is 10 dB and the input power into the base is up to 1 watt. The drive conditions will affect all the usual difficult to achieve stuff like harmonic distortion and efficiency.
Anyone using one of these transistors is dealing with a fairly competently designed power amplifier, and the real test of the device is to install it and measure the power output from the power amp. Any thing else is just guessing.
I note you ask these 'difficult to answer' questions but the use of this device requires some more competency. I hope you are not in over your head. I note too that the first posting was 12 months ago, so you have been battling this problem for some time.
hope this helps.

 

[unclejed613]

the proper pinout for a 2SC1971 is BEC, but there are counterfeits out there because this device has long been discontinued.... the only proper way to test the device is to test it with a beta tester, and make sure it has somewhere near the expected Hfe when connected properly... the counterfeits have C as the center pin and heat sink tab, and will not provide any gain with C and E reversed on the tester. the purpose for using the center tab as E is quite simple. the heat sink tab makes for a rather large stray capacitance. if this stray capacitance were in the collector circuit, it would make the proper design of an RF amp very difficult. using E as the tab simplifies things, since in most RF amps E is always grounded, and the stray capacitance between the tab and heatsink doesn't matter.

 

[Willen]

Main problem is I am going to buy same C1971 transistor from same shop. (Only a shop has these transistors.) I have no devices/tools to test the transistor except digital multimeter. I don't want to loss my dollars if they are damaged transistor. Also there is no tools to measure its input and output power to find out its hfe (ac gain). Multimeter is not able to test its hFE (DC gain).

 

[Willen]

Hi all,
look these bias, recently I tested the transistor like this way. I think this situation is transistor failure, isn't it? I think transistor won't work in Class C RF amp in this situation. Supply will short instead, isn't it?

Then should I have to say- it is 100% fake transistor?