quick question about transistors

[09mike69]

i checked the transistors in a circuit board of a faulty subwoofer and found three that gave no measurement at all, ive taken one out of the board and it still has no measurement, is this broken?
also three trannys seems like a lot to be faulty, well to me in my very limited experience, all other caps and transistors gave readings. unfortunately i dont know what caused the sub to fail, it was broken when i got it. if anyone can shed some light on this please.
cheers, mike

[Arkless Electronics]

i checked the transistors in a circuit board of a faulty subwoofer and found three that gave no measurement at all, ive taken one out of the board and it still has no measurement, is this broken?
also three trannys seems like a lot to be faulty, well to me in my very limited experience, all other caps and transistors gave readings. unfortunately i dont know what caused the sub to fail, it was broken when i got it. if anyone can shed some light on this please.
cheers, mike

How are you measuring these transistors? We need more info to ascertain that you are measuring them properly before giving much further in the way of advice.
I suspect that you are not measuring them correctly as it's far more common for a transistor to fail short circuit than open circuit so to have 3 like that is virtually unheard of!
For a start, you shouldn't get any reading from a good transistor if using the Ohms (resistance) ranges of a digital meter. You need to use the "diode check" function which most have. This will usually have a diode symbol to select it and usually just one range/setting. Expect between 0.530 and 0.780V (diode check displays voltage drop) from base to collector and from base to emitter and no reading from emitter to collector if the transistor is good. The polarity of this test tells you if the transistor is NPN or PNP and you should only get a reading when polarity is correct ie red lead on base for NPN and black lead on base for PNP with digital meters. I've highlighted the digital bit as it's all a bit different if using an analogue meter!

[09mike69]

How are you measuring these transistors? We need more info to ascertain that you are measuring them properly before giving much further in the way of advice.
I suspect that you are not measuring them correctly as it's far more common for a transistor to fail short circuit than open circuit so to have 3 like that is virtually unheard of!
For a start, you shouldn't get any reading from a good transistor if using the Ohms (resistance) ranges of a digital meter. You need to use the "diode check" function which most have. This will usually have a diode symbol to select it and usually just one range/setting. Expect between 0.530 and 0.780V (diode check displays voltage drop) from base to collector and from base to emitter and no reading from emitter to collector if the transistor is good. The polarity of this test tells you if the transistor is NPN or PNP and you should only get a reading when polarity is correct ie red lead on base for NPN and black lead on base for PNP with digital meters. I've highlighted the digital bit as it's all a bit different if using an analogue meter!

thanks for reply. im using ohms, dont have the diode function. ive tried all variations, POS middle NEG outer, NEG middle POS outer, etc etc, and no reading.. on the board next to the one i removed there is an identical transistor C2240 BL5H and it gave a reading when tested.. thanks again for taking the time to reply.
regards, mike.

[Arkless Electronics]

thanks for reply. im using ohms, dont have the diode function. ive tried all variations, POS middle NEG outer, NEG middle POS outer, etc etc, and no reading.. on the board next to the one i removed there is an identical transistor C2240 BL5H and it gave a reading when tested.. thanks again for taking the time to reply.
regards, mike.

There's often other parts connected that will give a reading when the transistor is on the board. You need a meter with the diode check function.

[09mike69]

There's often other parts connected that will give a reading when the transistor is on the board. You need a meter with the diode check function.

ive got a MM with diode function and tested the transistor i took off the board and it seems to be working, thanks for that advice. i will remove them from the board to test them. am i correct in thinking that when i test them if the readings differ significantly on opposite legs that it is faulty. thanks again, mike

[Arkless Electronics]

ive got a MM with diode function and tested the transistor i took off the board and it seems to be working, thanks for that advice. i will remove them from the board to test them. am i correct in thinking that when i test them if the readings differ significantly on opposite legs that it is faulty. thanks again, mike

For a small signal transistor such as the 2SC2240 you'll probably get reading around 0.670 - 0.730 or so. It doesn't matter if they are different.

[09mike69]

For a small signal transistor such as the 2SC2240 you'll probably get reading around 0.670 - 0.730 or so. It doesn't matter if they are different.

hi, yes it measured 660 and 665 that was with the POS on the right pin and NEG on centre and left pin. it only gave a reading on this configuration, but thats normal for a working transistor, isnt it?, if i get a reading with it reversed doesnt that mean its faulty?. i will remove the others individually to test them..thanks again for taking the time to reply.
cheers, mike.

[09mike69]

hi, yes it measured 660 and 665 that was with the POS on the right pin and NEG on centre and left pin. it only gave a reading on this configuration, but thats normal for a working transistor, isnt it?, if i get a reading with it reversed doesnt that mean its faulty?. i will remove the others individually to test them..thanks again for taking the time to reply.
cheers, mike.

the right pin with the flat face facing upwards...

[09mike69]

there are two b647 transistors, one tested 565-618 the other tested 590-1690. both tests where done with the POS on the centre pin and the NEG on the outer pins. could the latter be the faulty transistor?.
thanks, mike.