I am following this forum now for some time and I am amazed how many people with little or no knowledge or experience dare to open and work on discrete (that is with transistors) audio equipment.
When it comes to transistors many people can recognize it as a transistor but assessing if the part is not defective is more of a challenge. A happy few have a transistor tester and some even know how to interpret the readings.
If not, using a multimeter to measure the 2 diode junctions is not sufficient to assess the correct working. This method below I learned over 40 years ago and never failed on me.
Note on lead polarity: For analog, not-electronic multimeters the positive voltage is on the common (black lead). For digital multimeters I have no clue. I don't know if that is even consistent for all types. Just find out using a diode.
Note on Hfe: different transistor types have different Hfe. Small signal transistors like BC550 have quite some gain. So pressing on the lead ends might already give the reading. For larger transisitors like TIP42 you have to press on the flat sides of the wires. For TO3 transistors, put your entire finger tip on the housing and the base stud. That is why those large transistors came in that case: more area to put your finger during testing. Just kidding!
Note on collector and emitter leads. Fortunately there are 42 different standards for lead position on the various transistor housings. Transistors, especially small signal transistors have gain when you swap collector and emitter leads and do the described test. If you are not sure what is C and what is E, test the transistor in both ways. The measurement showing the highest gain has is connected correctly.
Note on meters: Analog meters are preferred over digital meters for this test and in general. Even the cheapest functional analog meter gives you better information about what you measure compared to digital. If this becomes a discussion, start a new thread.
For power transistors the Hfe is quite low. But because the collector current is also very low during testing generally Hfe > 10 and you will get a usable test reading.
When it comes to transistors many people can recognize it as a transistor but assessing if the part is not defective is more of a challenge. A happy few have a transistor tester and some even know how to interpret the readings.
If not, using a multimeter to measure the 2 diode junctions is not sufficient to assess the correct working. This method below I learned over 40 years ago and never failed on me.
- Connect the lead of your multimeter carrying the positive voltage to the collector and the negative to the emitter for an NPN transistor. The other way around for a PNP.
- The reading should be (close to) infinity for Si and "large" for Ge
- Moist the tip of your finger just a little bit. I say "moist", not "wet". Moist like you want to turn a page.
- Press your moisted finger against the collector and base
- The resistance reading should drop to a few hundreds or few tens of kilo ohms.
Note on lead polarity: For analog, not-electronic multimeters the positive voltage is on the common (black lead). For digital multimeters I have no clue. I don't know if that is even consistent for all types. Just find out using a diode.
Note on Hfe: different transistor types have different Hfe. Small signal transistors like BC550 have quite some gain. So pressing on the lead ends might already give the reading. For larger transisitors like TIP42 you have to press on the flat sides of the wires. For TO3 transistors, put your entire finger tip on the housing and the base stud. That is why those large transistors came in that case: more area to put your finger during testing. Just kidding!
Note on collector and emitter leads. Fortunately there are 42 different standards for lead position on the various transistor housings. Transistors, especially small signal transistors have gain when you swap collector and emitter leads and do the described test. If you are not sure what is C and what is E, test the transistor in both ways. The measurement showing the highest gain has is connected correctly.
Note on meters: Analog meters are preferred over digital meters for this test and in general. Even the cheapest functional analog meter gives you better information about what you measure compared to digital. If this becomes a discussion, start a new thread.
For power transistors the Hfe is quite low. But because the collector current is also very low during testing generally Hfe > 10 and you will get a usable test reading.
