Transistor testing - what to look for

Discussion in 'General Electronics Chat' started by markkelsall, Jan 7, 2015.

  1. markkelsall

    Thread Starter New Member

    Jan 7, 2015
    2
    0
    Hi

    I am currently trying to repair a 1980 Luxman amplifier which has blown or damaged transistors on one of the power boards.

    In particular I am trying to ascertain if some of the smaller trannys are ok or not. I have already removed the main power and driver transistors which I have replacements for.

    The transistors are 2SC1940's, 2SA915's and 2SC734's.

    Now dead shorts i can detect and understand with my DMM on diode mode, but I am unsure what to expect otherwise. I am testing the trannys in circuit.

    So far I have checked the readings against the 'good' board (Rt blown, Left OK) and there are differences i.e. some readings are higher and some lower on the 'bad' board

    What should good look like for B to C, B to E, E to B, C to B etc. as read from the DMM

    Hope you can help!
    M
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,151
    3,058
    Does your DMM have a diode tester? The base-emitter should behave as a diode. No reading in reverse, ~650mV in the B-E direction. I'd also be very suspicious of a short in both directions across any C-E.
     
  3. markkelsall

    Thread Starter New Member

    Jan 7, 2015
    2
    0
    Hi

    Yes I have been using a DMM in diode mode. The trannys are still in circuit though so I am getting voltage drop in reverse in some cases. This is consistent in both boards in the main. however. Where diff I will prob replace. And yes I have found one short across CE on one tranny.

    Thanks
    M
     
  4. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    You can get in circuit transistor testers - otherwise its best to isolate it from the circuitry, with power transistors its often convenient to unsolder 2 of the leads and make sure they're not touching the edges of the pas while you make the tests.

    everyone else has already mentioned using a DMM diode test function to check the 2 junctions - also check leakage from collector to emitter, any leakage is a fail.

    If you encounter any MOSFETs - the gate capacitance can store a charge causing it to show a false leakage reading - short the gate to source while checking for drain leakage.
     
  5. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
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    ordinary transistors usually check like two diodes with the base the common lead. npn and pnp test reverse of each other. which ever lead combination works from e to b, leave the lead on b, and check to c, should be pretty close to the same reading. and most do not read anything between c and e in either direction.
     
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Soldering and unsoldering alot can soon damage a board and the transistors or equivalents are cheap enough.

    If you unsolder put a new, known good one back and have done with it.

    You can also test by injecting a signal at the beginning and following through the stages, perhaps at reduced rail voltage. You do not need the output transistors in circuit for this.

    Have you a circuit diagram, or can you post the actual amplifier model number, I'm sure someone will oblige.
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,151
    3,058
    +1
    Same advice for electrolytic capacitors. If you've bothered to remove it, just replace it.
     
  8. ramancini8

    Member

    Jul 18, 2012
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    In circuit testing for junction: step1; monitor the collector voltage and short the base-emitter junction. The collector voltage will rise if the transistor was conducting bias current. step2; calculate or measure the resistance of the circuit driving the base, then add parallel resistance of similar value and watch the collector voltage drop. I usually just use a 1K resistor from Vcc to the base. Now you know if the transistor is functioning for DC. step3; Touch you finger to the base through a 1K resistor and observe with a speaker or scope. If you get an increase in noise everything is OK.

    Out of circuit is simple with an ohmmeter. For NPN transistors put the + lead on the base touch the emitter lead with the - meter lead. A good transistor will measure about 20 to 100 ohms. Reverse polarity on the meter and it should read an open circuit (reverse meter polarity for the next test). Leaving the + lead on the base touch the collector lead with the - meter lead. A good transistor will measure about 20 to 100 ohms. Reverse polarity on the meter and it should read an open circuit. Now measure across the base to collector leads and you should get an open circuit regardless of the polarity setting. Reverse the polarity settings for PNP transistors.

    Remember, a junction transistor is simply two back to back diodes with a short base region.
     
  9. studiot

    AAC Fanatic!

    Nov 9, 2007
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    One note I think that diode connected small signal transistors has started to become popular by the day of Luxman.

    so one or more of these transistors may be deliberately shorted when in circuit.

    This is why a circuit diagram is essential.
     
  10. RichardO

    Well-Known Member

    May 4, 2013
    1,238
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    Keep in mind that there is one part in any electronic assembly that can _never_ be replaced -- the PCB.
     
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