Elenco DT100K Transistor/Diode Tester help

Discussion in 'The Projects Forum' started by bswarm, Apr 19, 2014.

  1. bswarm

    Thread Starter New Member

    Apr 6, 2014
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    I'm having problems with the DT100K kit. Everything works until the transistor switch is placed in the PNP position, then nothing works. The diode test function works, the transistor test works in NPN, it even correctly identifies an NPN or PNP transistor if the switch is in the NPN position. But as soon as its switched to PNP, it's dead. I've tested the switches, solder joints, polarity of all parts, all those are OK. I would appreciate any clues on what to replace.
    Schematic attached...
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Can you provide good photos of both sides of your board?

    Taking GOOD photos of electronic boards is a bit tricky, as you get the best photos using indirect lighting. Direct flash gives a horrific glare. The best lighting is outside on an overcast day; the lighting is soft, even and no shadows. You can get similar lighting effects using a white sheet between your interior lighting and your subject board.
     
  3. bswarm

    Thread Starter New Member

    Apr 6, 2014
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    Photos of front and back, best as I could get. I've checked for solder bridges visually and with a DVOM, all OK. I also removed the (DPST) "PNP/NPN" switch for testing out of circuit (tested OK) because when it was in the circuit in the PNP position it tested as shorted across all 3 lugs on each side, but in the NPN position it wasn't.
     
  4. bswarm

    Thread Starter New Member

    Apr 6, 2014
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    Thinking I overheated a transistor during assembly, I removed all of them and carefully soldered transistor sockets (on the bottom of the board because they would interfere with the case otherwise) put all new transistors in, and it still only works in NPN or Diode mode. Still nothing in PNP mode. It tested good known NPN & PNP transistors, identifying PNP and NPN correctly, but only in NPN mode. Even the original transistors tested OK. The load control works, LED's and Diode polarity are correct, I even tried a new 555 IC. Hooked up my voltmeter to each of the leads and there is similar activity in both PNP & NPN modes, not sure what it should be though. Really stumped on this one.
     
    Last edited: Apr 20, 2014
  5. bswarm

    Thread Starter New Member

    Apr 6, 2014
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    0
    I've found that when it's switched to PNP mode, its a short circuit somewhere. The battery voltage drops from 9v to 1v at about .75amp drain. Any suggestions?
     
  6. bswarm

    Thread Starter New Member

    Apr 6, 2014
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    Using a bright flashlight on the top of the board while looking at the bottom with a magnifying glass, I found what looked like a line about half the width of a hair between two circuit paths. It was under the green coating, I scratched through the coating, removed the imperfection, and all works now. It was next to the negative battery wire solder point, not sure if it was my fault or a bad circuit board, but it's fixed now. Attaching the picture of the board with the bad spot circled in case anyone else runs into the same problem.
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    That's a "whisker".

    It happens when the Chinese guy got a hair on the artwork when exposing the PCB before etching the tracks. The result is a hair-thin copper track shorting things out, under the green coating.

    I've seen them curved, about 10 mm long. Probably an eyelash (not a whisker haha). But seriously, it happens all the time.

    Congrats on finding what can be a nasty fault, that was on the PCB when they made it.
     
    bswarm likes this.
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