Power Transistor Substitution

Discussion in 'General Electronics Chat' started by jamus, Jun 24, 2015.

  1. jamus

    Thread Starter Member

    Feb 11, 2013
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    0
    I am repairing a power supply and I need to replace a couple of transistors that test bad (shorted between all terminals).
    Unfortunately the transistors are no longer available and I can not find any datasheets. I am relatively new at this so I don't know where to look.

    TO-3 package, PNP
    part number:
    STi
    108-020k
    0046
    transistor.PNG

    Edit: another note would be that the diodes and resistors are on the board, so I know that there are none internally. EBJ 0.512, CBJ 0.432 for the remaining NPNs
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
    3,235
    619
    You could use a substitute with a current rating of 2-3X the max supply current.
     
  3. jamus

    Thread Starter Member

    Feb 11, 2013
    53
    0
    I didn't think it would come up, but this power supply is kind of different, it is for a scientific instrument that is basically an ultrasonic element. It's output is a 20khz sine wave. I know that the supply input current is 4A, but that is it. Specifications are unavailable for the output, although I know that it is high voltage and has a maximum output of 400w. Schematics are unavailable and the PCB is a little bit complex. I don't know the principles of operation.

    Here is a link to the users manual, if it matters:
    https://medschool.vanderbilt.edu/cdb-core-lab/files/cdb-core-lab/public_files/250-450 User Manual.pdf

    If I sketch out my circuit I suppose I may be able to determine the current by looking at the current rating of other components such as diodes.
     
    Last edited: Jun 24, 2015
  4. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    Possibly an NTE cross match.
     
  5. jamus

    Thread Starter Member

    Feb 11, 2013
    53
    0
    Here is the logo on the thing. Anyone recognize it? Almost looks like STmicroelectronics.
    logo.png

    Would opening the transistor package tell me anything?
     
  6. jamus

    Thread Starter Member

    Feb 11, 2013
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    0
  7. ian field

    Distinguished Member

    Oct 27, 2012
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    As long as the chip isn't too melted - NS used to publish transistor data books with pictures of the die layout in the back pages, these all had a reference number and a list of typical devices that used that fabrication pattern.

    Can't remember whether there's any NS databooks on archive.org - but it can't cost you anything to look.
     
  8. jamus

    Thread Starter Member

    Feb 11, 2013
    53
    0
    You are right, I came across this. I think this is what I will go with. Thank you.
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    There's still a bit of guesswork - you can probably narrow it down to 2 or 3 dozen type numbers, then look up the specifications and select one that looks like it can handle it.
     
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