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Thread: Compaq Portable - Help

  1. #1

    Default Compaq Portable - Help

    Hi to everyone. I am searching this kind of transistor (photos). I dont know the type of transistor and i cant find any info. Can somebody help me? This Psu is from a Compaq portable . Do you know if schematics are available for compaq portable?
    Thank you in advance.

  2. #2

    Default Service manual

    Here found somewhere else on this forum. Not very helpful for Q3 though. Lists it as a 508 with no replacement data. Hope it helps with the detective work.

  3. #3


    You could try purchasing the SAMS computerfacts for the compaq portable. They will most likely have a schematic for the PSU in them.

    I had a look on SAMS website and they still offer it for $22. Link here
    System 80 Expansion Interface located! Thanks to all who helped out and the good people in the NZ vintage computer forums!

  4. #4
    Join Date
    Feb 2009
    Chattanooga, TN - USA
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    Ok, so no clue on the cross, apparently I chose poorly last year, which is why my repair failed.

    It's labeled just 508.

    Here's a link to the service manual.

    Last edited by dabone; February 15th, 2019 at 02:52 PM.

  5. #5


    Based on the schematic posted above it's just a BU508 power transistor. Still readily available.
    System 80 Expansion Interface located! Thanks to all who helped out and the good people in the NZ vintage computer forums!

  6. #6


    Hey guys. Thank you very much for the support. I was looking for this pdf but i couldn’t find it. Thank you very much. Before i read your answers i replaced the transistor with BD139 and i also replaced the diode next to it which was bad. After i replaced these components the psu worked with all voltages. Unfortunately one tantalum capacitor on cga exploded so i have to replace all these capacitors. Now with this pdf i will examine better the psu and i ll check all the computer.
    Thank you again for the support all of you. !!!!!

  7. #7


    Hello, I have a problem with an identical power supply. The power supply starts and has the correct voltage for 20-30 seconds after which it power off. If I power on the power supply without load , the voltage tend to remain stable, but as I add a important load(big HDD) voltage begin to fall and the power supply stops. I changed the capacitors on the output but in vain.Some suggestions?

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