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Thread: Compact Portable dead power supply debug and fix

  1. #1
    Join Date
    Apr 2019
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    Ottawa, Canada
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    Default Compact Portable dead power supply debug and fix

    Hi guys,

    I often find help on these forums without needing to ask any questions. I thought I would post how I brought my Compact Portable back to life. I received the unit from a friend at work, he had two and only wanted to keep one. I didn't see any other post pointing to this specific failure so here it goes.

    My debug process

    First thing I did was to try and power the unit up. No luck, the fan spun up but nothing on the display. Since the fan is run directly from 120V AC, it makes sense that it would work even if the power supply wasn't working.

    I opened the case up and noticed no red LED on the motherboard.

    I unplugged all expansion cards and the hard drive / floppy. Still no red LED.

    I tested the +5V and +12V floppy power cable and they both read 0V.

    I disconnected power to the CRT, still nothing.

    Disconnected motherboard power, still read 0V on +5V and +12V rails.

    At this point only the power supply was connected, so obviously something in it had gone. I followed the service manual to remove the power supply board.


    Debugging the power supply

    I checked the resistance to ground from the +5V and +12V rails, the 12V had a short! (0.1 Ohms or something).

    I did some research here and saw a number of people had the tantalum capacitors go bad on them. I tried using an ESR meter on the caps, but I really have no idea what I am doing with that thing yet. I removed the two 225 tantalum capacitors and the short was still there.

    There are also two 3300uF and one 470uF electrolytic caps, removed them and saw they had bulged but the short was still present.

    At this point there was not much left on the 12V output, some kind of coil and a TO-220 diode with a heatsink marked USD835.

    I removed the diode, bingo! The USD835 had gone bad. Not the 12V rail had about 70 ohm resistance to ground, which I figured was OK since there is a coil there.

    IMG_20190709_004944.jpg


    Repair

    At this point I enlisted help from another co-worker and ordered what we think is a compatible part (USD835 was not available anymore). Here is the replacement part I used for the USD835: MBR1635G (ON Semiconductor).

    I also decided to order new 225 tantalum caps, and all new electrolytic. Pretty much all of the electrolytic caps had bulged to some degree, so replacing them all made sense.

    Replaced all caps and the bad diode, connected 120V to the power supply and now I had +5V and +12V.

    I re-installed the power supply into the case and connected the motherboard / video card. Success! I had the little green cursor display.

    Ther hard drive spins, and the floppy does attempt to seek but I don't have a 5 1/4" boot disk and the hard drive seemed to time out.

    I ended up using a Gotek with FlashFloppy to get booted into dos 3.20 and that is where I am today.

    IMG_20190714_105054.jpg

  2. #2
    Join Date
    Aug 2009
    Location
    Oslo, Norway
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    1,250
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    Default

    Well done! Reading about successful repairs is enjoyable too. Thanks!
    Torfinn

  3. #3

    Default

    Quote Originally Posted by Cmolson View Post
    Hi guys,


    There are also two 3300uF and one 470uF electrolytic caps, removed them and saw they had bulged but the short was still present.

    ...Here is the replacement part I used for the USD835: MBR1635G (ON Semiconductor).

    I also decided to order new 225 tantalum caps, and all new electrolytic. Pretty much all of the electrolytic caps had bulged to some degree, so replacing them all made sense.
    I've had similar issues in getting mine working; I've replaced the 225 tantalums, but the voltage drops from what it should be almost immediately after flipping the switch on the power supply (with nothing else connected).

    Do you have a recommendation or record of which electrolytic caps you replaced? This is not my area of expertise and there are a lot of options online. I'm putting in a Mouser order for the diode you mention, and figured I might as well get the electrolytic caps as well.

    Thanks!

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