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Thread: Compaq Portable 386 dead power supply

  1. #1
    Join Date
    Nov 2017
    Australia, NSW

    Default Compaq Portable 386 dead power supply

    In my previous post, which I posted in the wrong section and can't move, see here ( I had analysed the power supply and found out that transformer T2 was faulty. It has four legs with two of them joined (3 and 4). Thus there are two windings inside it. Legs 1 and 4 are one of the winding, which measures ok, but legs 2 and 3 show open circuit. This Elemac transformer dates back to 1988, so there are no specs or information on it. Thus it would be hard to find a suitable replacement. With that in mind, I decided on a bit of surgery, with a hope of finding out what kind of windings it had at worst and managing to fix it at best.
    T2 Windings (1).jpg The T2 Transformer as I had sussed it out!

    It is potted via some epoxy into a plastic housing, so I decided to remove the epoxy in a way as to not damage it. I first proceeded to remove the plastic housing by breaking it off, which left me with the transformer in it's epoxy glue.
    20191016_113303 (2).jpg T2 with the plastic housing removed, except the bottom part which slipped off without any issues.

    Next I put it into our kitchen oven and heated it up to 100 Celcius (212 F) as suggested by others on the Internet. I found that this wasn't enough, so I ended up heating it up to 130 Celcius ( 266 F) before the epoxy became soft.
    20191016_114311 (2).jpg The transformer in the oven, sitting on a piece of aluminium foil.
    20191016_120045 (2).jpg I used a meat temperature monitor to keep an eye on the temperature.

    Once it hit the 130C/266F temperature, I took it out of the oven and used welding gloves and a plastic spudger to break away the epoxy. I was careful not to damage the windings or the ferrite core. I managed to get it all clear of epoxy, except in the centre.
    20191016_122420 (2).jpg Breaking off the epoxy with the plastic spudger!
    20191016_122810 (2).jpg The end results after the oven treatment!

    It was by then cool enough that I took it to my workbench and further removed most of the epoxy from it. The epoxy was relatively easy to remove, despite being cold as it had undergone some change through being heated up.
    20191016_132409 (2).jpg Finally it's pretty clean and easy to inspect!

    As you can see it's a ferrite ring. The legs 2 and 3 connect to a copper foil, which is wrapped around the ferrite ring and are supposed to be joined by a small solder joint at the underside of the ring opposite the legs. I managed to put some solder on it and thus bring it back to life!
    20191016_133316 (2).jpg You can see at the bottom where the copper foils are supposed to be joined via solder.

    After verifying it was fine via ohm meter, I simply put some heat shrink, which has glue, over it and the bottom plastic piece. I didn't want to put any epoxy on it, just in case!!
    20191016_172858 (2).jpg Finally, ready to go into the power supply again!

    Once I had returned it to the power supply and attached some dummy loads to the +12v and +5v power outputs, I applied AC to the power supply. Lo and behold, it powered up for a few seconds, then a big Bang!! and I then turned it off.. Further investigation revealed a tantalum capacitor had blown it's stack in the output section! Once replaced, it happened again!! I replaced that one as well and the third one, just to be sure! Once done it powered up again and now it works fine! So it was time to return it back to the Compaq Portable 386 and see if it could bring the computer back to life! Yes, it all came good!
    20191017_171539.jpg The computer had Windows 3.1 installed on the HDD!!

    This Compaq is maxed out of memory, it has 10MB which is the most memory it can be installed with. There are 2MB on the motherboard and then the memory expansion card has two 4MB rows of memory chips, making it 10MB in total. It also has the back for additional two ISA cards. I took those two cards out, as they were network cards, one Etherent and one Token Ring?? Why the two, who knows! Now I just need to take the keyboard apart to clean and replace the coiled brittle cable and then clean the main unit.

  2. #2
    Join Date
    Oct 2016
    Burnley, UK


    Well done and thanks for the write up!

    I have 2 Compaq Portable III 286 units, one working and one with a dead power supply.

    I will have a go at fixing it one day and refer back to this post.

  3. #3
    Join Date
    Apr 2015
    Austin, Texas


    Quote Originally Posted by simmiv View Post
    I took those two cards out, as they were network cards, one Etherent and one Token Ring?? Why the two, who knows! Now I just need to take the keyboard apart to clean and replace the coiled brittle cable and then clean the main unit.
    Back in the days when this computer was in use, there were several networking standards that were not all compatible with each other. Token ring is not compatible with Ethernet and vice versa so having both cards would let you connect to more networks than with just one card.

    There were also different physical connection methods like 10Base5 (thicknet), 10Base2(thinnet), twisted pair, AUI and IBMs 8P8C, which is why older network cards have multiple connectors.


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