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Thread: Troubleshooting Compaq Portable III power supply failure

  1. #11
    Join Date
    Feb 2016
    Santa Rosa, California


    I've replaced most of them, though still waiting on a couple to show up. More to follow.

  2. #12


    Well, I couldn't find any concrete rules about necro-bumping timeframes (I admit I did not try very hard due to being too sick to really care) and this thread seems to be adequately relevant to my current repair project to bump, so...

    I recently found my father's old Compac Portable III languishing upstairs outside of his office inside of its carrying bag. He says he got it right about when they came out, got the expansion unit, expanded the memory, got the 80287 math coprocessor, and got some networking or serial expansion card. Says he bought it for like $8k, which disturbs me because that's a lot even now and he basically just dumped it upstairs. I understand, as it's very obsolete, but at the same time...

    Anyway, I plugged it in to see if it would boot up. I could hear it tick a relay repeatedly and it turned on its fan, so I assumed it was a power supply issue. I got the power supply out and tried it again out in the shop directly in front of my father's "smoking desk" so if I got electrocuted to death I wouldn't be in the abandoned upstairs offices. The power supply was able to run the fan and weakly trigger the relay only. I removed all of the electrolytic capacitors (except for the weird axial paper/polyester/whatever cap) from the board after labeling each by their accompanying silkscreen numbers, as I have a Fluke that can do capacitance measurements but I didn't think it can do so accurately with the components in place. Almost all of these aluminum caps tested at slightly over their labeled rated uF values, which my reading suggests does not indicate that they are actually healthy.

    I honestly intended to just replace all of the electrolytic caps because I know that they have a somewhat limited lifespan and the electrolyte is not great for PCBs, but I have run into an unexpected problem: apparently these capacitors are not in physical sizes that places like Digikey and Mouser typically carry. The largest ones are so far where I'm getting stuck: there's a 6.3VDC 8200 uF that's 25mm diameter and 55 mm long, and another one that's something like 16V or 20V 1200 uF and 15mmx35mm.

    I was wanting to get replacements from Nichicon, Rubycon, or TDK. TDK carries similar to the first one rated for a higher voltage and I think in a bigger can. My father (an electrical engineer) said that getting a cap rated for a higher voltage should be fine, but at the same time he doesn't want to talk about this at all and he doubts me when I say that electrolytic capacitors can fail by losing electrolyte over time due to not being hermetically sealed.

    I will get all of the labeled ratings for the electrolytic capacitors, their dimensions, and also post the silkscreen labels for the wire connections (voltage and polarity mostly) as well as the wire colors for future reference or something (maybe tomorrow; I have written it all down and work is slow enough that I can also get out the digital calipers and measure lead spacing). Mostly, if anyone knows of where else I could try looking for these oddball sizes, I would really love constructive input. I would prefer that they be of high quality and not new-old-stock, since capacitors are pretty cheap as long as you're not buying hundreds or thousands of them.

  3. #13


    The size doesn't really matter. As long as the voltage and value are correct and they have enough space then you can just bend the pins on the new ones to fit.

  4. #14


    IIRC, even the voltage can be higher than the original.
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  5. #15


    Uh, wow, I didn't think that post was going to get approved. Thanks for the input!

    I fixed a Dell LCD monitor recently by replacing all of the electrolytic capacitors, but there was one problem: one capacitor replacement was smaller. This was also the "problem" capacitor, as it turns out, because my issue with the monitor backlight cutting out and turning back on persisted until I replaced that cap with another (slightly larger than the original). This suggests that going smaller than the original will cause problems, but I wanted to make sure that larger wouldn't also be an issue. Thankyou for clarifying there!

    My father also said the voltage can be higher without issue.

    The final capacitor that I've been looking at is a Sprague 430p metallized film capacitor with a 400V rating and 1.0 rating at +/- 20% tolerance. It doesn't say 1.0 what, but I think it's uF. I found a datasheet for the 430p series that does not include parts with 20% tolerance. The part number that I think is the most similar is 430P105X9400. If you look at its datasheet, which I don't seem to have on this laptop, it's about 12mm diameter by 45 mm long with axial terminations. The datasheet also specifies the rating as being DC with no mention of AC at all.

    I bring this capacitor up specifically because I read somewhere that if the capacitor is involved with the AC mains, the rating has to be special or something. I can't tell if the capacitor interacts with AC that's not rectified, but anyway, I found KEMET #C4GAMUC4100AA1J seems to be close enough in size and rated a "little" higher on DC voltage. Assuming y'all think that will work out fine and that it's not going to be some special snowflake cap, I'll happily move forward with this project and we'll probably find out that the plasma display board needs new capacitors as well, or all of the tantalums will catch fire.

    I did end up having to go with an 85C 63VDC or 80VDC capacitor instead of the gigantic 105C 6.3VDC 8200 uF monstrosity. I don't know if the conditions in the PSU will make me regret this. We'll see.

    My other idea to replace the power supply was to get a modern replacement that's capable of fitting inside the housing for the old one, and get another power supply to take care of the 200VDC that the plasma display seems to call for in the user manual I found.

    Specs and such for the caps etc. on the PSU are at my house on my desk, I hope I remember them at lunch.

  6. #16


    In general, modern electrolytic caps are more space-efficient and higher quality than most older caps. You can usually get the same value at a higher voltage rating in the same physical space, which is always a plus. Designs that run electrolytic caps really close to their rated voltage are asking for trouble, IMO. Replacing a non-standard value with the nearest standard value is usually OK too (you can replace a 50uF with a 47).


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