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Help! Compaq Portable Plus suddenly died


Experienced Member
Mar 18, 2022
Hello everyone,

Yesterday I dug my Compaq Portable Plus out of storage and tried turning it on. I heard the fan start spinning, but the floppy and disc drive didn’t engage like they used to, instead they sort of “tried” to for half a second before failing. I tried a few more times, waiting about 15 seconds in between, but it wouldn’t boot.

While the unit was still on I spun it around to try and get the back cover off and then suddenly it just shut off. Once the cover was off I could see that the power supply had actually become dislodged. I suspect that the centrifugal force of me turning the computer pushed the top row of pins on the power supply against the computer’s metal chassis, shorting it out, but I can’t be sure of this.

I don’t really have the expertise to diagnose the problem on my own. I do really love this machine and it holds a lot of sentimental value for me. I would appreciate any help anyone can offer.
Have you had the tantalum capacitors replaced on the mainboard and cards? The portable and portable Plus are norotorious for having tantalums short out. And it can happen at any time.
Remove every card and install one at a time. Keep your eye on the LED on the board and if it stays on, that means the card is good. If it shuts off right away, replace the caps on said card or every card. Even the motherboard and PSU.

Which reminds me. I should recap my Portable's PSU.
Thank you all for replying.

Verault, I haven't replaced anything in my Compaq (not least of all because it's such a pain to take apart!). It's never had any problems before apart from some minor geometry issues on the CRT, but I was fine with that.

Retro-pc-user, I'll try what you suggest but I have a feeling I won't see any LED activity. My beloved Compaq Portable seems to be completely dead now, not even the fan will come on :(

I think the power supply shorted against the metal chasis, I just hope it didn't fry anything else...
my money is on a short.. most likely tantalums.. as i mentioned the portable is nortorious for them.. they all need to be replaced.
Update: I took off the back cover and as I feared the motherboard LED doesn't light up at all with or without the cards inserted. I don't think any power is reaching it.

If it is a short that's causing all this, how would I go about locating it?
So I tested the +5/-12 volt rails with a multimeter like in this video, but unlike him I got absolutely nothing on my meter...
I'd say order capacitors based on the values and order a capacitor kit for the power supply. Console5 has a cap kit for the Compaq Portable 1 systems and I believe that includes the Plus model since they share the same chassis and boards as long as the PSU model matches 00003-001 Rev E.

Note: recap the motherboard, the cards (Compaq VDU CGA, floppy controller card, and any other Compaq branded anything), and the PSU.

I honestly haven't a clue about how to check for shorts and usually, removing a cap will determine that in some cases, but not all cases.
So after about two hours of dissassembly I finally have my prize: a liberated power supply! It seems it was only actually fixed to the board in one spot, the bottom left (from the back), and the other three corners instead had plastic stoppers to prevent the pins on the power supply from touching the metal chassis… in my opinion a very flawed and frankly lazy method of mounting. The plastic stoppers on the top of my power supply appear to have come loose which is what allowed the pins of the PSU to make contact with the chasis.

Anyway, there is some black discoloration in two spots along the top portion of the power supply (which is roughly where I believe contact with the chassis was made). These might always have been there, but I have a feeling they’re the byproduct of some kind of short I’m no doubt unqualified to identify on my own 😅

Does anyone know what might be the problem? Alternatively, I’m open to replacing the power supply outright. There used to be an ATX adapter available for the Compaq Portable Plus, but it seems to have gone out of stock on eBay.


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I've been looking into fabricating some kind of replacement power supply, but first I'd like to verify that the power supply is indeed the problem here. I was thinking I might temporarily use a desktop style ATX power supply for this, would that work?
Interesting turn of events, I managed to find the fuse compartment and the fuse had actually blown. The strange thing is that when I replaced it and tried the power again, the replacement also blew...

Does anyone know what might be wrong?
Those dark spots look like normal discoloration from hot-running components.

Keep in mind that any short on the motherboard will prevent a power supply like this from starting up.

You might check with an ohm meter to see if there is zero resistance between ground and any of of the power rails on the motherboard.
Those dark spots look like normal discoloration from hot-running components.

Keep in mind that any short on the motherboard will prevent a power supply like this from starting up.
Agreed. Likely the diodes you have circled were pretty hot (not really uncommon, but not desirable, either). If they check normal, I'd add some flux to the solder joints they're connected to and reflow some additional solder to them. I'd suspect the electrolytics failed more than anything. Normal for them to dry-out over the years and stop filtering the AC. That'd also make them (the diodes) run pretty hot. Beyond that (as mentioned), check for any low values of resistance between the supply lines and ground. Converting an ATX to an AT takes some doing. I'd personally try to repair the Compaq's P/S first.
o the mains fuse blows when powering up, is this with the PSU connected or disconnected ?

Sounds like the PSU has given up. Possibly the rectifier has gone short circuit which is that round black thing with 'GI' printed on top.

If you have a meter, you can try to measure the resistance between both ~ and + pins then both ~ and - pins. There should be some resistance and not a dead short.

Of course it could also be tantalum caps on the PSU too.

Can't find a drawing of that PSU though SAMS apparently did a manual (CSCS-15)
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You might check with an ohm meter to see if there is zero resistance between ground and any of of the power rails on the motherboard.
Ok, I've checked the +12v line on the floppy molex connector. With the power off and everything reassembled I get 18 ohms. on the +5v line I get 32.4.

Gary C, the fuse blows with everything reassembled, including the power supply. Question... how can I tell which exactly are the +, - and ~ pins?

I want to verify the power supply is absolutely what's at fault here before I order parts and replace components. I have a spare ATX power supply, and looking at the pinout I don't think it should be too difficult to make a quick little ATX to AT adapter. My question is: would that work? My power supply is 500w, does that present any sort of problem?
The rectifier has + - ~ symbols printed on the top. Its easier to test out of circuit but if you have a decent meter you should get at least some indication of resistance between the AC inputs and the DC + - outputs.

I just had a PSU fail because that unit went dead short on the bridge and effectively shorted out the AC.

Using an ATX PSU. You need to ensure it has all the supplies your machine needs, probably +12V, +5V & -12V, it might even have -5V, the wattage doesn't really matter as I imagine the compaq has a smaller PSU than your ATX PSU.

So no real reason you can't get it to work, but you would need to make sure all the connections are right, that you supply the PSU with a power good signal and that you wire up the PS_ON signal correctly to turn the PSU on.

Without a diagram of the Compaq's PSU I have no idea what standard it uses

The SAMS Computerfact book is what we need as they are generally excellent (still for sale too !)
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Update: it lives!!!

I hooked my CP+ up to a spare Dell ATX power supply and voila!!, it turns on again. Phew!

So this confirms the power supply is the problem here. Now to begin troubleshooting it.

GaryC, I’m still having trouble finding that rectifier… could you point it out on the picture I linked above? I’m still not sure I’ll even be able to test it though, as the PSU’s fuse blows instantly whenever I turn it on.


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So I reinserted the floppy card and POW! Blown tantalum. I'm also having a hard time getting the HDD card to work, though it's very possible my HDD has independently died already.
It would be better to test it out of circuit.

A meter should show conductivity from both ~ to + in one direction only and from both ~ to - in the opposite direction. Its a bridge of diodes so the meter should be on diode test (if it has one).