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Compaq Portable not powering up, but fan runs

db2

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MI, USA
Greetings all, first post here. I picked up a Compaq Portable I at a hamfest today - known non-working, but the price was definitely right to take a stab at fixing it. Fortunately it's almost completely dead, meaning it seems likely the problem is in the PSU. The line fuse is fine, and the cooling fan runs normally when I switch on the power, but none of the internals appear to get power. If I flip the switch while I've got the computer opened up enough to observe the insides, I can hear a little pop that might be coming from the speaker, and there's a red LED on the motherboard that flashes on for just a split second.

This one appears to have some added hardware, as it's got dual 3.5" floppy drives (not sure if they're 720K or 1.44M) plus a hard disk of some sort. There are four cards in it, two of which appear to be the factory-installed I/O and video cards, plus a hard disk controller and some other expansion card that appears to have some RAM and/or EMS plus more I/O. I've tried powering on the machine with all four of these cards pulled out, with the same results. When switched on, I don't read any DC voltage across any of the pins on the molex connector that powers the two floppy drives.

Any recommendations what I should check based on these symptoms? I'm absolutely not an EE, but I'm handy enough with a soldering iron and multimeter, and I've recapped a few things and built a couple computer kits (Altairduino, MiniPET), so dumb it down appropriately for me. ;)
 

db2

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db2 welcome to the forum!

- Alex

Thanks! Wasn't sure if I should put this here, the hardware troubleshooting forum, or the portables forum, so you certainly won't hurt my feelings if you move it somewhere you think is more appropriate.
 

SomeGuy

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A common problem that prevents machines from powering up is shorted tantalum capacitors on the motherboard or other devices. Try disconnecting the power from the motherboard and checking the resistance between various rails and ground. When this happens, it is usually along +12 or -12v rails.

The fact that you see/hear something going on for a moment suggests the power supply is detecting a short and shutting down.

Of course, it could be a hundred other things, but that is a good place to start.
 

db2

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That's a good idea, I hadn't considered it might be short-circuit protection shutting things down. It looks like the PSU connection to the motherboard is a connector that's buried underneath the various cages inside the machine, so it's going to take some excavating to get to it and disconnect the motherboard.
 

jafir

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Some things are easier to reach from the bottom. If I recall correctly the power supply connector is one. The keyboard connector definitely is.

EDIT: I just opened mine. Here is a photo.

38BA8601-5EC3-4851-96D3-316793206810.jpeg
 

db2

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MI, USA
Some things are easier to reach from the bottom. If I recall correctly the power supply connector is one. The keyboard connector definitely is.

EDIT: I just opened mine. Here is a photo.

Definitely looks a lot easier to get to from that angle. If I disconnect power from the motherboard and the hard drive spins right up, then it sounds like it'll be a recap job. If that doesn't change anything, then I'm guessing the PSU is to blame.
 

db2

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Progress! Sort of, anyway. I tracked down two caps on the motherboard that are reading as a dead short, at least when tested in the circuit: C83 (tantalum 10uF 16V) and C84 (ceramic 47nF). Both of these are right next to the power input connector, and when I disconnect power from the motherboard I see good voltages on the drive molex connectors. I have replacements for the ceramic cap, but not the tantalum one, so I had to order a box of assorted capacitors. We'll see if anything else is wonky after I replace those two.
 

db2

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Clip one side of them at the moment for a test. The machine will run without them.

I'll give it a whirl if it won't risk wrecking anything else. What purpose do those two caps serve? Just smoothing the DC power or something?
 

db2

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I have an analogy at [here].

So in other words, as long as I don't key up my ham radio, I'm probably fine to test it this way. ;)

I clipped one leg of the shorted tantalum cap (the ceramic one was fine - it's wired in parallel with the tantalum so it looked shorted at first), and that was enough to get the machine powering up. The hard drive is shot, which I fully expected, but it had no trouble at all booting from a 720 KB DOS 6.2 floppy that I whipped up in my 486. So that's something! I'm going to replace all the tantalum filter caps near the power connector on the motherboard just to be safe, but this was an encouraging test. Now I just have to deal with the keyboard, because only the 4 and f keys work. Time to hunt down some replacement foam discs. If I can get the keyboard fixed, then I'll get ahold of an XTIDE and see what I can make it do.
 

jafir

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Does the hard drive spin up or do anything at all? If so, maybe it just needs a low level format? If it’s a Compaq controller, I believe you’d need to make the setup floppy to attempt a low level format. You could probably try a third party utility like speedstor as well.

As for the keyboard, I bought the kit from texlelec. It seems to work ok, you just have to be very clean as the mechanism doesn’t seem to tolerate debris from the old discs or even oil from a can of deoxit. Or at least mine was picky. I had to take it apart a couple of times to get everything happy.
 

db2

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Does the hard drive spin up or do anything at all? If so, maybe it just needs a low level format? If it’s a Compaq controller, I believe you’d need to make the setup floppy to attempt a low level format. You could probably try a third party utility like speedstor as well.

As for the keyboard, I bought the kit from texlelec. It seems to work ok, you just have to be very clean as the mechanism doesn’t seem to tolerate debris from the old discs or even oil from a can of deoxit. Or at least mine was picky. I had to take it apart a couple of times to get everything happy.

I didn't hear anything from the hard drive when I powered on. The activity light came on when the machine attempted to boot, but that was it. I don't think it's a Compaq drive, looks like some kind of after-market thing (like the dual 3.5" floppy drives installed vertically in the other drive bay). The controller card has chips that say WDC on them. I'll have to give it a closer look and see if I can find some info about it. Not too worried about getting the hard drive working, though, since I'd rather install an XTIDE. It also came with an Orchid Conquest expansion card that seems to include some more I/O ports, and possibly EMS RAM. Hope I can track down drivers for that...

Texelec is showing the keyboard foam-and-foil replacement kit available on backorder, but I emailed them to find out when it might be ready to ship. If it's a couple weeks, I can live with that.
 

db2

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Ooh, got a tracking number for the key contacts already! So I guess I'll be rebuilding the keyboard this weekend. I'll just run it from floppies at first to give it a thorough testing, then order an XT-IDE for it. Thanks to everyone for all the tips and assistance with troubleshooting the electronics.
 
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