PDA

View Full Version : Help with a Compaq Portable



XS-Nitrogen
June 14th, 2010, 08:53 PM
Just a disclaimer: I've built a few 386s, but that's the closest I think I've been to working on a real "vintage" computer until I bought my Portable. So please, bear with me - I don't know a whole lot, and I might have missed something pretty obvious.

I bought my Portable about a year ago from somebody on Ebay. When I got it, it was fully usable. Computer would boot to a command line, the drives worked, all of that good stuff. As far as I could tell at, everything worked - The video board, floppy board, and HDD controller all worked fine (I couldn't test the printer port on the FDD controller though). It also had two more cards: An "AST Six Pak Plus" and a card with what looked like two Joystick ports. I don't know much about either, and don't know if they worked or not. The computer had obviously been opened at least once before I got it - The top cover wasn't closed properly, and there were lots of screwdriver scuffs around the video and floppy board's screws. I don't expect the AST or the Joystick cards were factory, and as far as I know, the only hard drives shipped with Portables at all were built onto the HDD controller card. Mine is in one of the main drive bays.

The only issue was the keyboard - The little foam pads inside were starting to fail on me. After awhile I couldn't reliably type anything anymore, and the computer basically just sat in my basement. I found some replacement pads a couple of weeks ago, and I figured while I waited for them, I'd start up the old suitcase just for fun. Didn't turn out so well.

Nothing showed on the screen, the drives didn't seek... no life at all besides the PSU fan. I stripped it down to just the mainboard and video board, and got nothing. There is a little red LED that blinks for a quarter-second or do when I start it up, but it seems to be only after adding or removing a card. If I disconnect the mainboard entirely, the PSU can power the internal HDD (Which powers up and like normal, although with no computer to access it), as well as a drive-bay cooling fan from a deskop PC and an old CD drive (The fan was being used to extend the internal power cord by a few inches so I could hook it up to the CD drive). The drives do nothing if the mainboard is connected, and I can't get a picture on the screen even with everything disconnected other than the mainboard and video card. I expect the PSU might not be putting out enough power, as it seems unlikely (Although possible) that the mainboard itself packed up, as the LED on the board does still light up, although only briefly. Is there anything I can do to find out what's wrong and hopefully save the old Portable?

Chuck(G)
June 14th, 2010, 09:22 PM
Do you own a multimeter (DMM)? A certain minimum of test equipment helps to zero in the problem.

Right now, I suspect a bad capacitor on the motherboard--but that's just a guess.

mark66j
June 15th, 2010, 03:55 AM
Is there a battery on the motherboard? If so the first thing would be to try replacing it. Some vintage systems act very oddly when the battery is dead. Maybe not so likely in this case, but always worth a try.

Do you have one of those power-usage meters like a Kilowatt? If so you could plug the machine into that and see whether it draws a lot of current when the motherboard is in place (compared to just the hard drive). Seems like that would point to a current leak on the mainboard (as mentioned, likely a bad capacitor).

XS-Nitrogen
June 23rd, 2010, 06:37 PM
I do have a multimeter. The Kilowatt I didn't have, but I managed to track down something similar. Here's what I got from it:

The computer with the board and drives hooked up pulled 25w on power-on the first time I tried this, and a few times afterward, then it would drop to 6-7 after a second. If not that, it would just pull 6-7 (I expect that, since the meter takes a second to update the screen, it just didn't catch the power-on spike), except once it went to 107 on power-on, then dropped to low numbers like 3-7, mostly hovering around 4. With just the drives, it started at 54-56w, then down to 46ish, then bounced back and forth between 24-25 and about 46 without showing anything in between. It did this consistently.

As far as the battery goes, I don't see one on the mainboard at all.

Chuck(G)
June 23rd, 2010, 06:56 PM
Use your DMM to check the power supply output with the motherboard connected. Make sure that all voltages are present (+5, +12, -12, -5).

XS-Nitrogen
June 24th, 2010, 01:39 PM
Using the power connector for the floppy drive, +-5v and +-12v are all present when the mainboard is disconnected, but I get nothing with the mainboard connected.

k2x4b524[
June 25th, 2010, 09:20 PM
me thinks there is a short in the motherboard someplace, preventing it from turning on, Guys correct me if wrong on this. but i think the MB is the culprit. I think next would be to look for a short on the traces, or a place in the chassis that may be doing it

wrljet
June 26th, 2010, 09:28 AM
Using the power connector for the floppy drive, +-5v and +-12v are all present when the mainboard is disconnected, but I get nothing with the mainboard connected.

Likely a shorted tantalum capacitor on the motherboard. Very common failure.

(please excuse this post if it came through twice, I thought I sent it before but don't see it)

Bill

XS-Nitrogen
August 3rd, 2010, 12:34 PM
I hope it's fine for me to revive this after so long. I was out of town.

Shorted capacitors are definitely beyond what I'm familiar with. How would I go about testing the capacitors? Can that be done with a multimeter? If one's shorted or damaged, is there a specific type I'd need to use for a replacement? I'll double-check to see if the chassis is shorting it as well, although from what I can see without completely taking it apart, it's not touching the chassis at all aside from the plastic fasteners holding it in place.

james1095
August 11th, 2010, 10:52 AM
I hope it's fine for me to revive this after so long. I was out of town.

Shorted capacitors are definitely beyond what I'm familiar with. How would I go about testing the capacitors? Can that be done with a multimeter? If one's shorted or damaged, is there a specific type I'd need to use for a replacement? I'll double-check to see if the chassis is shorting it as well, although from what I can see without completely taking it apart, it's not touching the chassis at all aside from the plastic fasteners holding it in place.

Tantalum capacitors are small "blob" shaped components with 2 or occasionally 3 legs. The most common value I see in this sort of application is 10uF 16V although it is usually not critical. Finding a shorted one can be tricky since there are normally several of them across the same power rails. Sometimes you get lucky and the bad one helpfully explodes :)

What I would recommend is use the Ohms setting on your meter to measure across each tantalum capacitor. If you find one that is slightly closer to zero ohms than the others, suspect that. You have to be careful desoldering parts from a motherboard because they are thick multilayer PCBs so you need a lot of heat and care not to damage the board. I use a Hakko 808 to make quick work of this sort of thing, but with enough care it is possible to do this with a Radio Shack desoldering iron and some patience. If you are pretty sure you found the bad one, another method is to clip it off close to the body of the component and solder the new part to the remaining stubs. It's not pretty but it will work in a pinch.