Compaq Portable III Screen Problems

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Compaq Portable III Screen Problems

Postby T-Squared on Mon May 30, 2011 2:27 pm

Greetings from the United States of America, everyone! :D

I recently bought a Compaq Portable III computer off of eBay, and it would be the best computer I've ever bought since my Apple II...

...if it weren't for a few problems my screen has. The screen itself is an orange gas-plasma screen, which I think is susceptible to severe burn in. However, it hasn't happened to mine, which is VERY good.

The problems I'm having are:
1. When I turned on the machine for the first time the screen had 3 black lines running from top to bottom.
Image

I thought that the problem was caused by bad ZEBRA contacts (the rubbery/flexible sandwiched contacts found in LCD displays), so I opened the screen module to see if I could fix the problem, but apparently gas-plasma displays don't have ZEBRA contacts. (I had experience trying to get rid of dark lines from a Zenith Supersport laptop because the ZEBRA contacts would not press together properly.) Then I tried pressing on the rubber strip that presses the cable contacts against the screen contacts. No change. Then I realized something that I thought was a fluke: DOS text would poke through the dark spots when the contrast was turned up high enough.
Image

Someone on another vintage computer forum suggested a poor solder joint. (To me, that means that a few pins of a few of the driver chips on the cable had disconnected.)

I decided to ignore the problem for now until I could get a precision soldering iron with a point small enough to reflow the solder on the chip pins. I was about to try transferring a copy of Arachne web browser for DOS today, until THIS happened:

2. I heard a *PTTfsssssHHHHHHH* sound and magic smoke came from the vents of the screen module. However, to my surprise, the screen did NOT FAIL! I shut it off anyways just in case.

When I reopened it, I tried looking for obvious signs of board component failure on the video driver board, but I couldn't really find any... Until I noticed a nice spray on the back wall of the module I had not seen the night before, which matched up with the location of a cap on the board, with a neat little hole in the plastic covering of the cap. I'm not exactly sure what kind of cap to get, since all I know about it is it's a 250v 1uF axial capacitor. I don't know the other specs, so I can't make a determination yet.

I wanna be able to fix this thing so I can play The Secret of Monkey Island! TT.TT
T-Squared
 
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Re: Compaq Portable III Screen Problems

Postby tezza on Mon May 30, 2011 3:24 pm

Hi and welcome to the forums,

First up a disclaimer. I don't have any experience with the Compaq Portable III.

However if you've identified the cap which has blown, that's a good start. Is it a filter cap from the power supply? Is it a square shape like the one blown in this article. If so, replacements are easy to get. Look for Metallised Polypropylene capacitors at a store near you.

Tez

P.S. Unless people are registered at the Vintage Computer Forums they won't be able to see your pics. Best use a URL from a web service like Flicker or similar if you want to show images in here (at least one that doesn't require you to be logged on and registered to see the photo).
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Re: Compaq Portable III Screen Problems

Postby T-Squared on Mon May 30, 2011 3:50 pm

tezza wrote:Is it a filter cap from the power supply?


No, it came from the video board of the gas plasma display. However, I was able to remove it without trouble, and got a piece of info from the plastic casing that might be useful: its operating temperature. :D

The chips, however, I don't see an obvious problem with. I could reflow some pins when I get a precision soldering iron to see if that will fix the problem with the dead lines.
T-Squared
 
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Re: Compaq Portable III Screen Problems

Postby T-Squared on Tue May 31, 2011 12:49 pm

So far, I've tried reflowing the solder between a few of the chip pins and the solder pads, but no luck. :< The lines will not go away. (yet)

I'll try again later, but if someone could help me, I'd really appreciate it.
T-Squared
 
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Re: Compaq Portable III Screen Problems

Postby T-Squared on Tue May 31, 2011 1:47 pm

PROGRESS! 8D

I disconnected a bank of screen driver chips (there are 4, two for the top and bottom, and two for the left and right) and found something interesting! (Don't worry, they're still on the board. All I did was disconnect the cable connecting to the video board.)

The top and bottom banks don't do what I thought they did!

To start, on my old laptop, the top and bottom contacts (and presumably the screen driver chips) control the screen like this:

(Each vertical line represents a 1-pixel-wide line going from the top or bottom to the middle of the screen, starting from the left)

TOP
1-2-3-4-...
|-|-|-|-...
-------------
|-|-|-|-...
1-2-3-4-...
BOTTOM

When one contact would not connect properly, it would cause a dark line to appear in the bad spot that extended to the horizontal center of the screen.

HOWEVER, on the Compaq Portable III,

The arrangement of the chips and contacts controlling the screen goes like THIS:

(Each horizontal line this time represents a one-pixel-wide-line going from the top TO the bottom, starting from the left)

TOP
1-3-5-7-...
|-|-|-|-...
-|-|-|-|...
-2-4-6-8...
BOTTOM

The top and bottom chip banks control respectively-alternating vertical lines of pixels! It works much like a comb-teeth-pattern.

Anyways, because of this, I've narrowed down the problem to the top bank of chips. Hopefully I can repair the problem better now.

EDIT: Unfortunately, the diagrams shown above might not suffice. I'll post better pictoral diagrams if anyone wants it.
T-Squared
 
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