Is this an LCD issue or graphics chip issue?

[Coder]

I'm looking at a Compaq Portable 386 on eBay. The seller mentioned that the screen is "blurry" sometimes. I'm trying to determine if the artifacts are from the LCD panel or from the computer itself. These screens are impossible to find replacements for. If it's something on the board, I can possibly fix, then I may go ahead and pick it up.

 

[Eudimorphodon]

I'm trying to determine if the artifacts are from the LCD panel or from the computer itself.

Nitpicky observation: that isn't an LCD panel, it's a gas plasma screen.

Those screens are pretty notorious about having lines on the matrix fail with age. If it's intermittent, though, it could be something as simple as a gimpy ribbon cable connection or a cold solder joint.

 

[Coder]

Sorry, yes, gas plasma. Dang. I'm thinking I'll have to pass. I already bought one that had it's screen damaged in shipping, so I don't need another with a possible flakey screen. The more I look at the pictures, the more I think it's what you're describing; lines in the panel itself, not a problem with the display circuits. Both lines that are garbled are doing it in line with each other. I would expect graphical glitches from an issue with say video memory to occur more randomly, not lined up like that. I'll just have to wait for a better one to come up. Thanks!

 

[Bob-O-Rama]

I say negotiate down the price significantly and live with it or repair it. Also ask for a full screen of text, because its hard to tell how bad it is. These displays have a flex circuit along the edges lined with driver chips. Each chip handles 32 rows or columns and share a common address bus in the flex circuitry. This lets you deduce some things: The HV PSU looks good, as that would impact many more rows / columns. The cabling to the display is good, as that would likely impact more driver chips. The width of your defect seems to be about 32, so issue limited to a single chip. ( Based on the text provided. )

Also if you do have an *identical* one with a broken screen, you have a lifetime's supply of driver chips to transplant. So for the right price, if you are vaguely good with a air rework blower, you may be able to fix it and get one working unit cheap. But a full screen of text, like an editor screen / something with colors... would help evaluate it. The mylar flex circuitry melts at about 600F, its not as unforgiving as you might think.

-- Bob

 

[Coder]

Thanks Bob. Are the driver chips on the Mylar flex circuit, or on a PCB? The unit with the broken screen is currently in eBay seller dispute limbo. I asked the seller for a 50GBP refund (they're in the UK, I'm in the US) to account for the broken screen and their response was "file a claim with the shipper", so I filed a claim through eBay to return it. So unless the seller changes their mind, I may be shipping it back across the pond and won't have it for parts. Are these driver chips available or do you have to have a parts machine?

 

[clearscreen]

Hi Coder,

i've another post more complete (it's in compaq 386 or search my messages). I've totally restored one including screen dismounting. Take really care of the plasma screen because there is some high voltage on strange molex from the power supply. Thanks god i've measured voltage before, because, well ...

Just a photo (all are on the other post)

this computer was a real puzzle, i wonder how compaq managed to put all in that. This is not a "portable" it's a transportable lol (aka a desktop in a box) but it was the most difficult (with the persona 1600 because of bad soldering) computer i restored (excepted tape recorder i do also as hobby)

regards.
(and again beware with the screen).

 

[Coder]

I somehow missed this message. Thanks for the photos! I passed on the one with the fuzzy characters and went with one that doesn't display anything at all (great idea, I know). What did you need to do to your plasma driver board that you had to remove it from the screen? So far I've found a bad capacitor on mine (also on the driver board), and replaced it, but still don't get anything on the screen. I've been working on other projects, so I haven't revisited this one yet, but any ideas are always appreciated. I assume it's something with the high voltage as this screen in a known good test Portable III, still doesn't even light up. The rest of the computer's components work fine with the test system's good screen, so it's definitely something with the display or it's driver board.

 

[clearscreen]

Hi,
sorry for the delay. In fact there was some really bad glued dust everywhere and i had to remove all parts.
As the screen had some glitches, i remove all the connectors, and all parts, clean and use some KF-F2 (this is a special use for contacts). It worked better afterwards.
I've checked all the tensions and signals too, and it was fine.