Compaq Portable III alternative internal screen

[CompaqSniffer]

The title says it all I'm trying to figure out how to replace the plasma screen of the Compaq Portable III with an LCD. The reason why is because I'm in the midst of repairing some and they are very finicky. Very finicky actually doesn't even begin to describe the Amount of frustration I have experienced trying to repair these things. I have yet to fully repair one, as all screens I have took a crack at have this weird flickering horizontal line issue after I have repaired them. I have spent literal months trying to fix multiple screens.

Today I attempted a repair that only had 3 dead horizontal lines. I managed to fix all of them, but when I left the screen on for a few minutes, BOOM! Bright vertical lines show up out of nowhere...

Suffice to say I've had enough, and I want to know if there is any way I can take the internal video signal and hook it up to an LCD.

I've done so much research myself to see if it's possible but every Google search leads to a dead end. I find myself going to the 20th page of google every time I try to find an inkling of information to try it, but no luck.

Some info that may be helpful: the Compaq Portable III uses the same plasma screens that the Toshiba laptops do, albeit with smaller video cable connectors than the Toshibas. I'm pretty sure that the plasma screen controller board is a digital to analog converter, and that the video signal may actually be VGA, and not CGA or EGA. I don't know for sure though. The video signal I'm positive is digital before it hits the controller board, and that this info could help.

Knowing this, would there be a way to figure out what the video signal is, or find a way to hack the controller board to grab the analog signal off of it if it has one? I've done a few composite mods but I don't know how to grab a VGA/cga/EGA/MDA signal, or what it would look like on a scope. Also my scope is from 1960, so it's bandwidth is pretty low for what I'm trying to test. I'm only 21 and I'm still somewhat brand new to electronics/vintage computing. I've only been doing it for a few years now, so even though I know my way around a soldering iron, I lack the hardened experience of many people who have this hobby. I was wondering if anyone would be willing to try to probe the signals who has experience with this kinda thing. I'll post pictures of the controller board in question, and a few others as there are multiple revisions. I wanted to know if there is any standard type of video signal I could grab off of this thing or the internal video connector. I know I could take the external signal, but I didn't want to have to switch the video signal every time I turn it on, plus I haven't the clue if the switching actually works, because pressing Ctrl shift < or > doesn't do anything noticable like it does with my Compaq Portable I.

 

[CompaqSniffer]

Some other things to mention, the internal video connector uses a 20-pin cable, but only 10 pins actually do anything

 

[CompaqSniffer]

Here's a guide I was working on for the plasma screen repair, It has most of the information I've compiled

COMPAQ PIII / general Gas Plasma display Repair

Still Mostly A work in progress ABSTRACT This document is meant to catalogue the problems/ solutions I have encountered with Compaq portable III gas plasma displays. This document also catalogues a general idea of how these screens work to better understand how to repair them. SOURCESGene...

docs.google.com

 

[CompaqSniffer]

Here's a Toshiba controller board,
NOTE: it's missing two capacitors in the middle I took off

 

[Eudimorphodon]

Have you found anything like a datasheet for the screen? Or even a connector pinout? It really is hard to even guess how hard it would be to adapt it to some other monitor without that.

 

[CompaqSniffer]

Have you found anything like a datasheet for the screen? Or even a connector pinout? It really is hard to even guess how hard it would be to adapt it to some other monitor without that.

All I have are the line driver IC datasheets for the screen itself, if I knew the pin out I could go from there. What I do know is that quite a few plasma screens use the same 20 pin connector some are fatter than the Compaq's but the design is pretty much the same. Only 10 of the pins are used, as the GPU only has traces on one side that connect to the cable, I can post more pictures, and test whatever is needed, I just can't find any documents that are useful myself and so I thought it wouldn't hurt to ask for help. I did find a forum post on here, I'll link to it in a reply. I found one of the ICs on one of the controller boards today, it was a tiny TI chip, the other TI chip, the main one on the controller board doesn't exist according to Texas Instruments.

I tried measuring the pin out myself, but my oscilloscope is literally from 1960 and it's the only one I have. It does not have the bandwidth for measuring any signals
The four pin pinout is literally just the HV DC

 

[CompaqSniffer]

Have you found anything like a datasheet for the screen? Or even a connector pinout? It really is hard to even guess how hard it would be to adapt it to some other monitor without that.

Here's a forum post about a similar plasma screen, maybe it's not to far off?

Post in thread 'Plasma Display PN Panasonic MD480L640PG3 - Help, trying to determine the pinout'

May 6, 2025

I'm glad I stumbled across this, do you know if this display is similar enough to the compaq portable III screen so that I can replace it with an LCD? I've been looking, and a portable III clone uses the exact display referenced in this post, but the portable III uses completely different ICs, and a completely different controller board. I've been thinking about figuring out the pinout myself for it, but I am lacking basic knowledge on what I'm looking for. All I know so far is that half of the pins on the PIII display are also ground by looking at the graphics card, and from continuity...

• CompaqSniffer

 

[CompaqSniffer]

The portable III clone I was referring to is from this thread:

The South Post in thread 'Looking for portable III parts'

Jun 4, 2025

If you're interested in it, make an offer. I'm planning to take it to the VCF East Swap meet on Saturday if not.

https://imgur.com/a/bfXLgiP

Ran when parked shortly before the pandemic with the exception of the HDD. Previous owner told me that drive was on its way out when this machine was retired in the early 1990's. I'd just boot DOS floppies.
IIRC 386 motherboard. It shows a VLSI chipset and "Copyright" but no date or manufacturer. Probably an IBM clone.
I think the neatest thing in here is the MIDI card. It was a kit available at radioshack and other vendors.
This magazine has the docs for it...

• CompaqSniffer

Fleetwood Mac and cheese offered me a portable III clone and it has the display referred to above, maybe this is useful.
Also thank you Mac and cheese for providing good pictures

You may have to scroll to find it, but he offered me a portable III clone and posted an imgur link a little ways up

 

[jssposato]

Hi there, CS!

I was wondering if you ever had any luck with your project. I'm having the same issues with the plasma screen and I thought it would be fun to install a modern LCD in its place.

Thanks!!
Jeff

 

[CompaqSniffer]

Hi there, CS!

I was wondering if you ever had any luck with your project. I'm having the same issues with the plasma screen and I thought it would be fun to install a modern LCD in its place.

Thanks!!
Jeff

I have not, I haven't really started working on that yet, I just moved and time and money to work on it are not really available. I think it is possible to use the internal connector since the signal might be digital. I think we would just need to figure out how to convert it to a usable signal. While I love the look of the plasma screens, they will soon go extinct in my opinion. The electronics on them are absolute garbage. You fix one chip, and the entire screen starts freaking out.
I think installing an lcd alternative is the only way to fully fix them.

 

[Istarian]

Assuming that you aren't dealing with the actual screen elements failing (possible), there is probably some missing detail which would explain your difficulties.

You are probably going to need to dig into the software end of it too, if you want to replace these displays with an internal LCD.

 

[CompaqSniffer]

Assuming that you aren't dealing with the actual screen elements failing (possible), there is probably some missing detail which would explain your difficulties.

You are probably going to need to dig into the software end of it too, if you want to replace these displays with an internal LCD.

The line driver ICs are toast on those plasma displays, so they need replacing. The problem is that you can barely source new ones, and the new ones you install fix the dead/ and or bright lines, but cause the entire screen to have flickering vertical lines. I have seen this on more than one. On the screen that have the flickering vertical lines the film on the ceramic resistor networks is degraded causing the resistors to short together momentarily making a vertical line briefly brighter. The horizontal lines don't do that because the horizontal ICs don't have series resistor networks to degrade. It seems like heat makes the resistor networks worse. Also, these gas plasma displays smell like soiled pants, so it would be nice on the nose to get a screen that doesn't smell like a toilet.

 

[Istarian]

It's entirely possible that the smell has little or nothing to do with the gas-plasma technology, but rather from heating up gunk on the display like dust, leaked capacitor, electrolyte, etc.

 

[CompaqSniffer]

It's entirely possible that the smell has little or nothing to do with the gas-plasma technology, but rather from heating up gunk on the display like dust, leaked capacitor, electrolyte, etc.

Yeah, I thought so too, but the ones that haven't seen heat also smell like that, so I think something is degrading like adhesive or the protective film on those thick film resistors. Those resistor networks have gunk u o under the protective green film that is causing the visual artifacts.
Some of the screens, like the ones with aluminum frames did not have that problem though.

 

[Eudimorphodon]

It's kind of funny, I guess, how you don't normally think of electronics (at least in the solid state area) as things that would just go bad with age, but time has clearly humbled us once again. Old LCD panels are succumbing to various kinds of rot between integrated capacitors and the adhesives/sealants/plastics used in the panels themselves, I suppose it's not unreasonable that gas-plasma units would too. It's possible the really old ones from the 1970's might hold up slightly better than the ones from the 80's because I suspect that they're closer to just being really fancy neon bulbs. (IE, the support circuitry is more discrete and I think their envelopes basically are just all glass.)

 

[CompaqSniffer]

It's kind of funny, I guess, how you don't normally think of electronics (at least in the solid state area) as things that would just go bad with age, but time has clearly humbled us once again. Old LCD panels are succumbing to various kinds of rot between integrated capacitors and the adhesives/sealants/plastics used in the panels themselves, I suppose it's not unreasonable that gas-plasma units would too. It's possible the really old ones from the 1970's might hold up slightly better than the ones from the 80's because I suspect that they're closer to just being really fancy neon bulbs. (IE, the support circuitry is more discrete and I think their envelopes basically are just all glass.)

Yeah the panel itself is fine, it just seems like the resistor networks for the vertical sync side are rotting. It reminds me of vinegar syndrome except this one smells like piss instead of vinegar. I vote if that is 100% thie issue, we should dub it piss syndrome.

 

[Istarian]

It's kind of funny, I guess, how you don't normally think of electronics (at least in the solid state area) as things that would just go bad with age, but time has clearly humbled us once again. Old LCD panels are succumbing to various kinds of rot between integrated capacitors and the adhesives/sealants/plastics used in the panels themselves, I suppose it's not unreasonable that gas-plasma units would too. It's possible the really old ones from the 1970's might hold up slightly better than the ones from the 80's because I suspect that they're closer to just being really fancy neon bulbs. (IE, the support circuitry is more discrete and I think their envelopes basically are just all glass.)

Character and graphics LCDs that aren't TFT mostly suffer from physical damage, excess heat/cold, and failure of the polarizer layers. While the connections between the panel and drivers/circuit board can fail, that's a lot leas common.

Even TFTs seem to hold up pretty well if they don't get cracked/smashed and are protected from sharp impacts. Most of them do need backlighting though (no rear reflective/transflective layer), so a failed backlight or backlight drive is a problem.

While most things do inevitably fail, I doubt that age is the primary culprit. Bring neglected and probably stored improperly causes lots of problems in the long-term. Little problems become bigger problems and not fixing them leads to secondary issues.

 

[T0u]

Yeah the panel itself is fine, it just seems like the resistor networks for the vertical sync side are rotting. It reminds me of vinegar syndrome except this one smells like piss instead of vinegar. I vote if that is 100% thie issue, we should dub it piss syndrome.

Hi, bro) Link that you shared - don’t work: https://docs.google.com/document/d/1JmzrH7B1WTx5McuQZ1Zxlcce4-KLS0PbwYedE62emJY/edit?usp=drivesdk I’m not an engineer, but love for these gorgeous machines - compaq portable, ibm portable, and other MASSIVE tech makes me learn to understand comp.engineering. I want to at least TRY to fix lines on my compaq portable III. I disassembled the display - there the legs of the controllers are clogged with some bad stuff. I cleaned them somehow, in the end I got even more stripes. New type - Black) I would like to read your file. Thank you.

 

[Istarian]

@T0u When you say 'stripes' are you talking about the dark lines or the bright ones?

The ICs closest to the display itself are probably not controllers,but rather a type of driver that generates the electrical signal to to drive the display.

No connection may lead to black lines, while a good connection could produce a bright line in some cases depending on how the display works exactly.

 

[CompaqSniffer]

@T0u When you say 'stripes' are you talking about the dark lines or the bright ones?

The ICs closest to the display itself are probably not controllers,but rather a type of driver that generates the electrical signal to to drive the display.

No connection may lead to black lines, while a good connection could produce a bright line in some cases depending on how the display works exactly.

I have no idea what happened to the doc, I'll see if I can find it, it's not showing up in google docs for some reason. sorry for the late reply.