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Compaq Portable CRT Issues

fluxcap

New Member
Joined
Jun 20, 2021
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9
Hi all -

I have a Compaq Portable (Compaq Portable Plus) and notice when the machine is CGA mode, after about 5 minutes the CRT can be seen to:
  • Flash in brightness - it goes high for a second or so then goes back to normal. The image can be a bit blurry when it goes bright.
  • Flicker in position - meaning the whole screen "jerks" left for a second or so and goes back to normal.
  • Sometimes a single vertical line can be seen, a little left of center, and it sort of glows bright for a bit then disappears.
Also all of this seems to happen at the same time (it the image goes bright, the image will flicker in position, and sometimes that vertical line comes in bright). Again this always seems to happen only when in CGA mode, and after being on continuously for 5 or so minutes.

I've done some research online re: CRT repair with similar symptoms and found that replacing the capacitors could fix the problem. I've eyed the board (haven't used any hardware) and all the caps look good, but I plan on replacing all the caps on the video board, CGA card, motherboard and power supply.

Any thoughts or suggestions would be greatly appreciated.

(this video shows some of the symptoms above, but these symptoms happen most of the time):
 
" and after being on continuously for 5 or so minutes."

Look for capacitors which are near a heat source. The heat tends to bake the caps and dry them out over many years of use.

Replace those first and see if it helps.
 
Hi all -

I have a Compaq Portable (Compaq Portable Plus) and notice when the machine is CGA mode, after about 5 minutes the CRT can be seen to:
  • Flash in brightness - it goes high for a second or so then goes back to normal. The image can be a bit blurry when it goes bright.
  • Flicker in position - meaning the whole screen "jerks" left for a second or so and goes back to normal.
  • Sometimes a single vertical line can be seen, a little left of center, and it sort of glows bright for a bit then disappears.
Also all of this seems to happen at the same time (it the image goes bright, the image will flicker in position, and sometimes that vertical line comes in bright). Again this always seems to happen only when in CGA mode, and after being on continuously for 5 or so minutes.

I've done some research online re: CRT repair with similar symptoms and found that replacing the capacitors could fix the problem. I've eyed the board (haven't used any hardware) and all the caps look good, but I plan on replacing all the caps on the video board, CGA card, motherboard and power supply.

Any thoughts or suggestions would be greatly appreciated.

(this video shows some of the symptoms above, but these symptoms happen most of the time):
Have you found a specific solution for this problem? I'm getting about the same symptoms, except my vertical line seems to wiggle side to side for a bit before it straightens out (a vertical damped sinusoid). Also happens to me after some time running the machine in WordPerfect 5.1 (which I assume runs in CGA mode, IDK), or just fiddling about in DOS.
 
If the image looks good for any length of time, I'd be inclined to check the CRT board for bad solder joints, faulty or dirty internal connector cables, corroded or dirty potentiometers, and if I recall correctly there is a physcial relay that switches sync modes that can get corroded or dirty.
 
@SomeGuy An interesting thought. I'll check continuity on the cables I can get to, and look at all the solder joints. As for the pots, I've adjusted all of them with the monitor running and seen no symptoms during that adjustment, so I don't think its them.

On my machine at least, The monitor will stop working after a period of continuous use, then start working again after the machine has been off for a while. I think this suggests a correlation with heat, so I favor mykrowyre's suggestion; though it might be the heat-producing elements themselves rather than the caps.

I've managed to wrench the CRT control board out of my unit (against its wishes), so here's some pics in case they give y'all any ideas.

(As for an introduction, I've filled out my profile description for those interested. tl;dr: engineering student who also likes music)
 

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Alright folks, I'm back.

I replaced all the electrolytic caps on the board with new ones (and filled out a Caps Wiki Page, so you can check my work there), except for the big one, which I didn't realize was non polar. I was already suspicious of that one because of its size and proximity to that heatsink, so I figured I'd give it a try with that one the same: if I had the same symptoms, I could know the cause for sure. Plugged it in, turned it on, got some wiggly diagonal (mostly horizontal) lines, then smoke. One of the new capacitors popped its vent (I wiped it off before the pic), and there is a burn mark on a wire.

I'm new to this kind of low level repair, as I am just a baby engineer, so I'm not sure what to make of this. Did I put a cap in backwards, could that have caused it? The one that blew was in right, I checked. Was the new cap faulty from the start? Did the old big cap cause it?
 

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Last edited:
The one you’re pointing at looks backwards from your other photo, the white stripe for negative on the other side.
 
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