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IBM CGA adapter, warped composite video

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themaritimegirl

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I've just discovered that the IBM CGA adapter in my IBM 5155 Portable PC can't seem to handle graphics modes and high-resolution text modes very well, such as in the MS-DOS shell. When the display is filled with stuff, it results in very warped video (see the attached picture). But in the low-resolution text mode, or when sitting at the DOS prompt, it's fine, even with a text-filled screen.

I have another, identical CGA adapter which has a bad memory chip, and I just tried it with the same program, and there is no warping at all. So there's something going on with the composite video output of the first card. Anyone know what it might be? Otherwise I'll just unsolder a memory chip from it to replace the bad chip in the other card, and use that and call it a day.

IBM Portable PC warped display.jpg
 
Is that screenshot one of your Portable PC's display?

Are you sure your CGA adapters are truly identical? There are differences between older and newer IBM CGA cards, see here : http://www.minuszerodegrees.net/5150/early/5150_compare_cga_adapter.jpg

Old CGA cards tend to cause issues with a display's horizontal sync. I have an Old CGA card and I can see bendy rasters like yours, although mine are not nearly as severe.
 
Is that screenshot one of your Portable PC's display?
Click to expand...

Yes.

The adapter with the bad memory chip but good video output is identical to the card on the bottom of the picture you linked. The card with the warped video output is green in color, but has the extra bracket support, and it has the four resistors at the bottom populated (but not eight resistors like the bottom card). My brown card was made early 1984, and the green card mid-1983.
 
Does the image get better if you turn down the "contrast" control?
Click to expand...

Nah, it totally retains its shape regardless of either control.

It just occurred to me though, this card indeed drives the monitor harder than the other card. I had to turn the brightness up quite a bit when I switched to the other card (the one with the bad memory chip, which is original to the machine) to test it.
 
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Your green card is an old CGA card. The 4 resistors closest to the RGBI video output connector do not effect its composite operation. You may be seeing normal behavior for an old CGA card. If you have another composite monitor, try connecting the card to it and seeing whether the raster is as bendy.
 
I think the giveaway here is the "sawtoothing" (for want of a better word) on the left-hand side. It's not the entire line being shifted (because that sawtooth pattern only happens on the left edge, not with other should-be-vertical lines).

That suggests that the horizontal sync logic is faulty, and is sometimes counting too many character periods before ending the sync pulse and restoring normal video. That would also account for the unstable horizontal sync. My best guess is that U64 (the LS164 which is responsible for sequencing the horizontal sync pulse structure) is faulty. But as Chuck said, it really needs a poke with a scope to find out for sure.
 
Thanks all for the responses. I should mention that the previous owner of the card used it with a CGA monitor, and they said it worked just fine. So the issue seems to be limited to the composite output. I should also mention that tantalum capacitor C8 exploded under the previous owner's ownership, and they had replaced it with a new cap.

I've just had a new development - when you connect an external monitor to the composite output on the back of the card, the image on the internal display becomes perfect! Unplug the external monitor, and the internal monitor is warped again! So it indeed seems like either something in the composite output circuitry or the CRT itself is being overdriven. But I can turn the brightness and contrast all the way down to where you can't see a picture anymore, and the image is still warped.

I'm extremely intrigued.
 
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TheMaritimeMan said:
Thanks all for the responses. I should mention that the previous owner of the card used it with a CGA monitor, and they said it worked just fine. So the issue seems to be limited to the composite output. I should also mention that tantalum capacitor C8 exploded under the previous owner's ownership, and they had replaced it with a new cap.

I've just had a new development - when you connect an external monitor to the composite output on the back of the card, the image on the internal display becomes perfect! Unplug the external monitor, and the internal monitor is warped again! So it indeed seems like either something in the composite output circuitry or the CRT itself is being overdriven. But I can turn the brightness and contrast all the way down to where you can't see a picture anymore, and the image is still warped.

I'm extremely intrigued.
Click to expand...

Here's a trick you could try, check the resistance of the input of your external display with a multimeter, then find a resistor with a similar value and connect it accross the composite output of the card, it might act as a dummy load and bring the output level down enough that the internal CRT is happy.
 
Or just get a suitable pot (say 150 ohms) and connect it across the video output and twiddle until the image is perfect. But this confirms my suspicion that the monitor is being over-driven.
 
I thought that IBM PC Portables came with old or new CGA. One of the chief benefits of new CGA is its support for 16 distinct monochrome shades, compared to the 4 shades old CGA could show. This would be extremely useful for the amber composite monochrome screen of the IBM PC Portable. Perhaps the screen was simply not intended for old CGA.
 
I did a comparison:

CGA card found in my 5155

* Under the load of the 5155's CRT unit, the composite output is about 1 volt peak-to-peak. See [here].
* Running all of the video mode tests in CheckIt does not show distortion.

Early CGA card, temporarily fitted into my 5155

* This is the top card shown in the photo at [here].
* Under the load of the 5155's CRT unit, the composite output is about 1.5 volts peak-to-peak, significantly more than the other card. See [here].
* Running all of the video mode tests in CheckIt does not show distortion.


To be noted, is that IBM's technical document for the 5155's CRT unit ("IBM Portable PC Display") indicates "1.5 Vdc (peak to peak)" composite video.
 
Retrom said:
Here's a trick you could try, check the resistance of the input of your external display with a multimeter, then find a resistor with a similar value and connect it accross the composite output of the card, it might act as a dummy load and bring the output level down enough that the internal CRT is happy.
Click to expand...

That is such a good idea that I went ahead and did it. I found a 290 ohm resistor in a parts bin, and found that when I stuck it across the external composite output, the distortion went away. Soldered it across the terminals on the card, and... problem solved!!

Great Hierophant said:
I thought that IBM PC Portables came with old or new CGA. One of the chief benefits of new CGA is its support for 16 distinct monochrome shades, compared to the 4 shades old CGA could show. This would be extremely useful for the amber composite monochrome screen of the IBM PC Portable. Perhaps the screen was simply not intended for old CGA.
Click to expand...

Eh? Shaded colors look the same on both of the cards I have. Only the two lightest shades are properly displayed, and all the darker shades are represented in a striped/checkered fashion, which often makes text and small figures illegible.
 
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