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PC-XT and Composite CGA problems.


jdgabbard
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So I have been working on getting my IBM 5160 restored. Most things have been cleaned or repaired at this point. I had to remove a few capacitors that were shorted internally, sand the badly pitted case, repaint, etc.

 

The biggest problem I had run into was how to get an image on the screen. It had a Paradise Hi-Res card in it, which had a cga 9-pin connector. But I do not have a CGA monitor. Which created a problem.

 

So I bought a later IBM full size CGA card with Composite out. This worked ok, but it did not display any color. I tried fooling with the trimmer pot on the motherboard. Unfortunately, this only served to adjust the horizontal sync. Colors display horribly, mixing both fore and background colors and was completely unusable.

 

After doing more research I discovered that most CGA cards have a header on the board for Composite out. The IBM card does, I looked, and the Paradise card does as well. The paradise card is obviously a newer card, being half-size and offers both color and mono mode by means of a switch. And also an addressable LPT port.

 

Ensuring that the card was installed correctly I sat down at my bench and fabbed a bracket with a RCA socket, and a wired connector for the composite header. Connected everything up, and fired up the machine. After a few seconds the video initialized, and I began to see the ram check. But this is where my luck stopped.

 

After making sure that the board and the card are configured correctly with the main board dip switches and the jumpers on the card I can not get this card to display colors. It displays them in greyscale. Which is better than B&W, but not what I want. I have a RCA broadcast monitor that I can't wait to get colors on, but I have also tested against another CRT and a modern LCD. All with the same results.

 

I know that the motherboard crystal is probably slightly out of spec. But the system runs fine, and if it were really bad I would just get garbage on the screen with the IBM card which uses the system clock for timing. The Paradise card has its own 16mhz XO oscillator, so color timing shouldn't be an issue. From what I understand these card do display color on the composite out. So I'm at a loss. I have used mode.com to configure the various video settings between BW40 BW80 CO40 and CO80. I can obviously tell the difference in display modes, but still no color. Any idea?

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That would definitely make sense. I'll have to source a part, the footprint is too large for the adjustable caps I have on hand. The cap appears as it does partial work, since I do see the horizontal sync change just a tad bit (wavy lines to the right of characters).

A CGA card runs off the motherboard crystal, which is 14.318MHz. An MDA or Hercules card runs off its own crystal, which is 16-16.257MHz.

 

Is your IBM CGA card an old or new card? You can find out here : http://nerdlypleasures.blogspot.com/2013/11/ibm-pc-color-composite-graphics.html

 

Old IBM CGA cards give a rather non-standard composite output, newer TVs can have trouble distinguishing color from sync in its signal. New CGA cards are much easier to work with when you are using display devices from the 21st century.

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I don't have it in front of me, but I believe it's the newer board for the IBM card. It is brown...not that this helps with the ID number. However the other card is a Color/Mono Paradise CGA card. It displays shades for colors, but not color. And has a 16mhz onboard XO oscillator. I'm guessing that it is for the Mono.

 

From what I can see when adjusting the Cap and your description above conceding the crystal frequency, it is likely a faulty variable cap. I'm going to have to find a replacement. The closest I can find in the proper footprint is overseas. I may have to breakdown and order it.

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Yeah, I've read through that page. I think I'm going to replace it. What I don't understand is that the CGA card doesn't have its own oscillator. So even the H-sync and V-sync would run off that signal, unless I'm missing something. And there is no problem with those, I get a sharp picture. Just no color.

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Yeah, I've read through that page. I think I'm going to replace it. What I don't understand is that the CGA card doesn't have its own oscillator. So even the H-sync and V-sync would run off that signal, unless I'm missing something. And there is no problem with those, I get a sharp picture. Just no color.

From one clock crystal, you can derive many clocks used in the system

 

14.318 - Used for dot clock in high resolution (640x200) graphics

14.318 / 2 = 7.16 - Used for dot clock in medium resolution (320x200) graphics

14.318 / 3 = 4.77 - Used for CPU and NPU speed

14.318 / 4 = 3.58 - NTSC Color Burst signal (color in all modes)

14.318 / 12= 1.19 - Programmable Interval Timer base frequency

 

Serial, floppy and monochrome cards require their own crystals, but you got a lot of bang for the buck with one crystal.

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I get that. What I'm saying is that I don't understand how the crystal could be out of spec, yet still offer the right pulse for the V-sync and H-sync.

 

Of course, I could also not be lazy and actually look at the schematic and figure out the theory of operation. Best guess is that it's not out of spec enough to cause disturbance in the picture clarity...

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