Odd, old IBM CGA board (1804464)

[VileR]

Here's a card that just came in the mail. 1804464 was one of those "unknown type" CGA P/Ns (see this old thread) - i.e., it wasn't known whether it had an older-style or newer-style composite video output.

Having tested it now on my TV, I can confirm that it definitely shows an older-style output. However, there are a couple of oddities about this board.
First, there's all this wiring around the back (click for full size):



I haven't the slightest clue what this is for, but it looks neat enough that I don't think it's someone's custom job. Could it have been shipped out of the factory like that? Correction for a defective batch perhaps?
Then there's the composite output stage (click):



Only 6 DAC resistors here - as far as I can recall, other old-style IBM CGAs have 7. This one is missing R17 (next to R1 and R4), and there's no silk-screening or pads for it either (as opposed to R9..R16).

Looking at the TV output, I can't distinguish anything really different from what I'd expect for an old-style CGA. That is, 8088MPH shows the right colors; the initial 'which CGA' screen reports "OLD"; etc. I could take a few TV photos if that would help, but I suspect it wouldn't, since I don't currently have another old-style CGA to compare against on the same TV.

What could be the significance of the missing resistor, then? @reenigne? anyone?

 

[romanon]

I have same model i guess
https://ibm-pc.webnode.sk/_files/200000465-a58afa6849/IMG_6476.JPG
https://ibm-pc.webnode.sk/_files/200000466-223b32335a/IMG_6477.JPG

Its curious that you dont have black sheet on card

 

[reenigne]

What could be the significance of the missing resistor, then? @reenigne? anyone?

Normally on old CGA, R1 and R17 form a voltage divider to reduce the overall voltage of the composite output to 3/4 of what it would otherwise be. I'm guessing this was a very early version where they didn't realise that reduction was necessary - perhaps after this card was designed they found some monitors that couldn't handle such a high signal. Strangely, the new-CGA schematic in the Options and Adapters manual also lacks this resistor, but has a higher value for R4 - I'm guessing that acts to reduce the signal in the same way, just with fewer resistors.

 

[VileR]

I have same model i guess
https://ibm-pc.webnode.sk/_files/200000465-a58afa6849/IMG_6476.JPG
https://ibm-pc.webnode.sk/_files/200000466-223b32335a/IMG_6477.JPG

Its curious that you dont have black sheet on card

Nice - yep, same yellow wiring and very similar date codes, so I guess that answers my first question. Although I was also curious about the narrow 5160-style bracket on my card... for a mid-1982 card I'd expect a black wide 5150-style bracket like yours.

Normally on old CGA, R1 and R17 form a voltage divider to reduce the overall voltage of the composite output to 3/4 of what it would otherwise be. I'm guessing this was a very early version where they didn't realise that reduction was necessary - perhaps after this card was designed they found some monitors that couldn't handle such a high signal. Strangely, the new-CGA schematic in the Options and Adapters manual also lacks this resistor, but has a higher value for R4 - I'm guessing that acts to reduce the signal in the same way, just with fewer resistors.

Interesting, thanks! Makes sense, even if it doesn't entirely clear up the odd timeline of those revisions...

 

[VileR]

A couple of observations after playing with this board some more. Compared with the output of a 'new-style' CGA:

- In color modes, the image has a much higher contrast.
- In B&W modes, the image has a much higher brightness (i.e. while color 0 appears black with the burst enabled, it's a mid-grey with the burst disabled).

I suppose this could be explained by the card giving out a 'hotter' signal, as mentioned? Since the presence of a colorburst also triggers some normalization of levels on the TV side, I could see how the two cases might be related.