Try to adjust the trimmer cap on the mainboard first. Oscillator may be running out of tolernace due to aging.

I would look at the caps before messing with the oscillator personally, if it's not at fault don't you need an oscilloscope to tune it back at the correct frequency so you get correct colors?

From my experience the trimmer cap changes between 'stable color', 'unstable color' and 'no color at all'. In the 'stable color' which is like 1/3 of the trimmers range there is no hue/tint change. Trimming is not really critical.

But of course you can mark the original position.

h-a-l-9000 wrote:

From my experience the trimmer cap changes between 'stable color', 'unstable color' and 'no color at all'. In the 'stable color' which is like 1/3 of the trimmers range there is no hue/tint change. Trimming is not really critical.

But of course you can mark the original position.

That is my experience with the trimmer, although I think the one in my 5150 is less precise than it used to be.

Ah, I was under the impression that clocks should be very precise to get correct colors, I stand corrected 😀
Is there a possibility to check with a loaner cga as well? Maybe it's the card at fault, could be the case some 74xxxx fails after a while and messes things up...

> Maybe it's the card at fault, could be the case some 74xxxx fails after a while
Could be - but the trimmer is more suspect and easier to diagonse 😀

Clocks are synchronized on the receiver by the color burst. Only if it deviates too much it will malfunction.

Btw Jinxter, how do you know that your monitor is not broken? Would be a good idea to check with another display device i.e. TV capture card.
The monitor may also have a crystal with a trimmer cap, the latter known from TV sets as failing over time due to corrosion.

Thanks for your help. Adjusting the trimmer worked perfectly. No i can enjoy the SD card filled with CGA/composite games.

The only game that did not behave as before was PakuPaku, where the colors are gone. All other games/demos behave as expected.

Jinxter wrote:

The only game that did not behave as before was PakuPaku, where the colors are gone. All other games/demos behave as expected.

If I'm not mistaken, Paku Paku uses a 160x100 tweaked textmode, which is based off an 80 column mode.
CGA does not enable the colorburst signal in 80 column mode over composite, because the fine resolution used in 80 column mode would cause a lot of artifacts, making text unreadable.
So it will not have colour on composite.

In 8088 MPH we hack around this limitation, but that's why the calibration screen is there: since 80 column mode is not designed for colorburst, the colorburst does not even occur in the right place, timing-wise. This leads to instability in the synchronization. A known workaround is to use a specific background colour, which is then 'mistaken' for a proper colorburst signal, and the screen can synchronize to that. Since this is out-of-spec, it is specific to each display, which is why we allow the user to fine-tune the settings to get a stable image and the best possible colours.

Paku Paku does none of this afaik, so it will just not have colour.