STF micro ATX projet - Noise on Video Output

[Giants]

Hi !

Always working on my Atari STF-ATX projet.
Now, keyboard work good and all games/demos seems working.

But... I have some noise in specific RGB 'pattern'

Ex : A full Red Color 256/0/0 give me a good color on screen
(Tested well : Red, Green, Blue, Black)

But... In another color, I have a 'noise' (The colored box is buzzing) with a Noise pattern in this colored box
Ex : 128/128/255
See, I created a picture on Degas Elite

Note : The ST_ATX board can be powered with AT-PSU or 5V 3A with a standard 5V 'phone dongle'

I have this issue with all PSU I have tested, present with my DEL LCD (It accept 15Khz) AND the Atari SC1535 monitor.

I have also tested 2 others Shifter, same problem.
I have changed and check, Beam, inductance, résistor, transistor of Amplification Video schema
Change 2N3904 with BC548, worse in 'white' color.
Try Add a 3 x 22pF on RGB shifter output, no change.

If any body have idea...

Thks a lot.

IMG_20260416_151852.jpg

IMG_20260416_151715.jpg

IMG_20260417_101545.jpg

[Giants]

Hummm !
I'v compared with a STE output and, that's I think, not a good idea.
STF output are on 3 bits
STE output are on 4 bits

Amplification schematic it's NOT the same.
Value vor component is Not the same.

The video output on the STF is known to have this kind of issue.
The severity varies depending on the machine's PCB, but it's nothing like the STE.

I took some measurements with an oscilloscope before and after the transistor, both on STF and STE.
I asked ChapGpt to help me with the analysis.

The output on the STF has 3% more noise; on the STE, it doesn’t.

I got some advice from him (I’m a bit wary… anything’s possible with ChapGpt)
to improve the STF’s output.
Nothing too complicated, just a few resistors to replace.
I’ll check it out…

Anyway, I was planning to add an HDMI output using an RGB2HDMI adapter,
but well...

[britelite]

The RGB-output on my ST ATX board is also pretty noisy, but the image via RGB2HDMI is clean and it works like a charm.

[Giants]

Ah ?

Is it possible for you, with Dega Elite (or another program)
to create a large grid with a 256x128x128 (or use the red color available on right degas)
and take a close-up photo?

ThisOne.jpg

Yep, I think RGB2hdmi will be solve the problem.

[britelite]

Is it possible for you, with Dega Elite (or another program)
to create a large grid with a 256x128x128 (or use the red color available on right degas)
and take a close-up photo?

Sure thing. I won't have access to my ATX ST until next week, but I'll take a photo then.

[Giants]

Well....

Add a RGB2HDMI this day, work like a sharm