what the heck is FBAS? did you mean CVBS?

maxtherabbit wrote on 2022-07-08, 12:18:

what the heck is FBAS? did you mean CVBS?

FBAS is CVBS in German . https://de.m.wikipedia.org/wiki/Fernsehsignal#FBAS-Signal

Farb-Bild-Austast-Synchron-Signal
translates to
Color image-Blank-Synchro-Signal
so essentially
Composite Video Blanking and Sync Signal

Translation is by Google as I don't speak German .

maxtherabbit wrote on 2022-07-08, 12:18:

what the heck is FBAS? did you mean CVBS?

I bet yes since the 5155's monitor is a composite one
the only replacement card I know of that supports this is the ATI EGA WONDER that has a dedicated connector for use with the 5155

There are VGA to AV converters..
If the 5155 has some free space inside, a converter's PCB could be installed inside.

VGA signals can be tapped via feature connector or by tapping the VGA connector pins on the solder side.

Technically, maybe it's also possible to force VGA to monochrome mode (via pin) and send a Mono H+V signal to the monitor (instead of RGB)?
https://www.geocities.ws/podernixie/htpc/cables-en.html

Along with some DOS utility, a VGA CRTC can be programmed to output 15KHz instead of 31KHz.

Not sure if that's reliable, though. Timing, I mean.
The converter board should produce a standard PAL/NTSC signal, with interlacing.

Which in turn cause a bit of noise on a monochrome screen.
A pure BAS/VBS signal would be better in terms of quality.
A low-pass filter maybe can used to remove colour information.

Or, just use the Luma part of the S-Video port, if the converter has one.

Anyway, I'd recommend testing with an external monitor/tv first.

Edit: Um, did I say something wrong? 🤷‍♂️