So now after having most of the BKM-129X compatible monitors tested out properly, and some minor hardware adjustments fixed, I’ve made a spin-off of the BKM-129X compatible board, a direct SCART version with audio passthrough, and selectable mono mixing circuit.
Now there’s really not much difference to this board and the BNC version; the BNC plugs have been replaced with a SCART plug (I’ve used a Ninigi SCART, https://www.tme.eu/dk/en/details/scart-17/euro-hdmi-connectors/ninigi/) and then the two RCAs are added which feature the stereo audio passthrough from SCART. Along is a selectable mono mixing circuit, which is activated by the push button switch between the RCAs. The switch is supposed to be added between the RCAs. When pushed in, the stereo sound from the SCART is mixed to mono, when out, it is direct stereo. The mono mode is meant for monitors with audio inputs (which is always mono afaik) so a short RCA cable should be connected between the black connector and the monitor.
I’ve created a 3D printable bracket to fit in the slots for these cards, which will make everything sit flush with the monitor. The files can be found here: https://www.thingiverse.com/thing:4460188
So not much to it, the Kicad project along with gerber files can be found here: https://github.com/skumlos/bkm-129x-simple-scart