Since the HxC2001 firmware can give OSD viewing now...
Does anyone know where the same points are on a 464/6128 plus?
https://hxc2001.com/download/floppy_drive_emulator/HxCGotek_AmstradCPC6128_OSD_Mod.jpg (https://hxc2001.com/download/floppy_drive_emulator/HxCGotek_AmstradCPC6128_OSD_Mod.jpg)
^^^ is the normal CPC6128
Looks like Green is on R77 and Sync is on R87 (R177 and R187 in the schematics). Connect to the side of R77 that goes to the monitor, and the side of R87 *opposite* the monitor.