I just acquired an Atari STFM with a SM124 and SC124 monitor.
When starting it up for the first time, the monitor does power on as well as the computer, it accesses the floppy drive, but the video output is not working.
See attached photo and video.
The SM124 monitor has the white line but the SC124 has no video at all.
Not sure if it's a monitor problem or an internal video card problem.