It might be my case as well (800XL NTSC). I'm curious now, I'll check the voltages and try to match these values:
https://www.maximintegrated.com/en/design/technical-documents/tutorials/1/1184.html
NTSC S-Video
PAL S-Video
Setup
53.57mV
Setup
0mV
Y
714.29mV (Peak Luma, 100% White)
Y
700.00mV (Peak Luma, 100% White)
C
626.70mVP-P (75% Color Bars)
835.60mVP-P (100% Color Bars
C
663.80mVP-P (75% Color Bars)
885.10mVP-P (100% Color Bars
Sync
-286.00mV
Sync
-300.00mV