, thanks for trying to help.
The monitor has no apparent way to accept sync on green. I've navigated the On-Screen-Display menu and explored every possible setting, but most of them are related to screen geometry, position, colour temperature, moiré, degauss, contrast, brightness.
The leaflet for the video adaptor is located here
(PDF file). My model is the first, "MAC/IBM ADAPTOR: 15DM/HF-10P", and I've tried every switch combination in MODE 6 (which according to the leaflet is for Sync on Green), but to no avail. The results are the same as in my previous post: either no image or green-tinged image. I even tried quasi-random combinations to see which switches trigger the image, but no change in the outcome.
It is very frustrating. I had managed to upgrade the RAM in the Quadra from 28 to 40 MB and was happy to revive the old beast in combination with this nice 19" monitor, so that I could get rid of the old 14" Apple Colour Monitor, which weighs like a car...
I could even look for another video adaptor, but in a situation like this how can one know if it works or not?