@moominpapa:
Many TFTs need atleast a 31.5kHz/56Hz screenmode. If you run PAL you will get 31.5/50Hz and if you run NTSC you will get 31.5kHz/60Hz. So connect a working monitor to it, set workbench to run in NTSC and then connect your TFT.
Btw, after turning that little screw, the image can be rather distorted, as it is the adjustment for the internal flickerfixer. Ie, you might need to turn it a bit to get a good image again (preferably after the computer has warmed up).
/Patrik