When looking for a modification to improve the video size/quality of DVC games on my PAL CD-i player, I’ve found that only two tutorials are covering that kind of modification (here / here and here). The modification enables a PAL player to display full screen video without the black bars on top and bottom. NTSC players benefit from this modification too, as there are PAL exclusive software titles that already have full screen video (e.g. De Zaak van Sam) – without the modification, parts of the screen are cut off.
Unfortunately, the mainboard of my CD-i 220 differs from those used in the tutorials, so I had to get a service manual to figure it out myself. The service manual I found is valid for the Philips CD-i players CDI 220/20 220/25 220/39 (PAL) and CDI 220/31 220/37 (NTSC). It says there is an unimplemented switch 1201 in square C6 of the mainboard: