Hi Vanthaelen, welcome to the forum.
I myself use a Panasonic TX-MD251 (25" SCART). I have the problem you mention and I think it's referred to as 'overscan'. You are correct in thinking that access to the service mode will correct this problem (if you get it), as most service modes allow you to adjust the horiz/vert pos/size. However you can actually adjust these using a piece of software called Powerstrip (get it here:
http://entechtaiwan.net/util/ps.shtm). Search around for a FAQ on that forum and you'll find some basic info on how to do this. Be warned that your TV may not take well to being tweaked with this software though, so be careful! Ideally it's best to use the service mode of your TV (provided you can find it...do a google search).
I was lucky enough to find how to activate the service mode of my TV, but then discovered that one of the buttons required (to activate the mode), was missing off the front panel of my TV. I can only assume that it was removed by a TV engineer etc (my set is ex-rental).
I still have the overscan problem, but since using Minwah's excellent resolution tool (
http://mamewah.mameworld.net/downloads.html) in conjunction with my ArcadeVGA card, the problem is...well, not really a problem anymore. I still get a very small amount clipped off the top of the screen with some games, but it's nothing I can't handle!
We here in the UK are very lucky that we have access to a massive selection of SCART telly's which can be picked up very cheap now (due to advent of widescreen). I'd recommend paying around