Vertical-Size adjustments will not fit the entire image on the screen

[repetto74]

Hi to all of you

As per title I have an odd problem with my 25" CRT arcade monitor (model Monitor from Electronic System the former Hantarex) a clone of the old Hantarex Polo. I cannot get the image to fit the entire monitor frame vertically. I have turned the V-size pot at its full position and still have about a 10% black area on top and bottom!
The monitor was built in 2005 and sold to me still sealed in his box but I had this problem since the first time I turned it on. It has great colors and a perfect stable image...

I am using it in a cab with an ArcadeVGA 9250 card+Groovymame. I tried to swap the videocard with another one but that does not make any difference 
The screen is mounted horizontally and I can adjust the H-size without any problem but the V-size as I said will not fit the image entirely on the screen...

I have noticed that any vertical game launched will fit the entire height of the screen
Any clue on what is going on?
Thanks