Okay lets clear up some confusion here..
If you are located in Europe or Japan then the chances are your monitor will be a NANAO dual freq monitor.
If you are located in the US or Canada then you will be unlucky as Sega US used the crap WG Med Res monitors.
If your monitor is a SU26 (26") then the monitor freq select will be in the middle of the monitor chassis (quite tricky to get your hand down there).
If your monitor is a SU29 (29") then the monitor freq select will be at the back of the chassis (very obvious to spot, there be a row of pins, and a plug will be on one row set of pins).
The new Nanao which is on some new Japanese sit-down cabs and newer games (Dedicated VF3, F355 DX and so on), is Tri-Sync and is awesome, it has OSD and and auto detects from 15/25 and 31.5kHz.