"A quick check of the service manual tells me that there is a pair of resistors that connect CPU pin 58 to either +5V or GND in map section C2. Try moving 3290 to 3203 for 60 Hz. Don't forget to buy a regular 10 kΩ resistor. Soldering wires to a SMD resistor isn't going to be easy... Most of the jumpers are resistors, at least on the Mono boards. Mini MMC = regular solder bridge to switch between 50/60 Hz. Mono I and II = 1 kΩ. Mono III and IV = 10 kΩ.
You'll have to remove DVC, CD assembly and the mainboard. The resistor pads are on the back of the mainboard. In the first picture I've marked the four pads (resistor 3290 has already been removed). The two lower pads go to CPU pin 58. Connect them with a 10 kΩ resistor to the center of the switch. The upper left pad is for 50 Hz and the upper right pad for 60 Hz."
More extensive info you can find in his article at retrostuff.
[Thanks, Rosewood]