I'm no expert, by any means, but here's a few things to try:
1. Are you using an isolation transformer?
2. Make sure your monitor is actually powering up. To do this, turn off the lights in the room, then (with the monitor not connected to your PC) switch on the monitor. You should see the end of the tube near the neckboard light up and glow. (You'll prolly also hear a staticy sort of sound, especially when you then power off).
3. Once you've confirmed your monitor is powering up OK, check your wiring on your cable hack - you can certainly try separating out the H-synch and v-synch. If you are using an ultimarc video amp, you have to cut a trace somewhere on the amp to split the signal. There's instructions on the ultimarc site
http://www.ultimarc.com/vidamp.html Also, my monitor (a kaga KZ-18en-H) has little row of switches that lets you select what sort of synch signal the montior expects. Might be worth checking this on your chassis.
4. Also, what video card are you using? People around here will waste no time in recomending ultimarc's arcadevga, but other cards work too (just with more fuss). If you are using anything other than an arcadevga, you will of course need special drivers.
However, even with an arcadevga, you may need to boost the vga signal level. This will mean using a video amp (either build your own, I think there are schematics somewhere, maybe the pc2jamma page?) or buy one from ultimarc.
5. Finally, have you tried fidlling with the brightness pot on your chassis? (It might be that you just can't see the picture).
Other than that, might be one for the experts round these parts!!
Good luck
cheers
bviking