Unused inputs on the 7474 should be tied high or low,don't leave them floating.
I'm not sure, but I think it MUST be done only for CMOS parts, the proposed circuit uses LS parts so it can be facultative...
Anyway I just wanted to point out that in Telis's circuit the outputs of the '74 ( pin 8 & 9) are both connected to GND which I don't think it' a good practice.
I thought using triple potentiometer (500 ohm, like piher pt10 used on the slg3000) would be ok, putting one after HD74LS125.
It won't work, this circuit is totally digital, it can only do on/off.
To do what you want you'll have to replace the '125 with an analog circuit (for example an op-amp).
800x600 scanlines don´t work and 1024x768 works but scanlines is not good. I´m wainting my new video converter to test via Component and 640x480 output.
I found this table with the timings of the VGA modes:
Mode Horiz Vertical Horiz Vert Horiz HSYNC Vertical VSYNC
Dots Lines KHz Hz Sync Pol Sync Pol
VGA-480 640 480 31.5 60 3.8 us - 64 us -
VGA-400 640 400 31.5 70 3.8 us - 64 us +
SVGA I 800 600 35.2 56 2.0 us - 57 us -
SVGA II 800 600 37.8 60 3.2 us + 106 us +
SVGA III 800 600 48.0 72 2.4 us + 125 us +
XGA 1024 768 48.5 60 2.0 us - 124 us
As you can see both the SVGA@60 and SVGA@72 modes use positive pulses for the Hsync, maybe that's why it doesn't work.