(Update: 05 jan 2002)

Here is the schematic:

Use a simple NPN transistor for this task. But be carefull when you are soldering on your ZX motherboard. The ULA is very sensitive against static electricity and may be damaged. You can take the video input signal from ULA pin 16 or directly from the modulator.
The photo shows the two connections to the modulator. At the left you find the video signal and the right wire holds the +5V supply voltage. Ground can be taken from the modulator case