Just a few parts (call it the $1 Video-Kit) are needed, to connect a Video-Monitor to the ZX81.

And it's easy to build:

1) open the ZX and remove the cover from the TV-modulator

2) inside the TV-modulator cut off the resisitor from the TV-output and solder the capacitor between

3) place the parts outside the modulator "on-the-fly" and connect all like shown

4) close the modulator

5) connect the diodes (1N4148) and the 120R

It doesn't care, which type of transistor is used. If the screen appears unsharpen, another type of transistor will be better (best is a RF-transistor).

In random cases the screen can be distorted and unreadable, due to a too high output. Test it with a potentiometer instead the 62R-resistor, say up to 5k or so, and then solder in another resistor instead the 62R.

That's all!