Sinclair ZX81 Video mod

In these modern times, it is becoming more and more difficult to plug an old computer with an RF modulator into a TV and actually see a decent picture. I have a couple of flat screen TVs that take analogue signals and sort of work, but the picture is always terrible. It also suffers from drift, with the picture sometimes now working the next time you plug it in and requiring a re-tune. Whereas I don’t like changing old kit too much, risking spoiling the original feel of it, there are certain mods, like this one, that make the thing much more usable!

There are a few circuits online that you can either build yourself with a couple of resisters and a transistor. Or you can buy a pre-built circuit. That is what I did, and it was really cheap. Very well-built and tiny.

The mod here converts the RF output to a composite video signal. Much easier to use and more stable. The goal here is to have a machine that just works without any re-tuning.

The RF module has a metal lid to form a Faraday cage, blocking stray RF interference. That needs to be removed to get access to the output signal. Here I will be reusing the RF connector and re-purposing it as a composite video socket.

Remove the lid from the RF can

Cut the lead on the RF connector, the one that comes from the resistor:

Keep enough length on the lead to take some solder and a lead later on.

Next cut the 5V wire from the motherboard to the RF Module and the input connection:

The ground connection here can be soldered onto a hole that is right next to the RF module:

The circuit board here has a 4 pin connector. Riffling through my part’s bin I found the perfect cable:

It is an old CD audio cable. The great thing here is that the ground is a shilded connection which is perfect for a video output. Just had to move the ground connection to the end as the pinout of the circuit is:





Making sure each of the connections has some shrink-wrap on it, so we can make it neat and stop it from shorting out.

Added a sticky pad to the bottom of the circuit board to install it nicely

Pushed it into place, added some foam pads to the RF modulator can to avoid and sharp edges on the output wire:

I’m really pleased with this. One of the things I was trying to do is to keep the wires as short as possible, but have them shielded. It would also be easy to revert this back to RF should the need arise!

So, does it work? Well, plugging it into a monitor and firing it up we get:

The picture does not look amazing here in the photo, but it is rock solid in real life. No tuning required, and it is very easy on the eye. So, my ZX81 from 30+ years ago has entered the modern (cough) world!

Next I have to install my 16K RAM pack and see if I can load some games!