ZX81 Composite PCB

I have a couple of ZX81’s both of which have been modded to output composite video. With the first one I applied the well documented capacitor mod with no problems and reasonable output quality. The second caused me a few more problems, I couldn’t get a stable output picture and ended up buying a PCB from Ebay. This implemented a more robust fix including adding the backporch signal for early boards that didn’t implement it.

After some research I found that the board was implementing a schematic designed by YouTuber jouselpercoulomb and well documented here. Armed with this information I looked for a PCB design to get them made for future fixes – I couldn’t find one. Time to open KiKad and design my own board.

ZX Composite mod schematic (V2)

This didn’t take long to design v1 and a couple of weeks to arrive from PCBway. The same as the ZX Spectrum composite mod board I designed it to fit into the original modulator case.

PCB

I double checked the circuit once it was populated and noticed that one of the connections was incorrect(doh). But it was easily fixed by cutting one track and adding a small bodge wire. I also updated the schematic.

Fitting into the ZX81 is a little ‘fiddly’ as the video modulator case can sink a lot of heat so de-soldering can be a pain. But after about 20mins it was free and the new board was wired in.

Modded ZX81

The output video quality was much better that the original capacitor mod – so job well done.

I have posted the KiKad files to my GitHub account if anyone else wants to create their own.