Sinclair ZX81 – composite mod and repairs.

… or, gee … this is a tiny computer


Poor joke time:

Deciphering INTRO …
INT RO -> int RO -> Read-only integer ! YAY!


ZX 81

If I remember correctly, it was on RetroKomp/LoadError 2014 where Bachoo brought his Sinclair machines collection. I was kinda amazed at how tiny the ZX81 machine is.
I needed one for my own collection ;P

Several CLICKS later it was on a way to my place.
I was like… WHERE IS IT !?

After a week, I finally received it.

Here it is, with a pen just to show how small it is. Raspberry PI is straight from the ’80s 😉


I was super happy that I received my lil machine and I’ve happily started to set it up. Unfortunately, after a while of fiddling with it, I’ve figured that PSU is kinda dead.  Disassembly followed shortly …
Here are all the chips and PCB overviews after opening it.

After a bit of looking at PCB,” It became obvious what caused a problem – Cold joints.

Keyboard membrane

I’ve noticed that there might be a slight problem with the oxidized keyboard membrane connector.
That required a bit of gentle work with sandpaper.
Also, a second strip of the connector was cut a bit and strengthened with a bit of thick PVC foil and Kapton” tape.

Video output

Once I’ve sorted out simple problems, I was ready to connect my little precious computer … and … F>A>I>L

I wasn’t able to tune my TV to the proper frequency. By proper, I mean ANY visible video output.
A bit of internetz lurking gave me a solution – composite mod 😉
At first, I thought it will be as simple as 48k, which is an easy thing to do and requires rewiring a few elements and that’s it.
That was not the case here, but still nothing fancy.
All I needed to do is to add an amplifying transistor circuit to a video signal along with a grounding resistor (schematics are popular on internetz 🙂
That was enough to get a decent video output on a modern TV
I’ve put it all inside an original modulator case.

Grand finale

All the above work finally let me power on that little machine made by Lord Sinclair on my modern flat TV screen 😀

… and here is the final video output 😉


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