Sinclair ZX81 - composite mod and repairs.

... or , gee that is a tiny computer

. .

INTRO

Poor joke time:

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

/INTRO

ZX 81

If I remember correctly, it was on RetroKomp/LoadError 2014 where Bachoo brought his Sinclair machines collection. I was kinda amazed how tiny ZX81 machine is.
Obviously, 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 !?

via GIPHY

After a week, I've finally received it.

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

PSU

I was super happy that I've 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 :/
Dissasembly followed shortly ...
Here are all the chips and PCB overview after opening it.





After a bit of looking at PCB," It became super obvious what caused a problem -
Cold-joints.


Keyboard membrane

I've noticed that there might be a slight problem with 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 in 48k, which is an easy thing to do and requires rewiring few elements and that's it.
That was not a case here, but still nothing really 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 above work finally let me to power on that little machine made by Lord Sinclair on my modern flat TV screen :D


... and here is final video output ;)

Drygol

Chaos is your redemption .... better run .... better hide

I come from Internetz :>