Sinclair ZX81 Repair
Can't Quite Tune The Telly
So I have two of these, managed to get the first one for a good price as it had been mislabled as a ZX Spectrum. This one did not need much done to it to get it working. First the PSU was dead, turned out the original transformer had burned out. Just replaced that with a new transformer salvaged from something else.
The composite mod just required a singal transistor. This was because it was a later 1982 model ZX81 sold for about £50 post Spectrum, I have the original reseet. The keyboard also needed trimming down as it had brioken. It has since broke again & will be replaced.
All you need for a composite mod is a 2N3904 transistor & a 100 ohm resistor.
ROM with a date code of 32nd week of 1982. Still using 1981 firmware from the copyright.
Ha a MOSTEK RAM chip that is not dead. Nice ceramic pakage, never seen an instance of this chip in any other ZX81.
A Second Real Rabbit Hole
So I got a second one for a good price, came with some homemade hardware, hand drawn scematic & a homemade keyboard I used for my Acorn Electron portable project. The ZX81 was in poor condition & has mod damage to the case. I had plans to restore it from the begining, maybe even tune it into a laptop. Restoring it is what ended up happening. The keyboard was the least of this machines issues.
This unit does not containe any components of note unlike my other ZX81.
The first issue was looking into why there was no one home, tuned out to be a dead CPU, probaly from the mods this machine saw. Anyway luckly I had a spare one from a bunch of parts I had salvaged at uni.
As vintage 78xx regulators tend to fail but outputting their inout voltage to the output, killing any 5V device. So i poped a newer one of from 2005 that was also salvaged. 78xx regulators really have been used for decades.
The real pig of this project was the composite mod. As this is an early unit ZX81 where all the ULA says on it is the date code, 8113. It lacks the back porch part of the composite signal. Lucily there is a circuit so I followed the guid & built it & it worked, kinda. It worked in the sence it proccess the signal properly & added the back porch which was clear on the scope. However it did not amplify the singnal to an extent that any composite device could even detect.
Despite countless hours of trobleshooting & checking over the my build with the one off the guide I was never able to figure out why, dispaite others building the same circuit & having success. So I moved to another tacktic.
So I soldered in a modual I got off of eBay, the circuit is the exact same as the one I built but the only diffrence being it magicly works.
The composite modual running on a CRT SONY Trinitron TV in my bedroom.
The ZX81 running on my work bench pluged into a modern LG LCD TV.
Fully fixed 1981 ZX81 PCB running on the bech, looks the same as a dead ZX81 board.This ZX81 does have an intresting qurk, it needs to warm up before it boots.
Keycap Fun
So both keyboards have arrived which will hopefuly have a longer shelf life, not that 30-40 years is a bad shelf life. But how long are these reproductions going to be made for.
Anyway I will cover putting these in one of the machines even though I will do both as the proccess is the same for both computers.
The condition of the keyboard membrains on both computers was about the same. The plastic has degraded to the point it is very brittle & breaks when flexed at the extream angals they sit at in the computer.
The new membrain despite being bought in 2022 uses a design from 2014. Guess there is no point updating it for a computer trapped in time. You can see the 40 odd years of fading here.
The underside of the new membrain. Same circuit but a diffrent layout. The original one was no dought done by hand, much like the computer its self. I don't think CAD software really exsisted in 1980 when this machine would of been designed.
The new membrain pluged into the original connectors, no point changing what ain't broke. The ZX81 PCB looking most snazy.
The keyboard fully installed, worked a treat on both ZX81's. You can see from the images that the new membrain is a lot less faded. 40+ years will do that to inks & paints. Looks bake to being really snazy smart machine. If only it had the horse power to be useful.