Building a ZX80 in 2019

Pretty much 40 years ago I was presented with a wooden box, with a set of keys on the top. The keys pushed with a click and stood proud with clear caps. Under each key was a printed letter, a word in red (if not two) and for some of them a symbol or ‘graphic’.

Inside the box on some long studding bolts as spacers was a Sinclair ZX81, with a Kayide 16kb RAM pack plugged into the edge connector. The wood was super thin ply, and the holes in the sides for the ingress and egress of cables were round-ish and splintered at the edges. A metal plate heatsink sat proud from the regulator.

I closed the box again and looked up with joy and awe.

This was my very first computer. It, like others, hand-me-downs (Sinclair QL and Nokia PC XT) – with three degrees of Sinclair Spectrium filling the void between getting to grips with BASIC and leaving all that behind with the PC AT.

Realistically there were three ‘moments’ – the Linux server that I built to solve issues at school (as a teacher), the PC AT and Windows, and this, the ZX81.

A little history

The ZX81 was the larger ROM’d successor to the ZX80 – where design and manufacture had evolved. Available from the 5th March 1981 it could be purchased for £49.95 kit, £69.95 assembled (£188–263 at 2019 prices).

The video output was by UHF – so you would have to tune in a television to a specific channel, and plug in the RF lead in the back. Back then this meant having a second TV – or indeed having to change the lead. For me, I had the wonderous OLD TV’s at first that smelt of damp hot valves, years of dust, and hummed with high voltage – the smell of danger. Extended use of the H-Sync, and watching the image collapse to a super bright line, then down to a dot before fading. There was no sound. None (although you could cause the CPU stress that would cause noise on the channel that could give the appearance of ‘sound’).

For me – it really started to get me asking why – and wondering how.

You have a whole Kilobyte to play with – so the world is not a big place – go – go explore!

If you were not a child born into this world – or just need a reminder – fill your boots here: https://en.wikipedia.org/wiki/ZX81

Why?

The kit was not cheap – not at all – but that was not the point. The answer was more of a complext one.

In short: Emotional response.

When I powered it on, I didn’t expect it to work. Not one moment. Why would it? I had just followed some instructions and thrown a bunch of components at a circuit board over a series of evenings with QUESTIONABLE skill. I was happy with it simply joining the other Sinclairs on my wall – it had been a journey in terms of memories, and that was enough alone. It was art if nothing else.

However apparently there was an expression of childlike awe when it fired up, and the cursor appeared (after a second or so more than I would have liked if I am truthful!).

For the sheer ‘Looking upon the face of your creator’ – for the simpler world we lived in – the memories – the feeling of enablement it gave me – for the sheer ‘managing to remember what the key layout was’ of it all… to say “I did that”.

“I. Did that.” – me. Damn.

Thank you http://blog.tynemouthsoftware.co.uk and the rabbit hole that led me to your shop.

How was it for you?

Well – my electronics is ‘minimal’ – and my soldering is ‘functional’, and usually rare and big – best described as of the ‘last chance saloon repairs’.

Muttered disclaimers about such things as ‘T state analysis’ or ‘Karnaugh Maps’ are more a product of a necessary passage than a working knowledge – so such things are not out of reach as such – but none the less it is apparent that due to the time and effort put into this kit – it is not at all hard to end a Sunday evening moved to tears by a small inverted K on a screen.

A series of anti-static bags, two PCB’s, a bag of chips on foam, and so so many tiny components.

Despite the rather concerning schematics, the reality is that there are parts, and values, and a picklist with everything you need to know to find those parts. Sooner or later you run out of parts and its time to make the magic happen.

An obvious choice was a little Hello World, and a loop.

Amazed that I knew where the cursor keys were without looking (function and numbers) and how to run, list, edit, and so on. HOW ON EARTH do I forget so much, yet this is etched in there after so long?!

You would assume from this that I am easily pleased – if anything the opposite – but wow – what an adventure.

What didnt go so well?

Me, my skillz ;)

Jumpers that need to be cut can travel a surprising distance – to the point of jumping into another dimension never to be seen again.

… be safe, be happy adventuring jumpers.

I resolved this with a phat solder trail in this case.

The only other disapointment was the perspex base – when it came time to attach to it – the holes did not line up.

I should imagine this had as much to do with me not attaching the keyboard pin outs properly or something – as they would have lined up if those had have been a few mm closer together.

While it went well it was a lot of soldering for something I had not even factored in – the keyboard. Youch! Thankfully this was the alst task I took on and felt a lot more comfortable.

Other than that – what a thing!

Overall?

To the person who picked out the tiny caps with illegible writing on them – I SALUTE YOU.

A bit of adventure, a lot of soldering, a megatonne of history – all at once. Damn. Balm for the soul

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