I released my ZX81+35 ZX-81 clone design files on Github

Discussions about Sinclair ZX80 and ZX81 Hardware
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mahjongg
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Re: I released my ZX81+35 ZX-81 clone design files on Github

Post by mahjongg » Fri Sep 06, 2019 12:07 am

I have drawn the keyboard schematic for my ZX81+35:

Image

I also did the placement, and checked it against my keyboard overlay:

details on my revspace pages.

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mahjongg
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Re: I released my ZX81+35 ZX-81 clone design files on Github

Post by mahjongg » Wed Sep 11, 2019 11:11 pm

And this is how the PCB looks, the component side

Image

and the solder side

Image

the PCB works with a real ZX-81, or my clone, and its dimensions are such that it will fit in the keyboard recess (if you round of the PCB corners somewhat) of a real ZX-81, as it is 6.2 inch wide, and 2,5 inch deep.

you can solder 5-wire and an 8-wire flat-flex cables to the PCB, and stick those trough the opening in the case and connect the flat-flex to the ZX-81 as you would with the normal foil keyboard.

The keyboard uses forty 6 x 6 mm buttons ( MCDTS6-2N or equivalent ) keys with a 6.5 mm x 4.5mm pad pitch, these are industry standard keys and about the most cost effective keys you can buy.

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mahjongg
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Re: I released my ZX81+35 ZX-81 clone design files on Github

Post by mahjongg » Mon Sep 16, 2019 8:04 pm

Well, from the few comments I got I conclude there is not really much enthusiasm to build my ZX81+35 clone, some people even supposing I didn't have a working prototype (but I do, and its working great that is why I have confidence its 100% compatible) .

But the reaction I got the most was, its (mostly) SMT and that's way too difficult for me.
I can see that now, I saw it from the start, but because soldering is part of my job I have no problem with SMT, but I do understand why it scares off many people.

also, PCB's have become much less expensive lately, so I'm not restricted to the scrap size of 10 x 10cm to keep things affordable.

So I think I will start over, and design a much easier to build simplified version, but one that is still 100% compatible with the real ZX-81, will drive modern TV's well, and load tapes with ease from the sound output of a laptop.

I will start by examining if every part in my design is still available in PTH, I think that shouldn't be a problem (knocks on wood).

hopefully there are still people alive when I finish with it that remember the ZX-81 :lol: (i'm joking, I guess it will take me only a couple of months to get a prototype ready) and hopefully there will still be interest in it. I'm gambling there will be, as RETRO computing interest seems to be on the rise again.

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Erm
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Re: I released my ZX81+35 ZX-81 clone design files on Github

Post by Erm » Mon Sep 16, 2019 8:25 pm

I'd love a new ZX81 kit and would buy a few off you. In fact, I actually went to the trouble of taking apart a fully working original ZX81 just so I could sell it as a kit and it was very popular, I got more for it than I would have if I'd left it built! But obviously it took some time to carefully take it all apart. But yes, SMT is too small for most people. I think it will do ok, if the final price is right.
I'm Chris, a Brit in Germany (Chr$ on the QL forum).

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Re: I released my ZX81+35 ZX-81 clone design files on Github

Post by nollkolltroll » Tue Sep 17, 2019 7:20 am

I've looked with delight at your different posts regarding your project over the years. It is a well documented build that I would love to get to build sometime. Even more, I'd love a version that is through-hole, hope there is a kit in the end.
I'd also say the silence regarding most projects is quite normal, don't expect too much cheers or you'll end up disappointed. I know I was.
/Adam

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mrtinb
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Re: I released my ZX81+35 ZX-81 clone design files on Github

Post by mrtinb » Tue Sep 17, 2019 7:37 am

Sorry I don’t see the point of yet another replica.

There’s already Grant Searle’s version, and ZX96, ZX97, ZX2000.

If I would build my custom ZX81 it will probably be ZXNU from ZXTeam. It also uses standard components and has build-in flash drive.
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube

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Daeve
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Re: I released my ZX81+35 ZX-81 clone design files on Github

Post by Daeve » Tue Sep 17, 2019 8:19 am

I guess ultimately there is little point in any replicas at all - it’s not like the ZX81 is in any way rare or expensive :lol: (Jupiter Ace/ZX80 may make more sense).

Most of these projects are personal challenges with mods and improvements that are documented then (hopefully) released to the public, I really enjoyed Mahjongg’s blog on the design of this one over a long period of time and it’s great that it’s been released - I’ll probably choose this one as a project to have a go in order to learn better how it all works - after a Gigatron, Ben Eater’s 8-bit computer and the PiDP-11 I’ve got waiting...

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mahjongg
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Re: I released my ZX81+35 ZX-81 clone design files on Github

Post by mahjongg » Tue Sep 17, 2019 2:53 pm

mrtinb wrote:
Tue Sep 17, 2019 7:37 am
Sorry I don’t see the point of yet another replica.

There’s already Grant Searle’s version, and ZX96, ZX97, ZX2000.

If I would build my custom ZX81 it will probably be ZXNU from ZXTeam. It also uses standard components and has build-in flash drive.
Doesn't matter, as I said this is a hobby project, my goal is not to create money, its to create what is in my eyes my perfect ZX-81 clone that anybody can build.

I see now that I have made some choices in my previous attempts that seem wrong now, so Ill just try again, just for fun.

Don't know much about the ZXNU, but what I heard its not really a ZX-81 copy, just inspired on it.

Using built in Storage device, or other "extras" doesn't seem to be in the spirit of building something that people are interested in because of nostalgia, building in the 16K ram pack, and a slightly better keyboard, are my only planned concessions to building a ZX-81 clone kit for nostalgic reasons. That and a few practical features because it has to be usable with 2019 equipment, like a modern TV and replacement for a cassette recorder.

martin de jong.

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mahjongg
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Re: I released my ZX81+35 ZX-81 clone design files on Github

Post by mahjongg » Mon Oct 21, 2019 6:06 pm

I can report that a "thomas" (from Germany) built my ZX81+35 from the files on GitHub, and apart from the fact that he discovered a small error on the component overview drawing (U13 is a 74HC00, not a 74HC32) he got it working (the error has since be fixed on GitHub, and I also uploaded an new BOM spreadsheet). He actually built two, and sent me one as thanks for my assistance in getting it working.

I made an adapter for my earlier prototype keyboard, and tested it, and its fully working! with very good picture quality.

He patched it for 16K ROM, and included a small assembly code monitor in the free 8K.

This is the result of the following small BASIC program, I used to check out the video quality of the character alignment, note that this ROM also has the character 6 patched, so it looks less like an 8.

Image

I'm happy, the characters are sharp and align perfectly, see the checkerboard pattern characters.

Thomas also reported that saving and loading using the cassette interface works very well, my cassette input circuit is doing its job.

I will order a keyboard PCB for this board, and continue testing it, for one thing I want to try if I can simply implement loading bars.

In any case this confirms my ULA replacement is working, so I can be confident when continuing with my new easy to build version ZX81+38.

P.S. my new keyboard PCB has been reported to work fine too!

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