My ZX81 clone ZX81+35 and a PSG sound expansion board for it (now released on GitHub)

[Andyroo]

...
I find SMD quite easy, apart from the really tiny components. It takes up less board space and you don't have to keep cutting leads. Just a decent magnifier and fines solder is good enough even with my old unsteady hands.

Maybe I'll give it try - I've seen lots of 'Unexpected Maker' videos so maybe a time to try a few test boards from China.

[mahjongg]

That looks a lot harder to solder than the ZX80 I bought so many (many many) years ago At least I could cope with through hole - cannot manage this new fangled SMD type stuff at all

Must say though - the board does look neat and tidy. Signs of true love.

I was surprised that you can still buy cassette recorders (even Amazon stocks some) - I know I marked mine with snopake and covered it in tape once I had the volume just right. Oddly enough, I miss the screech of these and acoustic modems

SMD isn't that difficult, just fix two opposite corners first, then add a lot of flux, and drag your iron across the pins while adding a bit of solder.
If you get the hang of it its quicker that soldering the same in DIP format. I tried to use somewhat manageable size components, 0805 size, smaller will be too small to handler most people, but 0805 is doable, even for a quite old guy like me (I'm 63).

If I had used standard pin through hole components the PCB would have to have been quite a lot larger, my re-implementation of sinclairs ULA takes a lot of logic, but it does have some extra's, such as 32K RAM, back porch logic, which means it works with a modern TV, it even has 8 GPIO outputs and a GPIO input, These can be used for several things like controlling a robot, or an LCD, and even 4-bit sound output. In theory it also has a turbo switch, although video is glitchy when using both acceleration options it has, it can run in double speed mode when using "fast mode", or with a 10% increase in slow mode (with stable video).

The cassette input is optimised for weaker signals, and a wider signal range, you can play wave files trough your smartphone to it, works great!

I now have also uploaded a programmable sound generator (AY-3-8912 PSG) expansion board for my ZX81+35 to GitHub so you can play chiptune music on a ZX81, should be compatible with software written for the ZON PSG board (anyone still remembers dancing demon?) As an extra I added a SD-card socked, using the GPIO ports of the AY-3-8912 chip. Hopefully someone will test it, and write a software driver for the SD-card, so you can use SD cards instead of cassette tapes with it. more info here: https://github.com/mahjongg2/PSG-for-ZX ... ZX81-clone

[mahjongg]

If you are truly nervous about SMT soldering, I advice watching this: https://www.youtube.com/watch?v=hoLf8gvvXXU it gives a small demonstration on how to do SMT soldering, and shows how easy it is.

[Andyroo]

The man with the BIG knife

Not seen that one of his but he makes it look simple

[mahjongg]

there are countless SMD soldering starter guides available, but this is a good one to start with, it handles most important topics, recommended!
https://www.youtube.com/watch?v=f9fbqks3BS8

[mahjongg]

I have given this a lot of thought, I would like my ZX81 clone to become a popular kit, but in its current form, with some 26 SMT IC's to solder) I don't see that happening.
All the logic needed to replicate the ZX81 ULA, its a bit much to ask people to solder on in its current SMT (surface mount) form.

Perhaps I can strip out from my current schematic, the logic that a ZX81 clone really doesn't need, and put that simplified logic into a simple FPGA or CPLD (preferably one that can be used in a PTH form, for example a 48 pin QFP socket, although non SMT versions become rarer than chicken teeth).

A much simpler to build version, with only a minimum of pin through hole classically solder-able parts will be much more attractive for builders.

perhaps convulsively keeping to a 10x10cm PCB is also less than wise, (PCB's have become much cheaper) and using a pinheader for the edge connector isn't that good of an idea either when you already have 16K RAM on board, so crashing through a wobbly edge connector isn't likely a real problem.

in other words, perhaps I should simplify it so it resembles the ZX81 in kit form much more.

That said, what I don't think I need to do is to build something that fits in the original ZX81 plastic case, the original keyboard is so small, it would not harm at al to keep it at its current slightly bigger, much less cramped, size,. Also new users are unlikely to have the original enclosure, and are more likely to build (from 3D print or use laser cut wood) their own enclosure.

I need something that triggers nostalgia, but leaves behind most of the negative aspects of the original, but is still fully compatible with the old and new software and (old and new) peripherals.

watch this space for news. comments are welcome.

[botk]

think my old zx81 is still in my dads attic - i might dig out next time i see him.

[mahjongg]

I now am designing a keyboard PCB for my ZX81+35 that can also be used for a real ZX-81.

I will fit into the 6.2 by 2.5 inch indentation that holds the membrane keyboard, but my PCB uses real buttons.

this is its schematic diagram:



for a real ZX-81 only J2 and J4 are used together with flat-flex connector cables, or with a piece of FST-22A-15 cut into 5 wire and 8 wire parts.

[mahjongg]

The keyboard PCB designed is finished, and production files are released on GitHub.
Here is a preview, its a single sided PCB. And can be used with a real ZX-81 too, and you can use my keyboard overlay with it (available on GitHub too) https://github.com/mahjongg2/ZX81plus35

[mahjongg]

And the PCB itself (preview courtesy of Eurocircuits PCB manufacturing) , the (mirrored) solderside.

[mahjongg]

As SMT is for many people a bridge too far I am currently considering to design a minimalist, but fully compatible, ZX-81 clone using only currently available PTH components, so no special soldering techniques are needed.

It probably won't fit on a 10 x 10 cm PCB, but as PCB's have sunk a lot in price, that might not be that important.

More important is that it is usable/compatible with modern TV's, and modern cassette replacement techniques, (laptop audio replay) and power supply solutions (USB chargers).

will start by finding out if all necessary parts can still be bought.

Temporary project name is ZX81+38, and you can follow my progress here: https://revspace.nl/ZX81plus38_simple_t ... X-81_clone

[mahjongg]

I measured a real ZX-81, and designed a PCB shape that would fit in its case, and use its mounting holes. Then I started on a schematic, and placed most of the logic in it with DIL footprints, and tried if the logic would fit on this PCB.

I am reasonably sure it will fit. see the link to my project page, (in my previous post) where I show the PCB size, and the logic that must fit on the board.
Obviously a lot of components are still missing, but it looks promising.

[mahjongg]

Readied the schematic of revision 1.0 of the new pin though hole full ZX81 sized board (ZX81+38).
It ONLY has pin through hole components, and fits on a PCB that can be used as a replacement for a real ZX81 (with all connectors on the correct places, and with correct mounting holes). It should be fully compatible with a real ZX81, and now supports on-screen loading bars, has 16K RAM built in, and supports a joystick, and either PAL or NTSC composite video output, and has improved cassette loading input circuitry.
It can be powered with an USB charger.

The schematic (updated to rev 1.3, open in new windows to see full size):

[mahjongg]

I received a physical copy (build up PCB) of my ZX81+35 (built from files released on GitHub) and I'm happy to report its working fine, the new keyboard PCB is also good.

so far, the only item I found that needs correcting, is the text printed on the PCB showing which switch position is for turbo mode.

I'm still testing it.

also see https://revspace.nl/ZX81PLUS34_ZX81_clo ... ts_working

this means I can be even more confident that my ULA replacement logic is fine, and my ZX81+38 PTH version will work also.

[mahjongg]

The same person who built my ZX81+35 ZX-81 clone has also reported my PSG sound expansion board is working fine with software written for the ZON board, Software like Dancing Demon, and sound trackers.
I will Probably create a ZX-81 (and ZX81+38) compatible verslon too, after I complete my new simpeler to build ZX-81 clone.