ZX81 Replica Board (With a few tweaks)

Discussions about Sinclair ZX80 and ZX81 Hardware
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BrettRogersUK
Posts: 13
Joined: Sat Aug 06, 2022 12:19 am

ZX81 Replica Board (With a few tweaks)

Post by BrettRogersUK »

Hi All,

About four weeks ago I decided that I wanted a ZX81 in my collection but couldn't really find one that I liked So I had a look round for a replica board. I found the likes of the Mahjongg ZX81+38 and the Wilco2009 boards but I wanted more of a replica board that was laid out more or less the same as the original i.e one that doesn't use the discreet logic to replace the ULA.

So in my search I came across this one: https://github.com/Bluescreen2001/ZX81-PCB

So I got the gerber files and sent them off to JLCPCB to be manufactured. I chose the ZX81b versiion which is as the original but with a couple of tweaks to allow a W27C512 ROM IC to be used and also a 62256 32K RAM IC as well without having any mods done with wires to get it working. I also used the fantastic vLA81 and a zilog Z80 IC (purchased this Z80 from Retroleum).

So when the boards arrived I couldn't wait to start building it. I never had a ZX81 so it was all new to me. I'd seen lots of YouTube vids about memory mods and composite mods etc so I couldn't wait to start my build.

Thankfully it took about 5 days to build the board after waiting for parts to be delivered. Once it was built I had to decide how to power it. I decided to try and use one of my Speccy power supplies (UK1400) although I remembered that they are center pin NEGATIVE so I made a connector to put the center pin POSITIVE to a 3.5mm jack plug. I tested the board without any IC's in and I was getting 13.87v coming in and, as I had fitted a Traco 5v DC-DC Regulator, I was getting a solid 4.99v coming out to all places on the board that required 5v.

Then came the modulator. I wanted a more authentic look to my replica so I chose to source a UM1233 modulator as this was a common modulator found in many home computers and consoles of the time. After a while of tinkering and questions on Facebook I found that if I connected the output of Pin 16 of the vLA81 directly to the RCA Socket on the modulator (after disconnecting the internal connection inside the modulator).

Setting the vLA MOD switch (Switch 1) to Off and BP Switch (Switch 2) to off it gave me composite signal with a back porch and I got a solid stable signal with great contrast between black and white.

All in all I was very happy with this build and i'm just about to finish off it off with my 3D printed case. Unfortunately the person who designed it made the screw holes a little too big for a replacement screw set that I purchased off of ebay so I plan to use brass inserts in these holes and use M2 size screws for screwing the case together, then the only thing that'll need doing is the ZX81 on the front of the case needs painting in a Red colour.

Hope you like the attached pics of the build and let me know what you think.

Thanks,

Brett
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Flatulentia
Posts: 93
Joined: Sun May 14, 2023 3:58 pm
Location: Cambridgeshire, UK

Re: ZX81 Replica Board (With a few tweaks)

Post by Flatulentia »

Very nice. I like the authentic look of it. 8-)
ZX81 with ZiLOG Z84 CMOS Z80, 32KB battery-backed Toshiba 55257 SRAM with charge status LED, Wilf's "Why Wait?" mod and switch-mode 5V regulator. 50/60Hz mode switch, composite video output with Zigg's back porch mod and 1080p60 HDMI output.
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Paul
Posts: 1511
Joined: Thu May 27, 2010 8:15 am
Location: Germanys west end

Re: ZX81 Replica Board (With a few tweaks)

Post by Paul »

Did you isolate the eight connections below the modulator (which gives it a vintage look :D )?
In theory, there is no difference between theory and practice. But, in practice, there is.
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