ZX81 rebuild
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- Posts: 2256
- Joined: Sat Nov 26, 2016 2:42 am
Re: ZX81 rebuild
Putting in Andy’s ULA solved the screen jump issue I was having in hi res games. Nice one Andy!
Original parts left:-
Motherboard - can’t do much about that
6.5 MHz resonator
Case - might try and make a custom one
I still need to get hold of a UHF modulator for less than £20 and put in an ear and mic socket.
Original parts left:-
Motherboard - can’t do much about that
6.5 MHz resonator
Case - might try and make a custom one
I still need to get hold of a UHF modulator for less than £20 and put in an ear and mic socket.
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Re: ZX81 rebuild
Good work.
BTW I’ve got a few RF modulators in my spare parts drawer. Some of them should work fine, the others are broken. If you need one drop me a line.

BTW I’ve got a few RF modulators in my spare parts drawer. Some of them should work fine, the others are broken. If you need one drop me a line.
Claudius
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- 1024MAK
- Posts: 5527
- Joined: Mon Sep 26, 2011 10:56 am
- Location: Looking forward to summer in Somerset, UK...
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Re: ZX81 rebuild
Go on, send him a broken one. He loves fault finding on this Zeddy


Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Amp
Standby alert 
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Spring approaching...
ZX81 Chip Pin-outs
ZX81 Video Transistor Amp


There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb

Spring approaching...
Re: ZX81 rebuild
Hi Mark 
Now you can laugh out loud, because Lardo asked me for a faulty unit


Now you can laugh out loud, because Lardo asked me for a faulty unit




Claudius
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- Posts: 2256
- Joined: Sat Nov 26, 2016 2:42 am
Re: ZX81 rebuild
I aged 5 years in the time it took me to get this one working!

I only need a broken one for the case.

ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
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- Posts: 2256
- Joined: Sat Nov 26, 2016 2:42 am
Re: ZX81 rebuild
I just got one of these off eBay, possibly from someone on here?
Loving the keyring! Nice bonus.
Loving the keyring! Nice bonus.

ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
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- Posts: 2256
- Joined: Sat Nov 26, 2016 2:42 am
Re: ZX81 rebuild
In place and working but I need to add the switch so I can play with the other ROM options.
ROM options:
Bugged ROM (the earliest official Sinclair ZX81 ROM with mathematical errors)
Standard ROM (the normal Sinclair ZX81 ROM that was fitted to almost all models)
Standing on the Shoulders of Giants ROM (improved 'Standard' ROM by Geoff Wearmouth, 2003)
Husband Forth (The Forth programming language for the ZX81 by David Husband, mid 1980s)
ASZMIT (Assembler ROM)
ZX81x2 (an improved and faster ROM AKA 'Big Bang' from 2018 by Zsolt Gaal)
ROM options:
Bugged ROM (the earliest official Sinclair ZX81 ROM with mathematical errors)
Standard ROM (the normal Sinclair ZX81 ROM that was fitted to almost all models)
Standing on the Shoulders of Giants ROM (improved 'Standard' ROM by Geoff Wearmouth, 2003)
Husband Forth (The Forth programming language for the ZX81 by David Husband, mid 1980s)
ASZMIT (Assembler ROM)
ZX81x2 (an improved and faster ROM AKA 'Big Bang' from 2018 by Zsolt Gaal)
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Re: ZX81 rebuild
A most excellent zeddy you have there Lardo.
Just for clarity there is at least one known maths operator bug in the h4th ROM which may or may not be present in your ROM depending on who programmed it. If it come from the German lads then it's probably not present, a thread here for clarity.
viewtopic.php?f=3&t=2632&p=28741&hilit=h4th+bug#p28741
The manual for the Azmic ROM can be found here.
viewtopic.php?f=7&t=1503&hilit=azmic+rom&start=10

Just for clarity there is at least one known maths operator bug in the h4th ROM which may or may not be present in your ROM depending on who programmed it. If it come from the German lads then it's probably not present, a thread here for clarity.
viewtopic.php?f=3&t=2632&p=28741&hilit=h4th+bug#p28741
The manual for the Azmic ROM can be found here.
viewtopic.php?f=7&t=1503&hilit=azmic+rom&start=10
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- Posts: 2256
- Joined: Sat Nov 26, 2016 2:42 am
Re: ZX81 rebuild
Moggy wrote: ↑Fri May 15, 2020 5:55 pm A most excellent zeddy you have there Lardo.![]()
Just for clarity there is at least one known maths operator bug in the h4th ROM which may or may not be present in your ROM depending on who programmed it. If it come from the German lads then it's probably not present, a thread here for clarity.
viewtopic.php?f=3&t=2632&p=28741&hilit=h4th+bug#p28741
The manual for the Azmic ROM can be found here.
viewtopic.php?f=7&t=1503&hilit=azmic+rom&start=10
Thanks for the links Moggy!
Forth up and running.

ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
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- Posts: 2256
- Joined: Sat Nov 26, 2016 2:42 am
Re: ZX81 rebuild
Soldering the wires to the switch was an absolute nightmare.
Z80 for scale....
My eyesight is nowhere near good enough any more.
Z80 for scale....
My eyesight is nowhere near good enough any more.
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA