ZX81 repair

This video shows a repair of the Sinclair ZX81 retro computer.
0:00 Project introduction
0:27 Unboxing ZX81
1:26 Power adapter repair
3:48 Turning it on for the first time
4:35 Composite mod
6:59 Troubleshooting
8:18 Electrolytic capacitors replacement
9:01 Z80 test circuit
10:44 Desoldering ULA
11:53 Checking the ROM
15:32 Testing SRAM
17:01 Replacing the ULA chip
18:11 Testing the keyboard
18:20 Heatsinks on the ULA chip
18:30 Closing up the ZX81
18:49 Final words
RetroWizzard post:
/ 1515465522871209990
Mitchell Dayton post:
/ 1517136562290626560
ZX81 composite mods:
www.bytedelight.com/?page_id=...
www.evolutional.co.uk/post/zx...
github.com/gpimblott/ZX81-Com...
Z80 Test circuit:
www.z80.info/z80test0.htm
Ben Eater's clock circuit:
• Astable 555 timer - 8-...
ROM adapter:
www.hp9845.net/9845/tutorials...
Music used in this video is from Nihilore:
www.nihilore.com/

[@ExplainingComputers]

Interesting video. Always good to see a ZX81 brought back to life. It was my first computer on which I learnt to program in BASIC, then Z80 machine code. :)

[@ezContents]

Thank you, Chris! It was a pleasure to work on it and fix it.

[@akrimpen2]

For some reason I find it very pleasant to watch/listen to your video's while working. Keep'm coming!

[@ezContents]

Thanks Arjan! I am very glad to hear that. 😊

[@SqueekyBums]

Iconic machine. I still have mine, and the ZX80 which introduced me to my first home computer. 👍

[@ezContents]

Indeed. I wish it was my first machine. My first baby steps were on ZX spectrum.

[@RudysRetroIntel]

Nicely done. Thanks for sharing

[@MoreFunMakingIt]

Superb! I love that you just build your own testers.
I have used a composite mod with backporch on one of my ZX81's. I will link it on twitter for you.
Looking forward to more videos :D

[@ezContents]

Thanks Lee! I had to keep myself busy while awaiting the ULA replacement.🙂

[@leesmithsworkshop]

@@ezContents I was just talking to other lee about building a Z80 tester for a spectrum 48k I have and then he told me about your new video.

[@ezContents]

@@leesmithsworkshop Cool! Check the link in the description for details.

[@Xoferif]

That was a really good video! I've subscribed.
I love the mini CRT TV, by the way. It really looks the part when used with a ZX81. 😄

[@mslyczba]

Amazing video 👍🏼

[@ezContents]

Thank you. 😊

[@JoeldaSilvaVicenteFilho]

9:05 I immediately recognized the clock module hahaha Great video o/

[@ezContents]

Thank you! Yes, if you watched Ben Eater's series it is pretty recognizable. 😀

[@oldblokeh]

Just a comment about the UK mains plug fuse. It's not there because we have 30A ring circuits - each circuit has a breaker - but to protect the device mains cable in case of a short inside the powered device. Many device cables, including in the EU, will melt and/or catch fire, or at least get very hot at currents allowed by the circuit breaker. The fuse should be rated appropriately for the device in question, regardless of the current rating of the supply circuit to which it is connected.

[@ezContents]

Thank you for clearing that up.

[@tenminutetokyo2643]

Jam in new ceramic caps too. 104's

[@maximjacquet8434]

you can test the sram with your eeprom programmer 😀

[@ezContents]

Thanks. I was making it too complicated, with the Pico and 3.3V to 5V conversions. Although I think that the problem might have been with the in/out pins, which had 2 functions.

[@kevincozens6837]

I wouldn't buy a non-working ZX-81 because the most likely source of the problem is the ULA and then good luck getting a replacement.

[@ezContents]

True. They are hard to find, however I am still thinking of a project where I build an easy to make replacement.

[@kevincozens6837]

@@ezContents FPGA replacements for the SID chip and the video chip in the C64 have been created. Perhaps someone will create an FPGA based replacement for the ULA.

[@spacedock873]

Pointless doing the composite mod until it is established that a video signal is being generated in the first place! Since you have an oscilloscope (the same one that I and countless hobbyists have!) that should have been tested before getting the soldering iron out. So, basically the first machine has a dead ULA (very common fault) and so is effectively a junk/donor board and the second one worked out of the box. Yes, Arduino is more appropriate tha n a Pi as a test rig for this generation of machines due to its 5V compatibility. On the ROM adapter it is often possible to push out the unwanted pins from the DIP socket which saves fiddly electrical tape insulation. Always Interesting to see a ZX81 since I helped a friend build his kit version back in the day 😀