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. :)
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
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.
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.
@@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.
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 😀
