Sinclair ZX80 / ZX81 / Z88 Forums

I wondered that too, for a second, when I saw it, then I realized Its probably used to generate the -5Volt from the Raw +9V supply by running it as an oscillator, then driving a diode doubler circuit to invert it to -9V and using a 5V1 Zener to smooth and regulate it. You can see the components to d...

I can report that a "thomas" (from Germany) built my ZX81+35 from the files on GitHub, and apart from the fact that he discovered a small error on the component overview drawing (U13 is a 74HC00, not a 74HC32) he got it working (the error has since be fixed on GitHub, and I also uploaded an new BOM ...

The schematic is now function complete, ready to start component placement: https://revspace.nl/images/2/2c/ZX81plus38_rev1.2_cropped.png added jumpering for alternative ROM's, and support to emulate the canonical on-screen loading bars. also added some texts to explain parts of the schematic, and c...

The schematics is almost finished, I only want to add some jumper options so you can use other kinds of ROM's, specifically the original ROM of a ZX81:

Found it, its called a "softy", and this is its "brother", the "menta" http://www.old-computers.com/museum/com ... t=1&c=1277

I have seen one decades ago, its not a ZX80 at all, although maybe inspired on one, but with a SC/MP processor its a different beast.
I remember it as a dedicated eprom programmer, unfortunately I forgot how it was called.

You could try to initiate a csave operation (blind), and monitor if sound is coming out of the audio output, to see if the ZX-81 is otherwise working.

Instead of UV EPROM or OTP EPROM, you could use EEPROM chips like the Microchip AT28C64B (8k bytes) or the Microchip AT28C256 (32K bytes) see https://uk.farnell.com/w/c/semiconductors-ics/memory/eeprom?memory-interface-type=parallel&memory-case-style=dip&range=exc-delivery-surcharge In fact, if you...

Thanks, I will post regular updates here:

For now I decided to name it ZX81+38. It will be a fully ZX-81 compatible alternative that fits inside the original case, or can be used with my own keyboard in a wooden or 3D printed enclosure. This time around I will only use easy to solder pin through hole components, no more SMT. I modeled a PCB...