A project log for ZX80 with 40 column text

40-column text, and square pixels mean 45 degree lines look right. Runs 13% faster as well!

keithKeith 09/16/2023 at 11:510 Comments

The ZX80 is more hackable than the ZX81 because all the logic chip signals are accessible.

I bought a PCB and populated it with good quality turned-pin sockets and new chips.

I noticed it was still prone to crashing even without an external RAM pack.

I suspected that socket connections were intermittent, especially when the PCB was being flexed by pressing the keyboard.

I removed the sockets of the TTL chips and soldered the TTL chips directly to the PCB. This was a great improvement.

Lessons learnt:

I fitted a 32K RAM chip, to banish RAM-pack wobble crashes forever.

I fitted tactile switches salvaged from scrap boards. The buttons are low-profile, so they don't click so well when pressed by a finger. Some kind of key tops, or higher-profile keys, would fix this.

For square-pixels, the clock was changed from 6.5 to 7.732 MHz. I used a DIP14 oscillator in a metal can, but changing the crystal would also work and look neater. The rest was firmware modding. I added a SPDT switch to select the standard or alternative firmware. I used my modified ZX81 BASIC firmware, but you could just as easily select ZX81 Forth or anything else.