Sinclair ZX80 / ZX81 / Z88 Forums

If pin 6 of the Z80 is stuck either high or low, then a fault in the clock circuitry is suspected. It’s best to use a ‘scope to test. This is supposed to be a 3.25MHz square wave (although the edges are far from being square). The next step is to test on ULA pin 14. Is your ZX81 using an original U...

Hello Mark, Welcome to our forum :D It’s a ceramic 6.5MHz resonator. The ZX81 uses a three pin version. They are still available (although may look different) from electronic suppliers. Do you have, or have access to an oscilloscope? If yes, what do you get on pin 6 of the ZX81? If you don’t have a...

Hi My first post on this forum, my first computer was a ZX81 back in 82, one of two people in the class at school to have a computer in those days... Any way trying to fix a ZX81 to get working, have progressed to a white screen on boot. I do have a Issue One which works no problem and has all but t...