Sinclair ZX80 / ZX81 / Z88 Forums

Remove the ROM chip, then power it up (yes, with no ROM chip fitted). Then use your ’scope to confirm what all data lines are doing (D0 to D1), and that all address lines (A0 to A15) pulse between low and high. They each should be pulsing at a different frequency. I've done this with the ROM in and...

The continuity mode on multimeters is great, but unfortunately there is no standard for what the resistance value is where the changeover occurs between it (the meter) indicating continuity and indicating open circuit... Each meter does it’s own thing... Mark Yeah that makes sense, I've measured th...

So apologies for my ignorance about using my own multimeter here, but some googling around suggests that this can imply poor continuity or just a comparatively high resistance, but the ROM 23->Z80 38 connection has a 1K resistor in between just like a whole bunch of other pins going straight to the...

Hey Augusto, nice to meet you!

Good luck with your clone build - I'm attempting to rehabilitate and then upgrade a ZX81 that my dad had lying around in his attic!

What kind of wave do you see off the Z80's pin number 6 on an oscilloscope screen? Can you post a picture of that wave here, please? Thank you very much. z80pin6.jpeg What are you expecting to see here and what does it indicate for you? As this happened after you had the ROM chip out of the socket,...

Ah that's a really good point! I'm going to go ahead and test continuity on the pins of the plugged-in ROM to other points on the board with it powered off tonight and report back. What is it about the diagnostics I've posted that leads you to that suspicion? In the case that the connections are bad...

Hey team! So I've been playing around a bunch with my zeddy in order to get it back to health and then beyond. My goal with this thing is really to make it useful to me and as a learning device. So far I've built a power supply for it, done an RF->composite mod and built a new keyboard. This was all...

Hahah, yeah I totally should - to be fair I've just got the kit I need to ramp back up on trying to debug why the hell my BBC is so unstable and likes to crap out sometimes.

Ahh, in that case I must be mistaken. It must be another user with a similar but different user name. One place you definitely will not find me is on Apple Fritter. I have nothing against classic Apple computers. I just don’t have any interest, as I already have too many fingers in too many pies. I...

I thought TMS2532As were 4k EPROMs not 8k's, used as replacements for the ZX80 ROM? Even when used with the 80 they do need a bit of modding. It is entirely likely I got the wrong end of the stick when looking for compatibles and bought the wrong thing, but they were a few pounds and would take wee...