Yeah that makes sense, I've measured the resistance on all the connections and with most of them I'd chosen test points where there were no resistors between. I'm getting 0 on most points and 686 ohms and 995 where I was expecting to see 680 and 1k so I guess that's all within tolerances. I'm pretty convinced by this stage that it's not the ROM or the ROM socket.1024MAK wrote: ↑Fri Feb 19, 2021 10:52 pm 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
This is at the bottom left for both channels, so both channels were on 100mV which turns out to be pretty academic because I powered it up again and re-took the measurements and got this instead:
I don't know if I maybe caused a short or something that caused the Z80 to get in a wedged state or if I did something else dumb which a reboot fixed but it looks a lot more like how it's supposed to (2V scale for both traces, both bottoming at 0 and capping at 4.08V).
Any ideas of where else to look or something that I've missed so far?
Thank you all so much for your help and especially your explanations, I've learned a bunch about how the ZX81 switches between onboard functionality and peripherals from the edge connectors, using resistance measurements instead of continuity on a multimeter and how to use half of the functions on my scope this evening, let alone everything I learned about the zeddy's layout and functioning!