Sinclair ZX80 / ZX81 / Z88 Forums

Do you understand the arrangement of the how the different bits are stored in DRAM? In what way, how the ICs are organized internally? I do know which DRAM chip corresponds to what bit, I needed that when I used my logic analyzer to find out which chips were (more thoroughly) broken to make the RAM...

Could have been bumped, the area I live in is known for its seismic activity. ;) But since only 2 bits total were visibly affected, and the machine still worked fine before I reset it (though it's obviously impossible to tell whether other, not visible bits were flipped, but at least the ones of the...

And I'm not talking about the wobble. I've got a stock Sinclair ZX 16k RAM Pack, and right off the bat I had to replace two of the 4116s because they were clearly bad (found out with a logic analyzer, wouldn't start otherwise). But now I accidentally left the ZX81 on over night, and when I came back...

That table is a good reference. Internal RAM access was easier to understand even with the old schematic, which already shows the full signal path (well, together with knowing where RAM and ROM are shadowed to determine that the ULA really only looks at A14). I think I now have everything to make a ...

Ah, thanks a lot, that version of the schematic seems to be the same as the one I had, but with little boxes added where the edge connector pins branch off. That makes things much clearer! And now from a quick look at the 16k RAM pack schematics, which does not use /RAMCS' except to override it, it ...

Thank you for the warm welcome! Mark: In the 16k RAM pack, it’s a straight connection between +5V and /RAMCS’ (edge connector pin 2A). On the ZX81/TS1000 board, this is on the SRAM side of resistor R2, hence overrides the ULA /RAMCS signal on IC1 pin 12 but without damaging the ULA output pin circui...

I just finished repairing, restoring, and video-modding a ZX81 that I got off eBay. Now the only thing left is probably attempting to repair that 16kB RAM pack that came with it, because when I plug it in, either nothing or just stripes happen, even after cleaning the contacts. Anyway, while doing r...