Sinclair ZX80 / ZX81 / Z88 Forums

OK, if I understand right, the only inexpensive step I can still do is to sort out DRAM chips and disable ROM. The only source of DRAMs I found is AliExpress and even there only 4164s are available. Of course there are a lot of reviewers saying that the chips are used and not all are working which m...

1024MAK wrote: ↑Wed Sep 20, 2023 7:15 pm The ULA uses page mode when reading screen data, yes.

So was my result helfpul? Does ULA works OK? What is the next step?

For longer traces, my logic analyzer has top resolution 8MHz. In the beginning I see some writing, later only reading. Wide look: wide.png Narrow look: narrow.png However I can make my logic analyzer to read very short traces at 24MHz. very_narrow.png If each bit is read in 375ns, that is about 3 bi...

I’m presuming you have carefully inspected both the underside and top sides of the board, including the IC socket contacts? No, I did not systematically check all the contacts. But visual inspection suggests that the board is in a good condition and that there were no repairs concerning main and DR...

1024MAK wrote: ↑Tue Sep 19, 2023 4:29 pm Okay, can you remove all the 4116 DRAM chips as well, then post up a video of the resulting screen display.

Here it is from the reset on.

https://youtu.be/sN2nRGOs_vY

1024MAK wrote: ↑Tue Sep 19, 2023 2:47 pm As the ‘upper’ DRAM is in sockets, the first thing to do is to remove these, if only to eliminate them as the cause.

I did. Almost nothing changed. Shall I will leave them out until the fault is found/corrected (or the fault is not found and ZX Spectrum gets thrown away)?

Someone has worked on this board before. It's unusual for the 'lower' 4116 DRAM to be in sockets. The 4116 chips also have a very late date code for an issue two board. So they may have been replaced during 1983 or later. Judging from the back side, sockets were there from the very start. There are...

I recently dusted off my ZX Spectrum, which my father bought me and smuggled to Eastern Europe in the first half of the 1980s. What a story. I made my first programming steps in Basic and Assembler on this computer. Those were the days. But it does not work. I checked the -5V, +5V and +12V lines and...