Sinclair ZX80 / ZX81 / Z88 Forums

Given in the topic I linked to earlier: "The ULA only uses address line A14 (and the Z80 CPU control line /MREQ) to decode the control signals for the ROM and RAM". Both A14 and /MREQ come from the Z80. Although the ULA may include slightly more complex circuitry for timing purposes and addition lo...

The only chip that isn't socketed on this zx is the Ram, it has a single 24pin ram chip. I don't have any other ram chips on hand but I might as well desolder it, socket it and buy a new chip and try that out

I appreciate your help, I'll try getting my hands on a ram pack or try making my own and I'll report back if anything changes. Thank you!

Given that the /ROMCS signal is generated by the ULA, what conditions have to be met on the ULA for that pin to get stuck HIGH apart from the ULA being faulty itself?

Before I give the readings, I know for a fact that the z80 CPU is in working condition as I swapped it in another zx81 and it worked fine. Here are some suspicious things I found: 18 /Halt -> 20231225_235652.jpg 22 /WR -> 20231225_235941.jpg Address lines: A0 to A7 look fine and active, A8 to A15 Be...

I swapped the modulator with a 555 timer mod to generate the back porch, the readings I took where with the input to that circuit plugged in. I do suspect that something else is wrong on the board given that I see no attempt by the ULA to display black cursor text as you said. I guess next step is t...

From what I've seen online the sync signal should not be 0v. It’s because the ZX81 does not follow the composite standard. It sends a signal where the difference from bottom of signal to top of signal is ok. Old TVs think this is ok. It does this to avoid making a more complex circuit that sends ne...

It was still a blank white screen before the mod, that's why I did it in the first place, just to check if the cursor was just washed out due to the missing back porch. From what I saw on the scope there doesn't appear to be a cursor being drawn else I think I should see a small dip in the waveform ...

Hi all, I have a ZX81 which displays only a white screen. I attempted a back porch mod and got no output, I'm using a scope to probe pin 16 of the ULA and I get a 15.6khz square wave formation with lows of 0v and highs of 4v. Is this expected from what the ULA should output? From what I've seen onli...