Sinclair ZX80 / ZX81 / Z88 Forums

I wasn't sure if the two chips needed to be swapped together or what would happen in they weren't. What symptoms would I get if I used the chip with the daughter board on a non-550 ROM ? The other board doesn't have the 550 rom and altered processor but, as I've said, I've swapped all three at the s...

I'll look at swapping out the video mod board. It was bought new but it's obviously possible it could be faulty - but at least I'll know either way.

I'll use the 3 main ICs from the working board for further testing as these obviously work in that board.

Yes, I have a composite mod ready for that board. Just need to get round to fitting it.
But I was just trying to find out what could be wrong with the other one which seems to actually be faulty.

I do have one working board, it just has a very dark screen which I'm hoping a video mod will fix. This is the board I am swapping ICs to/from.
You can see the cursor so I know it's working, just very dark.

Thanks

I don't have a logic probe. There's a setting on the multimeter marked as Hz but never used it - don't know if this is usable ? I don't know if there is actually more than one problem now. If I use the 3 ICs from the faulty board in the good board, I don't get a display. If I put back in the Rom IC ...

I'd like to try putting 1K of Ram back in to see what happens but I don't have the original chip. Can someone tell me what chips are compatible with a Rev1 board ? The other ZX81 I have has a mk4118an but these are going for over £20 online. It's not cost effective to buy one just to see if it helps...

Hi I checked the connection between the top of the ULA and the board underneath and all checked out OK. But I touched up all the solder points anyway. No change. I've also now swapped all 3 of the socketed chips from the other working board, at the same time, and still the same issue. What should I ...

Hi Just swapped the ULA with the one with the dark screen and got the same blank screen. It's completely blank. As I said, I can adjust the pots on the mod board but all this does is make it go between completely white and black. Strangely, or maybe not, the keyboard also then doesn't work in the wo...

Hi

I'd already tried that but I'll do it again just to be sure as it was a while ago I did it and had other machines on the go at the time.

Thanks

Hi I just did both mods at the same time. I hoped the screen issue be would resolved with the video mod so I just did the Ram at the same time. But I also thought that if it was actually a Ram fault, then this would be getting changed as well anyway. Yes, I know, don't make too many changes at the s...