ZX81 blank screen

[jarweb]

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. Are there cheaper alternatives ?

Not particularly scientific when they're on the board, but I compared the resistance of all the resistors on the working board with the dodgy one and they all matched. Checked the diodes as well. The capacitors which gave me any readings on my multimeter also matched up.

If swapping all the socketed ICs makes no difference, which it hasn't, is there anything else I can check, apart from the Ram ?

UPDATE - I forgot I had a Ram pack kicking about so I've just tried all 3 socketed ICs plus attached the Ram pack - same issue. I've read that attaching the Ram pack disabled the onboard Ram but does this still apply after doing the 16K Ram mod ? I also tried removing the Ram chip anyway and plugging in the Ram back - no joy.

Thanks

[msknight]

As well as a multimeter, do you have a logic tester, to see if the processor is running and lines are being used? I'm using an LP-1 logic probe.

[jarweb]

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 get the cursor back. It's not a white screen but I haven't done a video mod on that so it's still going via the TV tuner. This board has the dark screen problem anyway.

But with all 3 of the good ICs in the faulty board I still get the blank white screen.

I'm now thinking of taking out the video mod but could end up going round in circles.

[msknight]

If the RAM mod respects the chip select, then the RAM pack is supposed to disable the on board memory by holding chip select high, so the on-board memory never gets a chance to respond.

Hz setting on multimeter... no clue.

If you're getting an all-white screen, that's usually BIOS or RAM, but there is also a chance that the 1uF 50V capacitor has dried out. Try changing that capacitor and if that doesn't work, then it's probably a case of checking lines to RAM and ROM for shorts and opens.

I know that when I de-soldered RAM on a board, my desoldering wasn't very good and I'd left some solder crossing to another pin which was causing my problem. Fortunately some kind soul on here spotted it in my pictures. A bit of a clear up and it lived.

EDIT - Once you've got one working zeddy, then you can swap out and test a little better.

[jarweb]

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

[msknight]

Have you thought of trying another composite board to see if that solves the dark display issue?

[jarweb]

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.

[Lardo Boffin]

I may have some removed RAM chips from doing internal 16K mods if you need them. I will need to check what I have left. They won’t be £20!

I also have some ROMs.

Are you in the UK?

[1024MAK]

Changing the RAM chip will not make any difference if the ULA or its support circuitry is not functioning. And an external RAM pack also will not make any difference.

All external 16K (or larger capacity) RAM packs will disable any internal RAM controlled by the ULA.

You need to remove the added video board, connect it to your working ZX81. Set up the two variable resistors/pots. Then put it back on the faulty board.

At least then you will know that the added video board is at or near the correct settings.

Then constructive fault finding can start.

Also, have you proved that the swapped chips work? Mark which ones work and which ones don't.

The range on a multimeter is for measuring mains or audio frequencies (unless it's a £300 plus multimeter in which case the frequency range may be far better) so is of no use on a ZX81

Mark

[jarweb]

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.