ZX81 blank screen

[jarweb]

Hi

I've got a couple of ZX81s that I'm looking to get running again.

One has a very dark screen but you can see the cursor. I'm hoping this will improve with a video mod ?

But the other I've already done a composite video mod on but the screen is completely blank. There are pots on the mod board but these are making no difference apart from going from completely black to completely white.

Screen was already blank before adding the mod but was hoping the mod would fix it. I've also done the 16K Ram mod.

I've tried swapping each of the socketed ICs but has made no difference. I only have a multimeter and not a scope so I know I'll be a bit limited for troubleshooting.

But can anyone suggest what I could look at ? I've attached a photo of the board.

Thanks

[Lardo Boffin]

The one with the dark screen is most likely an early ULA with no back porch signal so a composite mod which includes a back porch should sort this one out. I always use the ZX8-CCB but other boards are available.

Did the blank screen zeddy ever produce a picture? Nice kludge fix zeddy by the way!

Edit - just read that it was blank before. Short term memory loss…

What order did you do the mods?

[jarweb]

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 same time !

I don't think I've still got the original Ram chip to put back in and I think I had to cut the legs to get it out anyway. I assume all versions of the board support the Ram mod ?

Any advice gratefully received as to what to check.

Thanks

[msknight]

If the screen is completely black, one thing to do is try swapping the Ferranti chips between boards... if the other board is also socketed.

[jarweb]

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

[1024MAK]

When swapping chips, it's important to try the suspect chip in a known working system. Then you will find out if that chip Is working or not.

By blank screen, I presume you mean black screen. And I don't mean the 'no signal' screen that some displays show.

With a ZX81/TS1000, a black screen is normally caused by the ULA or its associated support components. Also visually inspect the ULA socket (using a good light and a magnifying glass) and the 6.5MHz resonator (X1).

A faulty ROM, RAM or processor will cause a white screen, as the ULA produces white unless instructed otherwise.

Mark

[jarweb]

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 working one one with the swapped ULA from the blank screen one. Swapped a couple of times to confirm and it's the same. Put the original ULA back in and the dark screened one works again.

The two ULAs are both Ferrantis - one 8202 (black screen) and other 8134 (dark screen but seems to work OK)

I'll double check the ULA socket for anything obvious.

Thanks

[Lardo Boffin]

The ULA controls the keyboard in effect so if that part of it is faulty then the keyboard won’t register. I have had zeddies where it all works fine until a key is pressed and then it dies. Replaced the ULA and all was well.

[jarweb]

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 be checking for with X1 ?

Also, is the mod on top of the CPU to fix an issue with the ROM on the board, or an issue with the CPU ? The other board I have is also an issue 1 but doesn't have the mod on the CPU. Just wondering if this particular CPU and ROM need to go together on a board ?

Thanks

[Moggy]

The board on top of the CPU is there to help cure ( but not completely) an error in the floating point division routine in the 550 issue of the ROM as fitted to your board.

I don't know what would happen if this mod is used with the 2 later ROM editions, certainly wouldn't cause any damage I would have thought.