ZX81 video issue

[jarweb]

Hi
Have a ZX81 Issue 3 with a weird video fault.
I have installed a composite video mod (replacing the modulator) and added a 16K Ram upgrade.
It powers on OK and seems to recognize all of the memory OK. Software loads OK but there is a strange corruption on the video.

I've attached a couple of images. This is from the 1K "bowling" game. One shows how it is supposed to look - there is a bowling ball that moves across the bottom of the screen with skittles/pins at the top. The other is a picture with the video corruption. The corruption is constantly moving so I don't know if it's related to a refresh issue with the moving ball at the bottom ?

I have swapped the ULA from a working computer but I get the same issue. I don't want to swap too many components between machines as I've done this previously and ended up with two broken computers !!

Is it possibly a duff memory module I've installed ? Or could it be something else ?

Thanks

[1024MAK]

Does it work okay in BASIC?

Mark

[jarweb]

Hi Mark

Do you mean does it load or save Basic programs ? Or just does it run Basic at all ? I have run the basic program that does the 16K memory check and it worked OK.

Thanks

[Moggy]

I ask in all ignorance but do things that are saved in 1k not run correctly on a 16k computer, different display file size or something.

Perhaps post a link or attach the game so we can test it on our Zeddies.

[jarweb]

Hi
The second picture is from another computer with the same 16K Ram upgrade
Thanks

[jarweb]

Hi
I've added another few pics. One just shows a listing of a Basic program (some kind of adventure) - don't know if this is what Mark was looking for.

Also added a couple from 3D Monster Maze. Don't know if this is fully Basic or not as the listing doesn't show "normal" text all the way through.

Thanks

[1024MAK]

The reason I ask, is that some modern SRAM chips are a bit too quick for the ZX81. A small value capacitor is sometimes needed for hi-res graphics to work correctly, but BASIC and other programs that use SLOW mode do work correctly.

The required value for the capacitor is small (tens of pF) and different ULA chips do appear to behave slightly differently.

Mark

[jarweb]

Hi Mark
Where would the capacitor go ? Happy to try if it's a cheap option and I know a value and where to put it.
Thanks

[1024MAK]

Try connecting a 33pf to 100pF capacitor across GND and /RAMCS (pin 20 or pin 22) or equivalent location. Start with a 100pF if using a modern SRAM ( -70) or similar chip. Start with 33pF if using an older, slightly slower SRAM chip. You may need to try different values to see if you get any improvement.

See photo (not mine) for a good position for the capacitor:

Mark

[jarweb]

Thanks - will give it a try