Sinclair ZX80 / ZX81 / Z88 Forums

So I have now had a number of zeddies with ‘sparkly pixels’, by this I mean that the screen shows occasional flickering and small groups of pixels randomly appearing where they should not.

In the past changing the ROM has sorted this. I now have an interesting one:

1) With the original ROM and no ‘spand the picture is rock solid. With the ‘spand it goes sparkly.

2) With a different ROM it is sparkly without the ‘spand but rock solid with it plugged in!

The zeddy in question has a vLA81 ULA in it and 32K of RAM.

Is there a scientific explanation for this or is my house built on an old native American grave yard (unlikely given that I am in the UK)?

I assume that because the character information for the screen is being read from the ROM if there are any issues that would affect the output.

One thing I have noted is that I currently have the second ROM plugged in so the screen is not stable without the ‘spand. If I load one of Dr Beeps amazing 1K hi res wonders onto the zeddy the screen is fine! Is this because the ROM is mostly not being looked at?

I guess the next question is where can I get a reasonably priced ’new’ ROMs from on a regular basis? By this I mean a modern one or at the least a recently burnt EPROM.

Any information appreciated!

Thanks

Lardo

It sounds like the problem I had when connecting the ZXpand to a Lambda 8300. Then random chars would appear someplace on the screen. They weren’t wrong in RAM but only on screen.

I guess it’s some sort of timing issue with the ZXpand, as there’s no issue when a regular Sinclair 16K is used.

Timing I think is the issue.

Some of the ROM chips that Sinclair used were just about fast enough. But as soon as the expansion bus is loaded, the stray capacitance changes, hence the change on screen if the ROM chip is not turning off or on fast enough.

To be continued...

Very interesting thanks Mark! Its a bit weird getting used to the idea that a supposedly digital process is so prone to interference like this.

Its a little strange that the ‘spand actually cures the issue on the zeddy with the current ROM installed i.e.

Zeddy + spand = no sparkles
Zeddy without spand = sparkles

Both ROMs I have tried are from different manufacturers.

Just to add my 5 bobs worth.

I too had this happen with an original spand inside a Memotech case. I attached the stick on Velcro strips that came with these devices which meant the spand is never fully pushed home on the expansion slot.

In this format....sparkle sparkle!

With strips removed and spand pushed fully home no sparkle!

So who knows?!

Clearly there are multiple sparkling factors to be considered!

Nice one Lardo.

Also I think Rich might do an EEPROM replacement as he supplies the 8k replacement for the ZX80 xpand conversion kit and I'm sure it could be programmed with the ZX81 code.

Barring that if you ever get hold of any of the old 21v 2764's I will happily bang an image of choice onto it for you. All my zeddies now have EPROMs fitted with the double speed BASIC ROM image and they work quite happily with the spand.

Moggy wrote: ↑Thu Jan 23, 2020 2:42 pm Nice one Lardo.

Also I think Rich might do an EEPROM replacement as he supplies the 8k replacement for the ZX80 xpand conversion kit and I'm sure it could be programmed with the ZX81 code.

Barring that if you ever get hold of any of the old 21v 2764's I will happily bang an image of choice onto it for you. All my zeddies now have EPROMs fitted with the double speed BASIC ROM image and they work quite happily with the spand.

Thanks Moggy!