ZX8-CCB vs ZXVid

[Lardo Boffin]

When I do a screen full of E it goes from rock solid to wavy:-

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When I change it to inverted E the screen gets destroyed. As it draws down the screen it gets dark (i.e. the brightness is reduced) about halfway down. It goes slightly further then completely distorts as shown below:-

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[PokeMon]

Thanks - I am pretty sure now the ULA is buggy. And the more black pixels on the screen the more wild the picture gets.
You could try to print just one or two E chars in each line and I think the picture would be still fine.

[1024MAK]

That's the result of the TV not being able to properly synchronise the display drawn by the electron beams. With analogue video, there is a synchronisation signal to signal the start/end of a line (line sync or horizontal sync), and different, longer one to signal the start/end of a field (field sync or vertical sync). Wikipedia explains a bit about it here.

As the TV is okay, I suspect that the ULA is not able to output the 0V level needed to properly generate the correct synchronisation signals. Do you have, or know someone with an oscilloscope?

Mark

[PokeMon]

As the TV is okay, I suspect that the ULA is not able to output the 0V level needed to properly generate the correct synchronisation signals.

Yes - I am pretty sure about it as the picture is moved to the left which tends to a missing sync pulse and interpreting the back porch as hsync. I wonder how the vertical sync is working.

[Lardo Boffin]

That's the result of the TV not being able to properly synchronise the display drawn by the electron beams. With analogue video, there is a synchronisation signal to signal the start/end of a line (line sync or horizontal sync), and different, longer one to signal the start/end of a field (field sync or vertical sync). Wikipedia explains a bit about it here.

As the TV is okay, I suspect that the ULA is not able to output the 0V level needed to properly generate the correct synchronisation signals. Do you have, or know someone with an oscilloscope?

Mark

Unfortunately not. I suspect that it will end up being sent off to Mutant Caterpillar Games to be fixed - this sort of thing is way outside my comfort zone!

[1024MAK]

Is the ULA soldered or in a DIL socket?

If it is in a socket, it's not too hard to change it.

Although, if you want the lowest cost option, that would be to buy another ZX81. Of course, unless you buy a known working one, you are likely to end up with another faulty ZX81, but most only have membrane failure, or have one of the earlier ULAs and the seller could not get a picture on the (modern) TV they tried it on (assuming they actually tried).

From what I can see, ULA failure, although not rare, is low compared membrane failure. When I bought a bunch of ZX81s some years ago on eBay, most had a faulty membrane, only one had a defective ULA. One had a faulty Z80 CPU and another one had the ROM inserted in the wrong holes in the DIL socket.

Mark

[Lardo Boffin]

Is the ULA soldered or in a DIL socket?

If it is in a socket, it's not too hard to change it.

Although, if you want the lowest cost option, that would be to buy another ZX81. Of course, unless you buy a known working one, you are likely to end up with another faulty ZX81, but most only have membrane failure, or have one of the earlier ULAs and the seller could not get a picture on the (modern) TV they tried it on (assuming they actually tried).

From what I can see, ULA failure, although not rare, is low compared membrane failure. When I bought a bunch of ZX81s some years ago on eBay, most had a faulty membrane, only one had a defective ULA. One had a faulty Z80 CPU and another one had the ROM inserted in the wrong holes in the DIL socket.

Mark

The ULA is in a socket so easy to replace if I can find one also in a socket!

The price of zeddys seems to have gone up of late as well.
Untested ones ('were working when put in the loft 30 years ago') are going for around £20 and working ones for around £40.

I'm sure they were going for around half this not that long ago!

Regardless I will keep my eyes peeled.

[1024MAK]

The price does vary. Around five years ago, "untested" or "not working" / "spares/repairs" ZX81s without PSUs were around £6 to £12.

In the last few years the price on eBay has gone up. But be patient, or try car boot sales.

The two issue 1 boards that feature in the new thread I started today, both have the ULA and the Z80 socketed (that's how they were before I touched them). New thread here.

Mark

[britain4]

I'd advise patience on eBay. I picked up 5 "untested" ones from there for £40 lately and aside from having to trim down a couple of membrane tails they all work perfectly!

All but one of mine have socketed ULAs (the exception being the issue 3)

[Lardo Boffin]

I'd advise patience on eBay. I picked up 5 "untested" ones from there for £40 lately and aside from having to trim down a couple of membrane tails they all work perfectly!

All but one of mine have socketed ULAs (the exception being the issue 3)

Blimey! If you happen not to want one of those...