ZX81 Dark screen, could it be the ULA?

Macc · November 2011

Trying to get a ZX81 going for a mate of mine, already replaced the membrane (Cheers Rich!) but now its the old "very dark screen" problem, probably caused by the ULA and fixable with the composite video modification, which I have ordered from Dataserve Retro, he however expresses concerns that this could be a damaged ULA that his mod may not fix, could this be the case?

ccowley · November 2011

Macc wrote: »

Trying to get a ZX81 going for a mate of mine, already replaced the membrane (Cheers Rich!) but now its the old "very dark screen" problem, probably caused by the ULA and fixable with the composite video modification, which I have ordered from Dataserve Retro, he however expresses concerns that this could be a damaged ULA that his mod may not fix, could this be the case?

With the qualification that I'm not vaguely, remotely a hardware person at all, I thought a "very dark screen" was 100% the expected behaviour of a working ZX81 when plugged into a modern-ish LCD telly? Or are you getting that behaviour on an old-school CRT and I am just being a numpty?

EDIT: My implication being that the ULA is perfectly fine and not damaged at all. So any composite mod will do exactly whatever it's designed to do. I really am not hardware-ish though (did I say that already?)

Macc · November 2011

Thats also my assumption, its just when a guy who really is a hardware guy casts doubt, you begin to doubt! lol

gtsamour · November 2011

I would say its the known ULA issue, I have one of those... the regular composite modification doesn't fix it unless its another kind of mod you're talking about.

My composite modded ZX81 (actually a PAL reversed Timex1000) displays a dark screen on one CRT (Sony) and a perfect bright screen on another CRT (Philips).

If your ULA's model fits the known "dark screen issue" one then I would say its not defective.

beanz · November 2011

Be careful of doing a composite mod btw if you do it, I fried 3 before I gave up. I think the ULA in the ZX81 is extremely sensitive to heat (from the solder iron), I never actually touched it but assume the heat traveled down the trace and killed it...3x.

Macc · November 2011

Im going to do it from the underside up with the ULA out, precisely because of what you described there.

So the comp mod wont cure this then? hmm, going to have to see when it arrives and fit it before exploring other avenues

guesser · November 2011

Is it an issue 1? I believe the early ZX81 ULAs had a similar issue to the ZX80 didn't they? i.e. not putting the correct sync pulses out.

gtsamour · November 2011

I modded two zx81 for composite (changed the board inside the modulator completely) and also all capacitors on them plus the 7805 regulators. Didn't have any problem with the ula going dead one me.
Also transformed a Timex1000 to PAL by disconnecting a resistor on one of the ULAs legs.

The easiest way to solve the dark screen issue is to find a newer ULA issue (2C210) and replace the one you have (ULA 2C184 or 2C183). But i dont think they are easy to find anymore.

ALso read this thread http://www.worldofspectrum.org/forums/showthread.php?t=32682&highlight=zx81+dark+screen

rwap · November 2011

A composite video fix does not resolve the common issue of black text on a very dark screen.

That is caused by the early ULAs on the ZX81 (and is known as the missing back porch!) - and as new ULAs are not readily available, that is why the ZXvid has been developed - this replaces the insides of the UHF modulator, to provide a very stable composite video out, plus adding a back porch latch for the original ULAs

See http://www.sellmyretro.com/search/naturalSearch?keyword=zxvid

gtsamour · November 2011

Very nice board Rich, but too expensive for what it is supposed to do, for me at least.
A new ula if found would be a lot cheaper.

Macc · December 2011

Im pleased to report that the comp mod arrived today and once fitted, bang, very bright white screen with a little black K in the corner, nice! (and thats on a Samsung LCD!)

rwap · December 2011

gtsamour wrote: »

Very nice board Rich, but too expensive for what it is supposed to do, for me at least.
A new ula if found would be a lot cheaper.

I agree, but then new ULAs are rare as hens teeth !!

There is actually a new ULA which has been developed - see http://www.rwapservices.co.uk/ZX80_ZX81/forums/ula-revistited-t445.html

beanz · December 2011

rwap wrote: »

I agree, but then new ULAs are rare as hens teeth !!

There is actually a new ULA which has been developed - see http://www.rwapservices.co.uk/ZX80_ZX81/forums/ula-revistited-t445.html

Are you going to be carrying those replacement ULA's Rich?

rwap · December 2011

Not sure at the moment how it will operate - the designer will probably just list them under his own steam on www.sellmyretro.com

guesser · December 2011

rwap wrote: »

There is actually a new ULA which has been developed - see http://www.rwapservices.co.uk/ZX80_ZX81/forums/ula-revistited-t445.html

I can't see that page... :(

You have ZB Block, and for some reason best known to the bloke who runs it I'm on the blacklist. :evil:

ccowley · December 2011

guesser wrote: »

I can't see that page... :(

You have ZB Block, and for some reason best known to the bloke who runs it I'm on the blacklist. :evil:

That's a shame, it looks right up your street - veroboard and oscilloscope porn. And something to do with "back porch generation" :-o I don't actually know what that is, but it sounds filthy.

ZX81 Dark screen, could it be the ULA?

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