ZX81 grey vertical lines on screen

Discussions about Sinclair ZX80 and ZX81 Hardware
bbg384
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Joined: Mon Mar 30, 2020 3:09 am

ZX81 grey vertical lines on screen

Post by bbg384 »

Hi
I've got a ZX81 which has the composite video mod, and another which just has the standard TV output. See the photos
When I connect the ZX81 which has the video mod, I get the K displayed on the bottom left, but also grey vertical lines on the screen.
What is normally the cause of this? I read somewhere online that it is the ULA chip.
Are these inter-changeable between ZX81's?
thanks
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1024MAK
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Re: ZX81 grey vertical lines on screen

Post by 1024MAK »

Hello and welcome. You may wish to properly introduce yourself in the welcome area ;-)

Can you please post up a photo of the display image that you are getting on your TV so that we can see the problem that you are talking about.

There are three different versions of the Sinclair ULA. Any ULA version will work in any ZX81/TS1000 board. The first two versions produce a video signal that is less compatible with colour TVs. The last version (2C210E) does include extra circuitry so that it is more compatible with colour TVs. The ULA fitted in your unmodified ZX81 is this later type.

However the ULA in your photo of your modified ZX81 has another chip soldered on top, so I can’t see the ULA type number/code. Having an extra chip mounted on top is unusual. Can you please take some more photos at an angle showing the connections from each side. And can you please tell us what the type number/code is on this extra chip.

Mark
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bbg384
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Re: ZX81 grey vertical lines on screen

Post by bbg384 »

Hi
thanks very much for your reply. and yes I'll introduce myself on the forum.
I have attached some more photos. The one from the screen isn't very clear, but you can see the vertical lines, ignore the dark diagonal one.

I have connected the zx81 to a commodore monitor which has a composite input, if I connect the ZX to my LCD TV's input, I also get the same result.

The small chip on top of the ULA has 5CALY7K written on the top of it, there is another below it but I'm not able to read it.
thanks once again.
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Moggy
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Re: ZX81 grey vertical lines on screen

Post by Moggy »

There is also some further writing on the chip just under the orange capacitor lead which I think would better help identification. Is it possible for you to post that?

Regards and welcome.

Moggy.
bbg384
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Re: ZX81 grey vertical lines on screen

Post by bbg384 »

I'll also attach a photo, but I think the extra characters on that chip are
3N74L3VZN
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1024MAK
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Re: ZX81 grey vertical lines on screen

Post by 1024MAK »

SN74LS02N is a standard logic gate (Quad 2-input positive-NOR gates) and is my best guess...

The ‘5CALY7K’ is a manufacturer batch or production number/code.

I’m also guessing that there may be wiring between it and the ULA.

What sort of power supply unit (PSU) are you using to power your ZX81?

Mark
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bbg384
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Re: ZX81 grey vertical lines on screen

Post by bbg384 »

I have the standard ZX power supply, AC adapter, outputis 9v DC AT 0.7A
Model UK 700
GTS1000TX
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Re: ZX81 grey vertical lines on screen

Post by GTS1000TX »

Mind if I tag along? Yesterday, I did the transistor and 2 resistor composite mod on my TS1000. And it has the same vertical gray bars. It has the 2C210E ULA. I'm connected to an LCD display via composite. Looks much better than RF could produce. I think RF had them too. But the picture was very poor.

This occurs with the adapter it came with. I didn't trust it. So I've removed the 7805 and am powering it directly from a 5V at 1A USB charger. And still have the vertical gray bars. They appear to be column width.
Lardo Boffin
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Re: ZX81 grey vertical lines on screen

Post by Lardo Boffin »

It may be worth trying one of these? https://www.sellmyretro.com/offer/detai ... -zx81-3712

I have fitted 30 and no vertical grey bars.
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
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1024MAK
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Re: ZX81 grey vertical lines on screen

Post by 1024MAK »

The vertical lines are almost certainly digital switching noise on the +5V rail.

Either ignore it, use a ZX8-CCB to clean up the video, or try soldering 100nF or 470nF or 1uF ceramic capacitors across the +5V/Vcc and 0V/GND pins of the ULA, Z80, ROM and SRAM chips.

Unless of course you are using a external RAM pack, in which case, the electrolytic capacitors in this may need renewing.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
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