ZX81 black screen

Discussions about Sinclair ZX80 and ZX81 Hardware
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Zx81BASIC+
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ZX81 black screen

Post by Zx81BASIC+ »

Hi. Thanks for the add.

Probably been covered many times over, but I have a black screen when trying to use my ZX81 with an known working mid-80’s CRT B and W TV.

Some research has led me to believe the composite mod may fix this ?

I’ve attached a pic of the ULA.

Any advice is appreciated.

Thanks in advance.

Mike.
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1024MAK
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Re: ZX81 black screen

Post by 1024MAK »

Hi Mike

I see no attached picture :(

Unfortunately with a black screen on a monochrome (black & white) TV, it’s unlikely that a composite mod. on it’s own will fix this.

If you turn the brightness and contrast up, do you see any image?

Mark
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Zx81BASIC+
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Re: ZX81 black screen

Post by Zx81BASIC+ »

No, nothing.

I was hoping the mid would by-pass my problem and allow use on a modern screen...
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1024MAK
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Re: ZX81 black screen

Post by 1024MAK »

A ZX81 is both complex and simple. It’s simple because it only uses four or five chips (depending on which type of RAM chips are fitted).
It’s complex because it does not use a dedicated video chip, but instead uses the Z80 processor to fetch video data from the memory. The ULA then takes this data and turns it into a serial bit stream, which it outputs to the modulator.

Normally if the Z80, the RAM or the ROM fail, the ULA will produce a white screen with no other video data. But if you get only a black screen, that unfortunately often means that the ULA is faulty.

The job of the modulator is just to take a composite like signal (from the ULA) and to mix this with the output from a ultra high frequency oscillator so that a suitable signal is generated for input to the aerial input of a analogue TV. If your TV can be tuned to a black screen, then the modulator is working.

I’m not trying to put you off, I just don’t want you to put false hope in a composite video modification that in your case, I don’t think will fix the problem.

There are very occasionally, original ULAs for sale on eBay. There is also sometimes a limited supply of replacement ULA modules. But COVID-19 and brexit have messed up production and shipping of these :(

Do you have, or know anyone who has any test gear. As a very bare minimum, that would be a digital multimeter. But a logic probe or oscilloscope would be even better.

Mark
ZX81 Variations
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bola_dor
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Re: ZX81 black screen

Post by bola_dor »

I would try a simple transistor/resistor mod just as a diagnostic tryout. just to rule out that the problem is not between the video modulator and the TV tuner. What Mark says is unfortunately true but the only hope in the face of a non-working ULA is right there.
Ernesto
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Zx81BASIC+
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Re: ZX81 black screen

Post by Zx81BASIC+ »

Thanks all. Much appreciated. I will try the simple transistor / resistor mod. If all else fails it’ll become a wall clock 😂. I think replacing the ULA is way beyond me... thanks again. Mike.
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