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.
ZX81 black screen
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Re: ZX81 black screen
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
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
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.
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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Re: ZX81 black screen
No, nothing.
I was hoping the mid would by-pass my problem and allow use on a modern screen...
I was hoping the mid would by-pass my problem and allow use on a modern screen...
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Re: ZX81 black screen
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
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
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.
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.
Re: ZX81 black screen
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
ZX80 USA, ZX81UK, ZX Spectrum, ZX Spectrum+, ZX Spectrum 128+ UK, ZX Spectrum +2/A, Sinclair QL, CZ1000, CZ1500, CZ2000, CZ1000Plus, CZ1500Plus, CZ Spectrum, CZ Spectrum Plus, TK83, TK85, TK90X, TK95. TS2068. And more to come
ZX80 USA, ZX81UK, ZX Spectrum, ZX Spectrum+, ZX Spectrum 128+ UK, ZX Spectrum +2/A, Sinclair QL, CZ1000, CZ1500, CZ2000, CZ1000Plus, CZ1500Plus, CZ Spectrum, CZ Spectrum Plus, TK83, TK85, TK90X, TK95. TS2068. And more to come
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Re: ZX81 black screen
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.