ZX81 no picture

Discussions about Sinclair ZX80 and ZX81 Hardware
poglad
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Joined: Mon Mar 24, 2014 3:11 pm
Location: Aberdeen, Scotland

Re: ZX81 no picture

Post by poglad »

Isn't there also a diode that lets the ZX81 know whether it's 60Hz or 50Hz? You can PEEK a system variable to find out which system it is, so it's definitely not just a matter of the modulator alone.
yukin
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Joined: Fri Jul 18, 2014 2:47 pm
Location: Québec, Canada

Re: ZX81 no picture

Post by yukin »

thanks for the information guys. I try connect the machine with my rf adapter right to the VHF input on my tv and when i put on the power of the computer, the screen change from white snow to black snow. I still hear the sound of the snow. i try changing the v-sync, the color, the bright, the contrast etc... Still not doing anything. So there is a reaction but it still doesn't work. So like RWAP said my best chance could be if im going with the ZXvid module.
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1024MAK
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Re: ZX81 no picture

Post by 1024MAK »

On your TV, in the picture that you posted, the TV/Cable switch is the only one that is likely to change between VHF and UHF.

Mark
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1024MAK
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Re: ZX81 no picture

Post by 1024MAK »

yukin wrote:I try connect the machine with my rf adapter right to the VHF input on my tv and when i put on the power of the computer, the screen change from white snow to black snow. I still hear the sound of the snow.
The ZX81 does not produce a sound/audio signal, so even a correctly tuned in video signal will still have "the sound of the snow" as the TV audio demodulator will only find noise...
The modulators are known to drift, so are often not at the specified frequency. So please tune through the whole band.

Mark
poglad
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Re: ZX81 no picture

Post by poglad »

Ah - well, my ZX81 in the 80s didn't produce the sound of snow... when properly tuned, it was very quiet with a low-volume buzz. If you detuned slightly, the buzz was louder and would change as things were printed to the screen e.g. as a listing was produced. When in FAST mode, the buzz would switch to complete silence whenever the display was off.
yukin
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Location: Québec, Canada

Re: ZX81 no picture

Post by yukin »

1024MAK wrote:On your TV, in the picture that you posted, the TV/Cable switch is the only one that is likely to change between VHF and UHF.

Mark
The tv/cable switch seems to only put back the TV to Channel 3.
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1024MAK
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Re: ZX81 no picture

Post by 1024MAK »

@ poglag, maybe your 1980's TV had a auto mute circuit.
Myself, back in the 1980's I had a Spectrum but I remember always turning the volume down.

Mark
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XavSnap
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Re: ZX81 no picture

Post by XavSnap »

Hi,
Yes, power on your ZX81 and scan all UHF frequencys.
If the TV picture is fully white or fully black... it's the ZX81 signal ! (if it's snowing, you able to see the univer's "sound")
Many ZX81, can hang his display during the power socket insersion!
The screen can be all white, or hang in the fast-mode.
Juste unplugg the power pin, wait few secounds and power on the ZX81, again.
The "K" cursor will be displayed correctly.
If there's nothing on the UHF bands, try the VHF frequencys...
Xavier
Moggy
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Re: ZX81 no picture

Post by Moggy »

poglad wrote:Ah - well, my ZX81 in the 80s didn't produce the sound of snow... when properly tuned, it was very quiet with a low-volume buzz. If you detuned slightly, the buzz was louder and would change as things were printed to the screen e.g. as a listing was produced. When in FAST mode, the buzz would switch to complete silence whenever the display was off.

He he that brings back memories :lol:
On my old GEC set when detuned slightly you could actually hear the various parts of the circuit working as the program executed.

"BRRR-DA-DA-DIT DA-DA-DIT-BRRR-DUGGA-DUGGA-DUGGA"

Happy days!
gozzo
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Joined: Fri Jul 08, 2011 8:52 pm

Re: ZX81 no picture

Post by gozzo »

hmm..you really need to 'tap off' a composite signal and feed it to a composite monitor (black&white, preferably) to see if the ZX is actually working or not, bypassing the modulator (you can get away with just an electrolytic capacitor of about 100uf, positive to the modulators input lead - leave it connected - , negative to the monitor - this is not ideal but ok for short term testing purposes), if you still don't get a K , there is definite problems, most probably ULA failure...
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