Streaky video

Discussions about Sinclair ZX80 and ZX81 Hardware
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Chalkie666
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Joined: Sun Apr 22, 2018 9:51 pm

Streaky video

Post by Chalkie666 »

Hi folks
Got a zx81. It works but video display into lcd TV with antenna in with manual uhf tuning is very streaky. Background is white and text black, so but corrupted with horizontal streaks.
What can I do?
Could rf modulator be bad?
Cheers
D
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1024MAK
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Location: Looking forward to summer in Somerset, UK...

Re: Streaky video

Post by 1024MAK »

Does your TV have a fine tuning control?

The modulators used in 1980s computers are analogue with a simple oscillator, so they do not produce an output at the exact TV channel frequency that modern LCD TVs expect. So see if using the fine tuning control helps.

Also note that because the inside of the case of a ZX81 warms up, this will also cause the modulator to warm up, this affects the output frequency, so it may drift off-tune.

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

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Chalkie666
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Joined: Sun Apr 22, 2018 9:51 pm

Re: Streaky video

Post by Chalkie666 »

Yes it does have a fine tune... I tried that for quite a while. There is no place where it looks good enough to read the screen clear enough. Attached image is as good as it gets.
IMG_20180422_223459.jpg
Anything else to try?
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1024MAK
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Re: Streaky video

Post by 1024MAK »

Can you solder?
Does your TV have a composite video input on a phono / RCA connector?

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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Paul
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Joined: Thu May 27, 2010 8:15 am
Location: Germanys west end

Re: Streaky video

Post by Paul »

Welcome Chalkie666,
I would also go for a composite video mod.
VIDEOAUSGANG FUER DEN ZEDDY (1).pdf
(448.9 KiB) Downloaded 160 times
I am afraid its only in german. Bit maybe still helpful.
In theory, there is no difference between theory and practice. But, in practice, there is.
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1024MAK
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Location: Looking forward to summer in Somerset, UK...

Re: Streaky video

Post by 1024MAK »

There are a number of options for adding a composite video output.

If you want a built board where you only need to solder some wires, have a look at this one from PokeMon.

If you are happy to build your own circuit, search this forum ;)

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.
dessony
Posts: 342
Joined: Tue Oct 29, 2013 2:26 pm
Location: Indiana, USA

Re: Streaky video

Post by dessony »

Also, may I suggest that you try 1024MAK/Dessony strip board video buffer amplifier?
http://www.sinclairzxworld.com/viewtopi ... fer#p25270
Mark designed the circuit. I made it simpler by using a Veroboard/stripboard.

Thank you very much in advance.

Cheers,
Dessony
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