Thread: Zx81
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Old 9th Mar 2020, 7:27 pm   #2
SiriusHardware
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Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 8,519
Default Re: Zx81

Hello again, John.

I imagine you have tried adjusting both the brightness and contrast as well as the tuning (if not, do so) but I suggest also turning the colour down to zero - yes, I know the ZX81 is not a colour computer, but presumably the TV is a colour TV? It might be producing colour fuzz / noise due to the absence of any colour signal to pay attention to.

There was an issue with early to mid period ZX81s outputting an imperfectly formed video signal - technically speaking the video output lacked the 'back porch' portion of the video signal.

The final version of the ZX81 ULA IC finally fixed that problem, but as time went by TVs became more fussy about the video input signal and the output from an early to mid period ZX81 with an earlier ULA can often look poor / dim on a later CRT or flatscreen because the TV is being fooled into setting the 'black level' to the wrong level.

If simple adjustments do not solve your problem, can you look (carefully, without stressing the keypad ribbon connector) to see which version of the ULA IC your ZX81 has?
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