ZX81 grey vertical lines on screen

Discussions about Sinclair ZX80 and ZX81 Hardware
bbg384
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Joined: Mon Mar 30, 2020 3:09 am

Re: ZX81 grey vertical lines on screen

Post by bbg384 »

thanks for the replies.
I have it working ia the composite input on a 80's computer monitor, though the lines are there, I can live with it.
rmzalbar
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Re: ZX81 grey vertical lines on screen

Post by rmzalbar »

I added a 33uF electrolytic cap across the +5VDC and GND near the regulator to get rid of some video noise. It also reduced the intensity of the vertical grey bars by about half. I imagine a larger cap would help even more so, as the bars are relatively low frequency.

I was surprised to find the ZX81 had no large caps to clean +5VDC. The 22uF cap on the board is for the unregulated 9VDC.
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Moggy
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Re: ZX81 grey vertical lines on screen

Post by Moggy »

Marks post on page one of this thread is very useful re capacitors and from experience I know it works.

As for why nothing added on the 5v line you obviously haven't heard the legend of Ebineezer Scrooge sorry i mean Uncle clive? The most parsimonious computer man on the planet! :lol:

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1024MAK
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Re: ZX81 grey vertical lines on screen

Post by 1024MAK »

It is also important to note that the 7805 voltage regulator is very good at ‘cleaning’ the noise and ripple from the +9V (nominal) input, but if the electrolytic capacitor(s) in the Sinclair ZX PSU (or equivalent) are life expired, then the 7805 can’t fix that.

In a normal situation, for normal use, that is well before widescreen LCD TVs were even a glimmer of an idea in the mind of engineers, keep in mind that these were low cost computers that were intended to be used with 12” or 14” monochrome (black and white) CRT TV sets. So there was no requirement for any electrolytic capacitors on the +5V rail.

If you are using a modern SMPSU (switch mode power supply unit), the cheap versions don’t always have enough filtering, and hence elements of the high frequency switching noise can get into the ZX80 / ZX81 / ZX Spectrum. The computer will likely work, but with lots more screen interference compared to normal.

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rmzalbar
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Re: ZX81 grey vertical lines on screen

Post by rmzalbar »

1024MAK wrote: Fri Jun 26, 2020 1:00 am It is also important to note that the 7805 voltage regulator is very good at ‘cleaning’ the noise and ripple from the +9V (nominal) input, but if the electrolytic capacitor(s) in the Sinclair ZX PSU (or equivalent) are life expired, then the 7805 can’t fix that.

In a normal situation, for normal use, that is well before widescreen LCD TVs were even a glimmer of an idea in the mind of engineers, keep in mind that these were low cost computers that were intended to be used with 12” or 14” monochrome (black and white) CRT TV sets. So there was no requirement for any electrolytic capacitors on the +5V rail.

If you are using a modern SMPSU (switch mode power supply unit), the cheap versions don’t always have enough filtering, and hence elements of the high frequency switching noise can get into the ZX80 / ZX81 / ZX Spectrum. The computer will likely work, but with lots more screen interference compared to normal.

Mark
My ZX shows some significant ripple leaking through the 7805, causing a high-frequency herringbone interference for the first minute or so until the 7805 warms up. I know it's the 7805 because if I blow on it and its heatsink for a bit to cool it off, I can bring the interference back. I'm using a 9V Atari 2600 wall-wart with probably pretty poor filtering of its own. So my 7805 is probably marginal, but I intend to replace it with an SMPS drop-in substitute. The 33uF cap I added across the 5V rail did clean it up completely.
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