Jailbars on ZX80

Discussions about Sinclair ZX80 and ZX81 Hardware
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albertusj
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Jailbars on ZX80

Post by albertusj » Sat Jul 13, 2019 7:22 pm

Does anyone have any suggestions on how I can eliminate or reduce the "jail bar" effect present on my ZX80s composite output?

http://www.myprius.co.za/jailbar.JPG
ZX80/ZX81/ZX Spectrum

Moggy
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Re: Jailbars on ZX80

Post by Moggy » Sat Jul 13, 2019 9:02 pm

This looks more like a monitor problem to me as the "jailbars " as you call them tend to affect the central display square which the zeddy outputs to the monitor and not the margins/top and bottom borders as seen in your picture which seems to show a global effect.
I think a comment from Mark would help as apart from superior knowledge to myself he has the knack of straight forward explanation and would have a more definitive answer.

Also I'm not sure what composite method you are using but I would seriously consider using this....

https://www.sellmyretro.com/offer/detai ... -zx81-3712

As well as a switching regulator and a Zxpand kit I use this on my ZX80 and it is picture perfect either on CRT or LCD display.

albertusj
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Re: Jailbars on ZX80

Post by albertusj » Sun Jul 14, 2019 12:40 pm

Strategically placed 100nF capacitors over the power rails of U9 and U20 made for some improvement. :D
ZX80/ZX81/ZX Spectrum

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1024MAK
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Re: Jailbars on ZX80

Post by 1024MAK » Sun Jul 14, 2019 8:14 pm

Assuming that the existing electrolytic capacitor in the external PSU is okay (if a Sinclair type), and the 7805 voltage regulator is okay, then it is just digital noise due to the use of low value decoding capacitors.

The other Sinclair computers of the time (ZX81 and ZX Spectrum) also suffer a similar problem.

As you have found, it is not so noticeable when the RF/UHF output is used, especially if used with a small screen CRT TV.

And as you have found, using higher value ceramic capacitors to decouple the +5V supply helps. Try replacing C1, C2, C5 with 100nF or higher values. On ZX Spectrum boards, 1uF multilayer ceramic capacitors have been found to work well.

Alternatively the video output can be “cleaned up” by using an extra circuit module like the Z8-CCB from Pokemon.

Mark

albertusj
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Re: Jailbars on ZX80

Post by albertusj » Mon Jul 15, 2019 1:00 pm

Thanks as always 1024! I'll try your suggestions.
ZX80/ZX81/ZX Spectrum

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