I have tried to eliminate "jail bars" on the screen after UHF mod to video out (based on that: http://zx81stuff.org.uk/zx81/ts1500_video.html) by adding 460uf 16V capacitor between +5V and GND but it didn't do anything to reduce them.

Any thought on that?

This is how it looks on the screen: enter image description here

  • You might want to try asking at electronics.stackexchange.com instead.
    – aybe
    Apr 28 '18 at 6:01
  • 3
    @Aybe Maybe not - The TS1500/ZX81 screen output is a mix of (very) tricky Z80 software and some (relatively) simple electronics. Even if I doubt someone would be able to actually get rid of the bars, the biggest chances of solving that would be with ZX81 experts
    – tofro
    Apr 28 '18 at 7:58

Pokemon on the German ZX Forum has designed a number of video improvement circuits (ZX81CPP, ZX81SCP) that allow connection of a composite video monitor and produce a crystal clear picture once properly adjusted.


The circuit basically completely re-creates the video signal from the ULA output.

Newer versions have an ATTiny(I think) on board that keeps the sync alive during fast mode which is a relief on modern LCD screens as they tend to get upset otherwise.

Every once in a while he offers PCBs and completely built circuits (which, BTW, nicely fit into the modulator case if cleaned up a bit).

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