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


closed as unclear what you're asking by brhans, Voltage Spike, Lior Bilia, Dmitry Grigoryev, Sparky256 May 12 '18 at 3:59

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  • \$\begingroup\$ I'm not sure, but I think you may be talking about the difference between interlaced and progressive mode operation? (p vs i.) A retro computer for NTSC would be targeting an interlaced TV and would add half a scan line to both fields. (There are 262.5 scan lines per field for a total of 525 scan lines.) Some receivers do recognize the extra half line as sufficient to switch to interlaced mode, automatically. Some don't and may require an added blob of memory-buffered hardware for this. (A de-interlacer of some kind.) A picture of what you see might be nice, though. \$\endgroup\$ – jonk Apr 26 '18 at 3:03
  • \$\begingroup\$ @jonk I have updated my question with the picture of those "jail bars" \$\endgroup\$ – JackTheKnife Apr 26 '18 at 12:29
  • \$\begingroup\$ that looks like the "text character" clock breaking through into the video ....... did you try to turn down the contrast setting? \$\endgroup\$ – jsotola Apr 26 '18 at 18:10
  • \$\begingroup\$ Possibly a dried-up/bad electrolytic capacitor in the scan derived power supply for the video or chroma circuits. See if you can find the ripple using a scope. Or just replace the electrolytics. \$\endgroup\$ – jonk Apr 26 '18 at 21:27
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    \$\begingroup\$ Overshoot on video. Could be that 460uf is too big. Try 0.1uf in parallel. Could also be inductance in wires - keep them short, especially ground. \$\endgroup\$ – glen_geek May 12 '18 at 4:13

Those 'jail-bars' appear to be a synchronous subset of the horizontal trace. They are very clean and well defined. Suggest tracking down horizontal circuits and replacing the electrolytics in just that section.

If problem still exist try to find a point where you can modify just the vertical bars and nothing else.

At some point you may stumble into a leaky transistor or bad IC. Bad resistors and capacitors tend to open up and loose value, so check those in the horizontal section.


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