I am trying to repair a ZX81 that I think was damaged by a lightning since the adaptor and the 7805 were defective, the reset capacitor of the Z80 was blown up and then I realized that Rams were defective.

In fact I had a white screen with a video signal that was discussed here

Concerning the cited question, I added the 555 circuit. First the signal was attenuated so I replaced the two diodes by a common collector transistor. Now the signal has a back porch but still not working.

I know that when the 16k is inserted it disables the 1k onboard by setting the RAM.CS however it didn't work until I removed the onboard Rams.

Now with the 16k of external Ram the screen is as follows when I enter:


enter image description here

And this is another output when running the following program:

10 FOR I=1 TO 23

enter image description here

What could still be defective? The instruction seems to be well excuted, the program well stored and runs normally, so Ram is no more the problem.

  • 2
    Almost as if the shift register got stuck when reloading in fast sequence. like with floating lines on the input side. More prominent on lines wth lots of dark content, so capacity could be a factor. Since most of that is inside the ULA, it might be hard to solve - unless it's some external pullup/down on data lines.
    – Raffzahn
    Commented Apr 25, 2021 at 12:39
  • (Sorry, typing on a phone, no access to schematics)
    – Raffzahn
    Commented Apr 25, 2021 at 12:40

1 Answer 1


Thank you @Raffzahn for the comment that inspired me to think of something blocking the video signal.

The initial problem described in the previous question was that the ZX81 was showing a white screen.

I have an old version of the ZX81, so there was doubt about the absence of the back porch in the video signal. This doubt was removed by adding the 555-based circuit proposed by that question's answer.

However, the problem was not the back porch being absent: the screen was still white.

I then solved the white screen problem by removing the on-board RAMs and connecting an external 16 KB RAM pack. This made another problem emerge: the wide black lines.

The 555 'back-porch-adding' circuit is a monostable with T=5µs, triggered by the end of the video signal's SYNC pulse. In turns out that this is sometimes triggered by black pixels.

So the solution was to remove the 555 circuit. Here is the final screen:

enter image description here

In conclusion, the only problem was the onboard RAM. The TV actually accepts the video signal without back porch.


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