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I have two ZX81, one is perfectly working and I am trying to repair the other one.

After repairing the external power supply (a wire of the transformer was cut near the connector. I think it was due to an overheating) and after changing the 5V regulator 7805, I got a flickering screen.

I checked and found that the power supply was noisy so I added a capacitor at the input and now the regulated voltage is good, the clock is working at the right frequency but I am getting a Blank screen (white screen) sometimes flickering.

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I think the ULA is not completely defective since I can see on the oscilloscope a normal RAMC.S. signal.

But comparing video signals between the working and the defective ZX81 I am getting the following:

  • For the defective one:

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  • For the working one:

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So in the defective ZX81 the Sync pulse is right but there is no backporch and the level is always white. I need more information on how does the ULA generate the video signal to continue troubleshooting.

So the question is:

How is the video signal generated by the ULA? How come that, in my case, the Sync pulse is generated but not the backporch?

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That portion of the video signal is generated directly by the ULA; early ZX81s just don't generate a back porch, which was seemingly good enough for the majority of televisions but problematic enough to be fixed in a later revision despite the machine's commercial lifetime being only a year. As you've observed, the absence of a back porch causes some displays incorrectly to calibrate their expectations as to signal levels.

The fix is to add a back porch externally, e.g. this guide offers a solution based on a 555, four resistors, four capacitors, two diodes and two transistors. Others have produced prebuilt versions of that schematic or even complete ULA replacements that are guaranteed to include a back porch.

In the mean time you should be able to confirm by switching the ULAs between your two machines.

So, in short: I don't think your other ZX81 is defective, I think it's still operating exactly as it always did.

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