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?


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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