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.
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:
- For the working one:
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?