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:
PRINT "HELLO WORLD HELLO WORLD HELLO WORLD"
And this is another output when running the following program:
10 FOR I=1 TO 23 20 PRINT I 30 NEXT I
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.