After deciding that I needed to change all logic chips from 74HC series to 74LS, I had to wait a week for the arrival of the new set. Nowadays I order all this from Reichelt Elektronik, a German company that has an amazing stock, and supplies components in any wanted quantity to anybody, business or private. ( Professionally I'm used to Farnell and RS, but these companies prefer only B2B) Great company, but delivery just takes 4 to 7 days.

So after swapping all the HC chips for their LS equivalent, I had high hopes when switching it on. But alas: nothing changed ! :-(

Well, the oscillator wouldn't start due to the different impedance, but after I just changed this specific chip back to HC, it worked again. All signals do seem a bit cleaner, but it does not solve the problem. The screen still looks like this:Again: it does react to the keys, so everything seems to be functional, and this confirms my earlier suspicion that the fault must be in the writing or reading of the video memory. Which it was. Another check of the schematics showed that I tied the /CS signals of both video RAM chips together. Obviously not correct. And after fixing this by disconnecting the CS pin on one RAM chip and wiring it to the correct port, the computer showed me this:
YES !. That is what I was looking for !. Now let's try the only command I know: VLIST.

EUPHORIA !. Finally, after more than 18 months, it works.
Now I still have to check the cassette interface, attach the speaker, and solve an annoying wavy pattern in the video image, but all that will be peanuts compared to the path I've traveled so far.