Hello, all you wizards
(Sorry for the maybe hard to follow description. It is puzzling to explain -- and English is not my first language)
When I boot this PAL Apple IIe, I get a scrambled GR mode screen like this:
It changes all the time, but stay very similar (hard to explain, but here is another picture of the same screen).
Notice the dots are not at the same place. It seems to be changing at every refresh cycle. Not all characters are changing (more on that later).
If I remove the CPU, I get the same kind of pattern, but it doesn't change. It is often less "random" (sometimes it is just the same pair of chars repeated -- probably just what is in memory at startup).
If, with the CPU inserted and I press Conrol-Reset, the display freezes (while I hold the keys). When I press Control-Reset several times in successing, I sometimes can get a TEXT mode scrambled changing screen. In some cases, I can get it to display what is obviously ROM content:
This screen is changing all the time, but
a) not all chars change
If I look in the Apple IIe charset, all non-changing characters have their bit 6 set to 1. All the others are created from a superposition of two characters that only differ by their 6th bit (like '%' and 'e') [I can check that by pressing ctrl-reset, which generally freezes the screen].I discovered that the parts that flips are characters exactly 32 bytes appart (two examples).
b) it loops over every second, in what looks the same pattern (there seems to be some wave of chars on screen for 500ms, then a more static display, and it restarts). I can try a video if people want.
My suspicion is that a broken "bit 6" RAM chip may give so of this behavior (CPU instructions are wrong and it crashes/loops, while the video reads garbage). Or some latch somewhere?
Here is the motherboard:
I tested the few LSxx that are socketed in my minipro, they seems to be good.
Any brillant idea of what is broken is welcome.If you don't get any brillant idea to spare, I take the stupid ones too :-)