So, I have de-soldered the parity chip on my SIMMs:

- SIMM.JPG (277.77 KiB) Viewed 286 times
and fitted everything back.
As not expected, the booster does not work
Happily there was no smoke, but something must be wrong.
Checked everything again but no luck. I only get a white screen with vertical bars, which disappear after some seconds (there is nothing on the screen if switched to HI-RES mode).
The Ultrasatan gets power but the floppy does not work if connected.
The worst is that if I get back to the original config, I also get a white screen with no bars...
I swapped to the old CPU but nothing changes. Could it be the memory, not working without the parity chip ?
As I have no spare RAM, I will have to buy some on the net.
I will definitely have to investigate further when I have more time.
If you guys have some clues, they are welcome.
Cheers.