Most excellent Béla !! I threw away the tube I remember it was on, and if you have it "extra" I would love to receive that one

Well.....it's dark now and it's getting colder and colder. I finished up the monitor and even managed to get it back in the cab. Bart had somehow managed to lift it out of it without removing the back panel (as the manual says you should

and even all that without breaking the neck

After a couple of tries I called him to ask how he managed to do that and then indeed it worked out (although it sure wasn't easy). I just didn't want to have to remove the back panel. Turns out that 4 of the bolts have been replaced with a regular bolt & nut, and they were TIGHT !
Anyway, it was back in there, PCB set back in and power up. It looks really MUCH MUCH better than it was. Also, as I expected all the jittering was gone so it was indeed the PSU. The picture is really bright now and totally workable.
But since this project always seems to be 2 steps forward, 1 step back there of course had to be one step back and it turned out that I don't have any sound anymore. The test screen gives "Sound board timed out". I checked the power- and flat cable but they are firmly in place....no idea what this is now and I was done with it for today....so rolled the cab back in the game room (yes this is not ideal, I have to drag it out and in to work on it every time).
Oh well....we'll see about this problem later.....