Archon wrote:
Tubes busted internally, if it was signal distortion the shape would be
the same top, bottom or middle of the screen.

You will most likely here glass tinkling inside the tube if you tip the
scope, also remove the blue implosion filter and check for tiny marks on
the phosphor from debris.

Yes you're spot on, I disassembled the tube to check if any damage was
visible, and there were little pieces of glass going around inside. One
of the glass posts holding the deflectors and electron gun was broken.
There are indeed several marks on the phosphor, as if it was scratched
from the inside.

Out of curiosity, do you know what element in particular causes the
strange distorsion ? Since the shape changes whith the y-position, I
figure this would have to be after the vertical deflection plates, but I
don't see what it could be.

v.