The ion trap is a magnet that fits onto the back (the neck) of the CRT.
Ions
are atoms, not electrons. When electrons go though a magnetic field,
they bend,
ions don't. The electron gun in old crts is deliberately cocked off
angle, and
the ion trap magnet corrects that by bending the electrons back towards the
screen. The ions cannot make the turn, and impact the side of the tube
(harmlessly).
Ions are bad because they burn a hole in the center of the screen over time.
Why do ions occur in the electron beam ? Because every once in a while
an atom of the material that makes up the electron gun assembly rips away
and joins the beam.
By the time of the ADM3a, ion traps were obsolete. Some genius engineer
figured out that if you coat the back side of the screen, behind the
phosphor,
with a thin coating of aluminum, the screen could take ion damage quite
well.
The notice on the tube of the ADM3a is saying to service people that putting
on an ion trap magnet, which is a cheap spring loaded job, would kill the
operation of the tube, because it has a straight-shooting electron gun.
The trap would cause the electrons to veer off the screen. I guess the
assumption is that some old kodger TV repairman would say "look, this thing
is missing the ion trap".
The odd epilog to the saga of the ion trap is that yes, even with aluminum
protection, ion burns still happen over time. You have to get out a
mangifying
glass to see it, but virtually all crts more that a few years old have a
small burn spot in the dead center of the screen. The spot is dead center
because the ions aren't deflected by the sweep magnets that move the
electron beam, but they continue to the very center of the screen.