17 Jul
2008
17 Jul
'08
7:05 a.m.
--- On Thu, 7/17/08, Eric <eric940 at gmail.com> wrote:
I've seen screen rot before -- even owning one
with
perimeter
symptoms, but this is an example of _extreme_ "screen
rot" (separation
of the protective plate from the CRT) on an ADM-3a.
I had an ADM3A with bad rot like that, but I never tried to fix that one. The rest of the
terminal was in such terrible shape, it just got parted out. I've got a couple other
terminal tubes with light rot forming around the edges though.
I'm unsure if we've ever come to the
conclusion if
this is just an
adhesion separation issue resulting in blotches resembling
mold, or
actual mold causing the separation. (Have we had some sort
of cause
for this problem?)
I don't know about the terminal picture tubes, or how they're bonded, but I do
know that this problem also affects old color television picture tubes as well. The old
color tubes from the late 50's/60's will develop similar rot all along the edges,
sometimes all the way to the middle. It's referred to as "cataracts". I
don't know if it's mold, but it basically results in the breakdown of the PVA
bonding compound that holds the safety glass to the face of the tube. To remove it, at
least on the TV tubes, you take the the tube out of the set, put it neck down in a large
bucket/barrel so that it's supported, and use a heat gun to heat up the screen. After
a while, the PVA will soften, and the faceplate can be gently separated from the tube.
Don't pry it up - try to gently induce air bubbles under it with wooden shims and
heat. Once the bond breaks over most of the tube, the glass should come off without
breaking. Then you clean out all the PVA gunk, and
reattach the safety glass with packing tape along the edges.
I've never run across a terminal that was bad enough to warrent fixing (and where the
rest of the terminal was intact enough to bother with). Fortunately, the 12" mono
tubes in the ADM3 and the like are pretty easy to find, so replacement is usually an
option. Now, with the old color TV tubes, it's a different story. Try finding a
replacement 21FJP22! (It's a round, 21" color picture tube, 70 degree deflection
angle)
Note that on some color TV tubes (Zenith, IIRC), the bonding compound is different and
won't come off with a heat gun. Those tubes require "persuasion", in the
form of a car battery and nichrome wire. I have no idea what class of bonding compounds
the terminals fall under. But, if anyone has a badly delaminated tube, I would be
interested to hear how far you get with the heat gun.
-Ian