Extreme screen "rot" on an ADM-3a (an ebay auction)

Mr Ian Primus ian_primus at yahoo.com
Thu Jul 17 09:05:30 CDT 2008


--- 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



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