From: Ethan Dicks <ethan.dicks@gmail.com> Subject: Re: [vcf-midatlantic] Museum report: terminals On Mon, Jul 9, 2018 at 12:27 PM, Jonathan Gevaryahu via vcf-midatlantic <vcf-midatlantic@lists.vcfed.org> wrote:
On 7/9/2018 11:39 AM, Herb Johnson via vcf-midatlantic wrote:
I saw many "moldy" CRT's - that's when the front of the CRT has a transparent cover that's separating from the glass. Repair means
replacing
the CRT, which can take most of an hour depending.
AFAIK, it's bubbles in the PVA layer between the face of the CRT and the implosion protection screen.
These CRT tubes don't have to be replaced, the CRTs can be very carefully heatgunned to remove the front protective glass cover and scrape out the 'cataract' goo. I think Ian has done it to a few of them already?
I've done it with a VT220. You can use a heat gun or a hot wire to cut the PVA to free the shield from the tube, then it's all cleanup. You do need to re-install the shield because otherwise the CRT clamps don't fit really well and the tube slides around. I used dots of a thick (1mm?) silicone "tape" on the corners and it worked well. Just a small airgap in front of the face of the tube instead of a layer of what is supposed to be optically clear PVA.
-ethan
We had a VT14 at the RICM that had cataracts. We used dental picks to remove the silicone holding the metal mounting band to the CRT. We planned on using a hot wire to cut the PVA, but when we started picking at the edges of the PVA we found that very little PVA was adhered to both the CRT and the shield. Cleaning up the PVA was tedious, but not difficult. We were concerned about putting a CRT without the PVA in a public place in the museum. We had images of a young visitor breaking the CRT and glass flying everywhere. We bought some 0.093" thick Lexan at Home Depot, waited for my wife to go shopping, and put the Lexan on top of the glass shield in the oven. When the temperature hit 325F the Lexan conformed to the glass shield. We trimmed the Lexan to the same size as the glass shield, and use silicone to hold everything together. This is probably a lot safer than the original PVA and won't degrade. There are pictures and a description of what we did here: http://www.ricomputermuseum.org/Home/equipment/dec-pdp-12/dec-pdp-12-restora... -- Michael Thompson