71B: repair/polish LCD screen scratches

[cdmackay]

hi all,

I've just picked up a 71B, which has some scratches on the LCD screen, including a bad one right by the prompt, which is a little annoying.

Is it possible to polish out such scratches, with Polywatch or even toothpaste, as one might on an acrylic watch "glass", or is that not advised?

I imagine a replacement LCD cover (if indeed there is one) is not something that's easy to find?

thanks very much for any ideas.

[rprosperi]

Displex works well on plastic display covers (e.g on old LED machines) though I've never used it on a 71B lens cover so can't say for certain.

Also Novus plastic polishes have a very good reputation (often mentioned here) but I have no personal experience with it.

[cdmackay]

thanks very much Bob, I'll look into those…

[Maximilian Hohmann]

Hello!

I use stuff called "Polywatch" for that purpose. And also for it's intended purpose of removing scratches from watches. It works with glass, metal and plastic and acts rather gently so there is no risk in damaging anything by overpolishing.

Regards
Max

[cdmackay]

thanks Max; I just wanted to check there wasn't anything unusual about the LCD cover on the 71B that I might ruin…

[Dave Frederickson]

Be aware that some LCD screens have an anti-glare coating and that it's actually the coating that has been scratched. See here:

https://www.hpmuseum.org/cgi-sys/cgiwrap...read=34762

Any attempts to remove the scratch will likely result in the removal of the coating. If the scratch is deep enough to have penetrated the coating and into the screen, then you really have nothing to loose. Even products like Brasso are reported to be effective at removing scratches.

Dave

[cdmackay]

interesting, thanks very much Dave.

[Hans Brueggemann]

fortunately, the HP-71B has an extra perspex insert in the display frame that protects the underlying LCD from polishing "accidents". the perspex has a thickness of approx. 1 mm. that should give plenty of margin for buffing out any scratches.

one trick i tried in the past with admittedly so-so success was to fill out a deep scratch with a drop of transparent UV-curable resin (comes in small bottles with a blue LED keyfob as UV-lighsource) first, then wet grinding it down to 2000 grit, and then manually polishing it with acrylic polish. don't use power tools as they might burn the perspex surface because of too high rpm.

[cdmackay]

thanks very much Hans.

Might an alternative be to cut a new piece of perspex and use that instead?

Any tips on removing the bezel, please?

thanks.

[Hans Brueggemann]

the bezel is held in place by a very strong self-adhesive coating on its back. i managed to dissolve the coating by repeated careful application of lighter fluid along the edges of the bezel (that took me a couple of hours, though). you need apply some gentle force to convince the bezel to come off, and in due course you will ruin the perfect flatness of the bezel. in hindsight, it was not worth trying because of all the consequent hassle to get it back into shape.
also, don't try to use heat to soften the adhesive coating, or acteone to dissolve it. both methods will ruin the plastic of the calculator's top shelf.

[cdmackay]

thanks for the warning Hans; I will stay well clear of the bezel

I tried some Polywatch on the scratched perspex, but I struggled to see that I was making any difference. Now pondering whether to try again, pressing harder, and/or polishing longer, or just leave well enough alone.

[Zaphod]

(05-29-2019 05:32 PM)cdmackay Wrote:  thanks for the warning Hans; I will stay well clear of the bezel

I tried some Polywatch on the scratched perspex, but I struggled to see that I was making any difference. Now pondering whether to try again, pressing harder, and/or polishing longer, or just leave well enough alone.

Confucious says "one must have patience" , you’re not polishing, you’re removing a microscopically thin layer of material with a very very fine abrasive, it’s not gonna happen instantaneously , it takes time....

..... then a little more time.

[cdmackay]

OK, thanks for the encouragement, I'll try again

[Hans Brueggemann]

(05-29-2019 05:32 PM)cdmackay Wrote:  thanks for the warning Hans; I will stay well clear of the bezel

I tried some Polywatch on the scratched perspex, but I struggled to see that I was making any difference. Now pondering whether to try again, pressing harder, and/or polishing longer, or just leave well enough alone.

ah, i see.. you're just just using polywatch and nothing else. that will take forever.
pressing harder won't help at all in the process.

before you are going to ruin the perspex of your calculator, try your polishing method on some scrap piece of perspex:
1) purchase "wet" (never use "dry") sanding paper with 600, 800, 1000, 1500, 2000 grit , 1 sheet each.
2) fill a cup of cold water and add a squirt of liquid dish washing detergent.
3) starting at the lowest number, thoroughly sand the perspex in circular motions with snippets of the sand paper, using a generous amount of the dish washing solution as lubricant. if the sand paper seems to get "blunt", rinse it under fresh water. do only apply very slight pressure. objective is to get an obsolutely evenly sanded surface. check the perspex against the light. it should have become evenly "foggy" or semitransparent, with no more individual scratch marks visible.
4) thoroughly rinse the perspex under fresh water. also, discard the rest of the dish cleaning solution, clean the cup, and make some fresh solution. we want to get rid of all sanding particles from the sand paper, before we go on with the next higher grit number.
5) repeat from step 3) with the next higher grit number until 2000 grit. dry the perspex with a lint-free paper towel.
6) now, use the Polywatch and a package of q-tips to give the perspex the final touch. done!

now that you got the hook of it, apply this procedure to the perspex of the calculator.

as i said before, in case the scratch is very deep, you may to try to fill it with a drop of UV-curable resin. you can determine whether the scratch requires filling if it is still visible after using the 600 grit.
this stuff is available at brick&mortar stores, including the UV light source for curing it. just leave the repaired area untouched for 24 hours, befor you start again at step 2) with the coarsest grit.

[cdmackay]

great! thanks for the detailed note, Hans, that's really helpful.