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I've been restoring a couple of HP-97's. As I wrote earlier, I can see why people like restoring this machine. It completely strips down to the point that the keypad can be washed in soap and water, etc.

In order to try to reduce the extreme yellowing and darkening of the case. I placed the case in a solution of dish soap and bleach overnight in the sink-- not the keyboard, as I'm afraid it would dissolve the paint. (Cleans the sink, too!)

After a little scrubbing, the case does look much lighter and, of course, completely clean.

Do any of you engineers out there know if this will harm the plastic at all? I don't see any evidence of damage and am hopeful the solution is only cleaning the outer molecular layers and not reaching the bulk of the plastic. In other words, I don't think it's doing any harm.

Any comments would be helpful.


Hi, Doc; how are you?

I am an engineer, but I know nothing bout industrial chemistry. I believe Chemical OR Industrial Engineers/Technicians will be of help.

I am AWAYS afraid of using chemical products to clean plastic houses. Mostly if they need to be sank overnight. But you are talking about an (even) used HP97's back assy, I believe this if fine.


It should be okay for a one off cleaning. I would expect any problems to show up immediately or shortly after drying. The problem that I would expect to occur would be a chalky appearance to the plastic, but if it came out OK it sounds like the plastic is not affected by chlorine. Perhaps a little Oxyclean would help also. I know it works wonders in some strange applications.

I have tried glass bead blasting in the past, and have had pretty good results. You need a real fine bead though or you will change the texture of the plastic.

I have a yellowed Acorn Electron (UK) and will try your clean on it, so thanks for the tip!

Actually, I just now pulled it out-- after 24 hours. Looks good. I have also had great luck with a very light coating of "Armor-All" on the plastics. Gives them a lasting lustre. I've also found this works great on areas near the "on" switches that show that whitish change to the plastic. Again, it's supposed to be good for plastics; preventing oxidation, etc. I've been using the light Armor-All coating for some time.... calcs I shined up 3 years ago still look great!



Many plastic housings get yellow beacuse of dirt, smoke or direct sunlight exposure. Almost all plastics include additives for UV light protection, color and molding viscosity reduction, among other things.

Sunlight usually degrades the polymer's molecular bonds, and sometimes even the best UV absorbers cannot prevent degradation due to long-time exposure. However, it is easier to deal with dirt or smoke.

HP handhelds housings are made of acrylonitrile-butadiene-styrene (ABS,) a high strength copolymer with good resistence to chemicals. Since it is intended for heavy duty applications, ABS is carefully additivated to reinforce its mechanical properties in the long run, including direct sunlight exposure, heat, abrasion and impact resistance.

Therefore, a soak in oxyclean and soap will not harm any plastic housing. It will remove surface dirt and nothing else. And, as an extra feature, many cleaning solutions have substances known as optic blankers, which adhere to the surface being cleaned and do not absorb visible but UV light, making the surface look even cleaner.

HP might have made somewhat fragile circuit boards or wheels that become gummy in time, but the plastic it used for the housing was the best for the job. Shout the dirt out and make it look nice and clean.

Hope this helps.


Really ? My first computer was a BBC model B... what a hell of a machine !

Not being an engineer at all I'm always scared of using cleaning products. I already had bad experiences of chlaky effects, so what I do and what is usually successful is a solution of water and... dishwashing soap... If you rub the calculator with it most of the dirt will disappear.

Thank you all for the discussion!!

My bleached case, on drying and closer examination, is definitely lighter in some areas and looks much better than before, but it does have some uneven areas... it's darker, now, rather than lighter where there were stickers. Presumably, there was enough residue where the stickers were to block the cleaning effects. I would guess that if I hadn't just soaked it, but had scrubbed it several times, it might be more even, but this is just a guess.

I'm glad to hear about the durability of the ABS cases! Again, a reminder to all to be very careful with the keyboard, as even a soaking too long in soapy water can disturb the paint... especially on the woodstock series.


I am a product design engineer, so I have designed and tested quite a few plastic parts. I have always assumed that HP cases were an ABS/polycarbonate blend, but I don't really know for sure.

I don't think you will cause any damage with a water-soap solution and light scrubbing. I have used a child's toothbrush and Windex to scrub HP's, and have never had any trouble. I dip the toothbrush in a puddle of Windex rather than spraying the Windex on the (or into the) calculator.

In testing various products, we have found that certain oils can degrade the strength of plastics. We use olive oil to simulate the effect of years of exposure to oils and grease from human skin contact. Also, sunscreen and some hand lotions have a very dramatic effect on plastics.

I would avoid Armor All on any parts that have stuck-on labels - Armor All has a lot of silicone in it which will migrate everywhere and completely ruin the adhesive bond.

About 20 years ago, I used an old HP workstation that had a painted plastic keyboard surround. Before too long, the paint peeled away where users rested their palms. Apparently, the paint had been tested with a variety of degradants, but not with human sweat and skin oils.

Well, it's done.

I don't know how to post a picture, but imagine this:

The 97 was awful. Dirty. Darkened. Sticking keys. Broken (gummy) card reader. Dead batteries.

After reassembling (second time, as the first time revealed a card reader still not in perfect adjustment...), it looks like a new machine. Some final scrubbing with cleaner removed the remaining sticker residue. The keyboard, after cleaning with soap and water (as the contacts aren't part of the assembly once apart) is perfect. Battery pack rebuilt with new NiCD's.

Now, picture an HP-97 that looks and works like new. Very cool.

Ah, I love this hobby! Thanks to all for your comments, and as always, support.


Best sticker residue/marker/paint remover I have found is white gas (cigarette lighter fluid). Does not harm cases but can leave water marks on the keyboard.