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Restoration of Calculators

by Bob Noble

Note from Guy -- Legal-lawyer talk time:
The information in this wonderful article is for public information. This information may cause death or disfigurement if the actionee is not careful. Some people could cut their fingers off with the X-Acto knives (or worse) or develop warts and other health problems due to the use of certain solvents. Anyone following any of the instructions below do so at their own risk. The writer of this story, the web-guy who is posting it, and their mothers will not take any responsibility for any damage, death, or bad things that happen to anybody doing the stuff below.
PLEASE! -- Life requires responsibility and responsible actions!
With that said, let's get on to some great information for those of us who restore old calcs --guy


My Story:

I remember the first time I saw a handheld L.E.D. calculator, it was around the Fall of 1973 when I was 12 years old and we (my parents and I) were over at a friends house for dinner. After the meal had ended my father and his friend went out to his workshop to look at a project that he was working on, so me being my fathers shadow tagged along. The project was a beautiful redwood water wheel/millhouse replica and the gentleman was having some difficulty calculating the proper spacing for the chambers that catch the water.

He had purchased especially for this project a Texas Instruments SR 10. At 150.00 it was a fairly extravagant purchase at the time but it helped him make the necessary calculations to finish his job. Needless to say when he let me fool around with it I was HOOKED!! The sleek case, glowing display and clicky keys held me in a sense of wonder for the remainder of the evening.

The feeling I had of being able to press a key and have the number appear on the display and then manipulate that number with other numbers is still, to this day, indescribable. I swore to myself that night that I was going to save every penny I could to own one and impress my Junior High friends and ACE every math test old Mr. Hoover could throw at me!!!

Well, it took me almost a year but I had scraped together enough money to finally purchase an SR 10, but by that time the price had dropped and other more elaborate and expensive models had begun to appear. I was torn, High School was around the corner and I thought I would need more power! (Ooh Ooh Ooh Ooh!!!) So instead of spending all the money I had saved I purchased a Unitrex Mini-Handy 80R which I summarily destroyed about one month later! (A stupid accident involving a shirt pocket, running and a sidewalk!)

Anyway, I was unable to get my next calculator fix, as it were, until the middle of my freshman year. Prior to this I had been exposed to the injustice of watching many upperclassmen walk up and down the hallways of my school with padded vinyl or even (shiver..... Hewlett-Packard) genuine leather cases strapped to their belts. Oh yes, I KNEW what each of them contained by name and model number, I KNEW the keyboard layout of each one, I KNEW I had to have one just to be part of the NERD crowd, I KNEW I would have to throw about1.0000000 10 newspapers to afford one!!!

Just to get a case of some type strapped to MY belt I knuckled under and bought a Corvus 411. I avoided showing off this calculator as it only had the basic slide-rule functions. What can I say? I was impatient and undisciplined!!

While I tolerated owning this (in my then humble teenage opinion) substandard calculator, I began a letter writing campaign to Texas Instruments and Hewlett-Packard requesting product information and pricing, thinking I would get a better deal by going straight to the source. Hardee har har!!! Little did my capitalistically naive mind understand the true meaning of free enterprise. Manufacturers Suggested Retail Price became a four-letter word to me!! But I did receive a lot of nice pictures of high-end calculators!! I wish I had all of the literature and advertising that I had amassed then today.

Things went on like this for months, I had my eye on a TI SR 51A but the cost was still too high so I bought gimmick calculators in the meantime because I was unable to manage my money and stay focused! A Radio Shack version of the Casio CQ- 1 comes to my mind, I took this one apart after a couple of months just too see what was broke. I then traded something, I don't remember what, for a 3 year old Commodore, (model unknown) I took this one apart broke. I quickly learned what THE WHOLE IS GREATER THAN THE SUM OF ITS PARTS meant!! I even overloaded the chips and L.E.D.s just to see what would happen!! I was SICK!! ( Although looking back, I gained a lot of knowledge that I use today in my restoration work.)

By my Junior year I had grown up somewhat and was earning more money, the SR 51A was mine!!! I loved this calculator!! I used it in all my classes, even Government! I sold this unit during my Senior year and purchased a TI 58. This was one slick machine, I recall programming it to spell out messages if you held the calculator upside-down!

All during this period of my life I still yearned for a Hewlett-Packard, they were to me then(and still are) the ultimate in technology and durability. Their quality and appearance were awe inspiring. I never went into a department store without stopping by the calculator counter to play with these machines. Only the more upscale stores carried the HP line and often they were displayed in velvet lined glass cases which seemed to add to their mystique.

It has taken me 20 plus years, but I am finally a Hewlett-Packard owner, I have 12 models in my growing collection of calculators and have the goal of attaining all HP handhelds from Classic to Present day. A pretty lofty goal, yet one that is attainable given enough time!

Having become a semi-rational adult I gave up the calculator crazies and got on with my life, but always in the back of my mind, the desire to own an obscene number of these marvels has haunted me! When I found out that there was an organization devoted to collecting calculators, the fuse was lit, and so I began my quest!


Restoration Tips

Having no experience collecting much of anything except dust, I was uncertain about how much I should invest and what condition I should look for when buying an old calculator. I learned quickly that spending more than $5.00 was almost unheard of and that condition is important, but not so important that you should pass up something just because it has a few flaws, even those that may seem major. After encountering many specimens that run the gamut from new to used/abused/landfill, I have combined my other hobby of scale modeling with calculator collecting to aid me in restoring the treasures I come across during my hunting and trading.

More often than not, a thorough cleaning may be all that is needed to make your latest find a presentable display piece. I have seen several of my specimens that appeared to be damaged far worse than they actually were just because they were covered with dirt and grime. Remember, calculators are operated by hands and you have no idea where those hands might have been! For surface cleaning I recommend Formula 409, Fantastik or a weak ammonia solution. NEVER use solvents of any kind for general cleaning as most calculator cases were manufactured from styrene, an easily molded, soft plastic that is readily attacked by petroleum based chemicals. Solvents do have their place in my restoration practices though. (More on this later).

One product that is safe to use to remove adhesive and other sticky substances is GOO-GONE, available most everywhere. This product is oil-based and should not harm most plastic cases, however, be cautious around screen-printed lettering as some fading can occur. Use this sparingly and only in those areas where the adhesive is stubborn.

When you have thoroughly wiped down your calculator and removed adhesive, etc. inspect the seams, grooves and other recessed areas where dirt and gunk have built up. To clean these areas, a Q-tip soaked in cleaner can be used. I like to take this procedure one step further by using a TUPPERWARE orange-peeling tool, this is a yellow-handled thingy that looks like a toothbrush without bristles and is flat like a screwdriver on one end. It is great for scraping out the seams and grooves without causing damage to the surrounding plastic. When things are really gunked up I disassemble the calculator and wash the case halves in a mild soap solution. I know disassembly is not for everyone but It is not that difficult if done carefully in a quiet well-lit area. Most calculators are held together with two to four screws and come apart easily. There are exceptions to this rule as some are press-fitted with locking tabs. For this I use a CASE-CRACKER that I purchased from my local electronics repair shop. I think you can also find them in electronics supply catalogs. I NEVER use a screwdriver or any other flat metal tool to pry the cases open, these tend to inflict more damage to the plastic.

Earlier I made mention of my use of solvents in the restoration and repair of damaged plastic. The solvent of choice for this procedure is Methyl Ethyl Ketone, or M.E.K. for short. This stuff eats styrene like acid and can cause extreme damage if not applied properly. M.E.K. is a component of most lacquer thinners and is highly volatile. It can be purchased at most larger home centers and hardware stores such as Home Depot. DO NOT use lacquer thinner or acetone as a substitute, these leave residue and evaporate much too slowly.

Once I have done my basic clean-up I then inspect the calculator for structural defects such as: case cracks, scratches, gouges, engraved ID (identification) numbers, etc. all of which can be removed or lessened to a great degree by using some of the following techniques:

For repairing cracked and broken cases use a SMALL drop of M.E.K. along the inside of the of the cracked area, it will wick along the crack and melt the plastic and virtually weld it back together.( I have purchased an assortment of small caliber hypodermic needles for this purpose. They can be found at veterinary supply stores.) You can also use a delicate paintbrush dipped in the solvent If some warping has occurred because of the damage, causing the crack not to stay closed during this procedure, use tape or a small padded clamp to press the sections together. Let this repair sit for 30-60 minutes to allow the plastic to re-harden. Any softened plastic that may ooze out of the crack onto the case exterior can be cut away with a SHARP X-Acto knife. ( See #2 for blending the repaired area). You can also use this method when replacing rechargeable batteries in those sealed battery holders that some calculator manufacturers used. ( I will cover this later). This type of repair has worked extremely well for me on several jobs, even those where structural components have broken. The mounting screw holes and standoffs for my PC 100C print cradle is a case in point. These were completely sheared off of the bottom and top of the chassis when I received it. They are now stronger than the surrounding plastic!

These problems are also easily remedied ( If not too severe) with M.E.K. Most case exteriors are of a pebble-type finish and can be reasonably restored to original by applying just enough M.E.K. to soften the plastic and smoothing the area with your fingertip. You may have to make several small applications of M.E.K. to the area in order to completely fill in the offending blemish. Once this step is complete you will then need to duplicate the pebbled surface, here again use the solvent to soften the area. Then using coarse-grit (about 100 to 220) sandpaper or steel wool sprayed with Sta-lube brand silicone lubricant, duplicate the finish. Do this by lightly pressing then quickly removing the chosen abrasive into the softened plastic. If you randomly alternate the placement of this pattern while the plastic is still soft, a decent finish can be obtained. I have also used my clean fingertip to do the same thing on finer-pebbled cases.

For the smooth, glossy finished type cases I use the M.E.K. to soften the plastic then smooth with my fingertip. After this I let the plastic dry overnight then I use varying grits of wet/dry sandpaper WET to smooth out the finish.

Use 400 grit paper first to remove the heavier defects then switch to 600 grit paper to begin the polishing. Once you have obtained a smooth yet matte finish switch to an abrasive liquid kitchen cleanser like SOFT SCRUB or BON AMI, apply sparingly to a soft slightly damp cotton cloth and rub gently in a circular motion. You will begin to notice a change in the appearance of the plastic as it gets smoother. After this step then switch to COLGATE brand toothpaste (the cheap white kind) applied in the same manner, this time buffing rapidly. Once the finish is near original then use a small amount of silicone type car wax. AUTOFOM is my personal favorite, FINISH 2000 is also a good choice. Remember, use only small amounts of these abrasives, the plastic is soft!

I have successfully filled them in using a mixture of M.E.K. and shavings scraped from and inconspicuous location inside the case or from a sacrificial parts calculator. ( Texas Instruments is the easiest to do as most of their cases are basic black.) Another source of filler material is the trees or spurs that model-kit parts are attached to, but color-matching can be difficult. Use just a small amount of shavings mixed with an even smaller amount of solvent, you only want enough to create a putty-like consistency not liquid. It is difficult to give exact quantities due to the fact that case quality, i.e. density of the plastic, varies greatly from one manufacturer to another. Hewlett-Packard and Texas Instruments tend to be of a harder styrene than say a Casio or Summit. You just have to experiment.

Scratched displays can be restored using the above techniques for smooth finish cases. Only begin with abrasive cleanser not sandpaper! For deeply scratched lenses that cannot be polished out completely, get them to look the best you can, then use FUTURE acrylic floor wax to fill in the scratches. When allowed to flow over the surface of the lens this will work wonders! A small 1/4 inch wide, fine-bristled model paint brush works well for flowing the wax over the surface of the lens, keep brushing to a minimum. Practice on a piece of clear plastic to perfect your technique!

A better solution than gold and silver model paint is to purchase a gold and silver-leafing kit from an art supply or craft store such as Aaron Brothers or Michael's. The results you obtain will be much brighter than paint but it will not be as bright as the original vacuum plating used on some calculators. I use the leafing kits according to their instructions and follow up by burnishing the trim with nylon pantyhose! Another alternative to painting is BARE METAL FOIL which can be purchased at most well-stocked hobby stores that cater to model-makers. This an adhesive-backed, extremely thin real metal sheet that must be cut to the exact width of the area you wish to cover, this tends to make it less desirable when the trim is narrow as in most calculators. Again experiment to see what works for you.

Most faceplates are made of painted or brush finished aluminum. In the case of painted faceplates I have on hand an assortment of high quality model paints that I blend to match the original color as near as possible. The brands I recommend are TESTORS MODEL-MASTER, POLY or FLOQUIL. These are also found at well-stocked hobby stores. My technique here is just simple touch-up, most faceplates are screen printed with numbers, functions, etc. that are nearly impossible to replicate using simple methods. I have a fairly steady hand and good eyesight and am able to hand letter some markings, but I limit myself to only the most simple of these. For wear areas around ON/OFF and other selector switches that are black I use a MAGIC MARKER to cover the scuffs, for other colors I start mixing paint! The secret here is to use the paint to fill the scratch, keeping as much as you can off of the surrounding paint. I have paintbrushes that are used for fine/superfine detailing, they are sized from 0 to 0000 and work well for this. Again, an art supply or hobby shop should carry these. For metallic faceplates a SCOTCH-BRITE pad or wet/dry sandpaper of appropriate grit size used WET will do the trick, just rub with the grain of the original polish and do not do too much! Most metal faceplates have a clear lacquer coating on them to preserve the finish, so once you have removed the blemish you will need to apply FUTURE floor wax to replace this.

Depending on what your particular find requires in the way of refinish work you may or may not need to apply all of the aforementioned techniques. When I have reached the desired condition for a calculator I then apply a nice coating of vinyl and plastic protectant like ARMOR-ALL, this does a nice job of giving a nearly new appearance to the entire calculator, try not use too much as this gives an oily, unnatural finish, spray a clean cloth with a moderate amount of protectant and buff the outside of the case, especially around and between the keys. You may also wish to use your TUPPERWARE orange-peeling tool with one layer of the cloth over the blade end to get the protectant into those narrow, hard to reach areas. There you have it, a clean good-looking display piece!

For calculators using rechargeable batteries in a sealed holder I use an X-ACTO chisel blade along the parting lines of the pack to separate the sections. Most of these were welded ultrasonically at the factory and are essentially one piece but can be opened if you use care, the example I will give is for the pack from a Hewlett-Packard HP25. This pack holds two AA size ni-cad batteries that can be purchased from RADIO SHACK for about 6.00. With the pack out of the calculator, lay it on a flat, sturdy surface with the visible portion of the batteries facing up. Place the X-ACTO knife (chisel blade) across the seam where the center divider meets the exposed contact end of the pack and tap it sharply and quickly with a small hammer or other weighted object, if done correctly it should separate with only one blow. Do not bang on it, RAP it! Gently lift up the free end of the divider and wiggle the batteries out of the housing. You may need to use your TUPPERWARE orange-peeling tool to aid in prying them out. Note: be gentle with the end of the divider that is still attached to the pack, you should be able to do this without breaking it. Replace the old batteries with the new ones, paying close attention to their polarity and press the divider back in place (It will probably stick up some). Next use a small C-CLAMP lengthwise across the pack while pressing down on the divider, the pressure of the clamp should hold it in place. Apply one drop of M.E.K. to the seam and allow it to sit overnight. I have also used a similar technique on an HP 41CV.

Other calculators may have their batteries sealed directly in the chassis, in this situation opening the case and replacing them is usually all that is necessary. At times your calculator still may not operate with fresh power, it has been my experience that most electrical problems are due to broken or corroded wires. The older ni-cads get, the more susceptible they become to leakage and this in turn can damage the internal components of the calculator. (Be careful when dealing with leaking batteries as they do contain acid and heavy metals, dispose of them as you would any other hazardous waste). Cleaning the inside of a calculator that has suffered leakage is as simple as washing with a solution of baking soda and water (About 2 tablespoons to 8 ounces of water). Do not immerse the affected electronics in this solution, just generously moisten a lint free cloth with it and wipe/blot the surface of the component, then wipe/blot with a dry cloth. Let the part sit for a time to allow extra drying. Inspect the battery connector wires for breaks and repair as required. I use a 25 watt fine point soldering iron and light gauge solder, be careful with the hot iron! Keep it in contact with the connection only long enough to flow the solder into the joint, heat destroys electronics quickly!

If you are not familiar with soldering, practice on some 24 gauge braided wire, not your prized Commodore! Most replacement batteries can be found at RADIO SHACK or other electronics supply houses, and even come pre-wired in two, three or four packs of AA size. Another source for odd type batteries is THE BATTERY STORE at I found the replacements for my HP41CV there. Great place!

Well I hope my modest experience gives you some ideas to assist you in restoring these past marvels of modern science, I realize that I have not covered all the possible refinishing and repairing problems you may encounter but these are as varied in number as the amount of calculators there are to collect and I can only write so much! Remember, start out with your restoration and repair work slowly, begin by using a calculator that you can afford to experiment on. I have the advantage of being a fine-scale modeler for the past 20 years and have used these techniques while assembling 1/48th scale aircraft kits with the occasional car kit thrown in. Just practice until you gain confidence enough to tackle that HP65 with an engraved ID number and hazy display window!


Vintage Calculators

Text & photographs copyright, except where stated otherwise, © Nigel Tout 2000-2024.