LogicDart and related maintenance questions



The LogicDart, as mentioned here and in another thread, was designed by Tony Duell with the HP41 in mind, right? Tony, will it be also useful for other applications, basically Hewlett-Packard calculators maintenance? I have three HP calculators still in the medical bay (1 HP15C that no longer switches to ON, 1 HP55 with bad contact in both ON/OFF and PRGM/TIMER/RUN sliding keys and recently an HP11C with a cracked LCD) and I'd like to know if the LogicDart can be built with commercial components. Is it possible to disclose this sort of information? If not, it's completely understandable. I'd just like to know.

Two questions: a few months ago, one of our contributors mentioned a chemical product that would help removing the aluminum bezels (it would dissolve the glue) present in some calculators. Does anybody remember which product was that?

The other question is about the lubricating grease used in the sliding keys of the old calculators. It seems the grease used in these keys has, at least, three major functions: extend metal-base parts by reducing rubbing action, protecting against chemical reactions and improving electric conductivity. Which grease is that? I believe the major cause of the bad contact in the HP55 sliding keys is because of this: no grease at all (previous owner "cleaned that dirt paste").

Thank you for any help.


The substance that causes adhesives to temporarily loose their stick is the solvent heptane. It is sold in small 1 and 4 oz bottles under the brand name "UnDu" for rather high prices ($5 for the 1 oz bottle, $10 for the 4 oz) at a lot of hardware stores, etc. I have seen some rubber cement thinners that were heptane (others were hexane) for around 5 dollars a pint. I don't think they contain any other solvents (none were listed on the label).

I would attempt the bezel removal by wetting a string thread with the solvent and using it as a "cheese slicer". Perhaps get a drop or two under the edge of the bezel. Try to keep it off the keyboard background as it can leave "water marks". It does not affect the case material. It also evaporates very rapidly.

As far as switch lubes, none are conductive. I use Permatex Suberlube (contains teflon). Radio Shack sells a tube of stuff called "lube gel" or mayby "switch gel" that is surprisingly good stuff.

Your HP55 switch problems will be cured by cleaning the PCB contacts under the switches and the switch conatcts themselves with alcohol. Lube the PC board pads. Also, retension the switch contacts by slightly bending them out (and make sure that they are ate the same level).


Hello, David;

thank you for the valuable info. Was it you the one who posted about the solvent heptane before? Your tips about how to apply the heptane are also much welcome.

Thank you for the HP55 guidance, too. If you don't mind, I have two more questions: "who" produces the mentioned lubricant (Permatex Suberlube) and in which state it comes: fluid or as a paste? (is this the correct way to ask about this?)

Thank you very much.


Yes, I was probably the one who mentioned heptane before. I have used it quite a bit. Works really well for removing the feet of HP41's. Place a drop on a small Swiss Army Knife blade, slide it under the foot (try to get under the double sided adhesive, not between the foot and the adhesive). Also works great lifting the corners of classic HP machine labels. I do the same as with the HP41 feet, but use a razor blade. They are thinner than the knife blade, but be very careful not to cut the label though.

Superlube is a teflon based grease mad by the company "Permatex". It can be found just about anywhere in the US (hardware stores, auto parts stores, etc). It is a translucent white substance. I think just about any teflon grease would work fine. Also the Radio Shack Lube Gel should be available all over the world.

Another recomended calculator chemical is white gas (cigarette lighter fluid, camping stove gas). It removes adhesive residue, grime, etc from cases and display windows with ease. Don't get either the heptane or white gas on the keyboard backgrounds though. They can leave "water marks".


Forgot to mention... white gas is also the best thing I have found for cleaning those rubber feet on HP41 and Woodstock machines. One swipe with a cloth and all the grunge be gone.

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