One of our visitors has put the finger in the wound - the outrageous prices that are now being paid in popular auction sites for non-working equipment. And it is not that you buy it and doesn’t work, no; it is already advertised as “defective”, “defekt”, “non-working”, “for parts”.
It is just that hp calculators of yore were so well built, that many people is taking a chance at buying defective equipment, to see whether they can repair it and get a fantastic calculator at good prices. But more often than not, you wind up as you started: with a defective calculator.
Let’s take, for example, the hp41c. I only buy full-nut hp41c’s, since they are the only model that can be converted into hp41cl. I prefer to buy working models, but sometimes there are some with an excellent body, but that just don’t work. And sometimes I take a chance. I have to say that there are in my parts box four “chances” taken that proved complete failures - so it is an avenue that I am very hesitant to recommend, and “no way” if it has to be a one off.
The most typical failure is a broken lower post. This calculator has two circuits: the one housing the keyboard, and another for the main circuit (this is the reason it is called “full-nut”, as opposed to the newer machines, that were made in a single, more integrated circuit). E main circuit connects to the keyboard and body through a connector, but connection is secured through pressure. This pressure is made by the two halves of the calculator pushing together; but for this to work, both screw posts need to be in good shape - and this is one of the weak points of the calculator. Again, that lower screw connection broken can come from two sides: the part where the screw “screws in”, and the part on the other calculator half where the screw head rests. I have seen both failure models but the first is by far the most frequent.
In some cases, it is enough to use a slightly longer screw to fix it - the original hp-provided screws were notoriously short. If it is the “head rest” that fails, it is a worse repair - whatever you glue here will have to withstand force for the whole life of the calculator - not a good prospect. (Somewhere else we have discussed glues too, and we’ll have to come back to that)
As frequent as the broken post failure is the corroded battery bay connector. This was a fatality unless you found a working replacement, but most of the times a non-working calculator will have corroded contacts, so you’re left with two non-working calculators!
Diego Diaz designed a flexible circuit to be applied to the battery and port module. He was successful in replicating the original, but care must be put in carefully bending it prior to application, sanding the old rivet heads, and cleaning thoroughly the assembly before adhering the circuit. Failure to do so will result in a working calculator for now, but the circuit will peel off in the medium term. You need to follow Diego’s manual to the T, if you want to succeed with it. You also need to be careful wi the tools you use to adhere the inter-port part - too much pressure with a sharp utensil may result in broken circuit paths, which will cause module bays 3 or 4 to malfunction. Happened to me once!
There are 2 authorities in the HP repair field that I turn to in case of doubt: Nacho Sánchez Reig in Spain, and Geoff White in HPmuseum.org. I can read the advice from the latter, but I can talk with the former. He has as formidable collection of hp calculators, and is always willing to get more. I feel envy, but I have a business to manage, so I need to forgive and forget. Smiley. If I have any model that is not a hp41, I send it to him - and he’s never failed us. If you need some repairs, please ask me for his address at email@example.com
As final advices for those who feel that they can buy a non-working calculator in an auction site and succeed.
-Hedge your bets: if the ad reads “for parts”, make sure it has the parts: all 4 port covers, the charger cover, the case, the battery contacts more or less clean…
-A defective calculator coming from a reputable calculator vendor is very bad sign. He most surely has tried all the tricks in the book to repair it and failed; why would you succeed?
-A module in one of the bays is a very good sign: the person has not made the effort to separate them to make more money, so he probably has not tried to repair it. Some modules command high prices - you know which ones. And modules seldom, if ever, break down.