Disclaimer - following are some tips that I've put together based on my experiences with troubleshooting some early-model calculators. Please use common sense when trying to clean, modify, open up, or otherwise repair an old calculator. The author assumes no responsibility for the accuracy or applicability of the following information, use this information at your own risk. Sorry, the lawyer made me say this.
So you've just picked up an early '70s calculator and you anxiously put batteries in it, turn on the power switch, and nothing happens. AAARRRGGGH!!! Don't give up yet on the machine, however, there are some simple things that you can try to possibly bring it back to life, without having to enlist an electronics expert.
Perhaps the most unfriendly element to calculators that have been sitting unused for a while is the oxygen in the atmosphere. Oxygen attacks the metal parts in the machine and can form an insulating oxide layer on contact points that prevent proper operation. In normal operation, this oxide usually gets cleared by use of the machine, but it builds up over time and can be a problem with a machine that hasn't been used for several years. Leaky batteries may also cause corrosion on the battery terminals, therefore, the first place to start your debug process is with the battery terminals on replaceable-battery machines. If there is visible corrosion it should be removed. Even if there is no visible corrosion the battery terminals should be thoroughly cleaned to remove any oxide layer that may have formed. I usually use a mild abrasive such as a pencil eraser or emery board, or gently scrape the contact points with a small knife blade or nail file. This is something that you need to use your own best judgment for, taking care to make sure the battery terminals make good contact with the battery but don't remove more metal than necessary, and don't risk ruining the calculator permanently.
Once this is done, put fresh batteries in the battery compartment - you also want to make sure the batteries themselves are clean. Some machines are finickey about how the batteries are inserted, particularly ones with 'spiral' contact springs - make sure all the batteries are making proper contact with the terminals on both sides. Sometimes the the terminals lose their springiness and may need to be pried outward with a small screwdriver or pliers to make good contact with the batteries. Now try turning the machine on. If the machine doesn't work, re-check the orientation of the batteries, the condition of the terminals, and the proper contact between the batteries, and try again. I've had several machines that were so particular about the batteries that after simply taking the batteries out and putting them back the machine started to work.
If the machine still doesn't show signs of life, the next area to try debugging is the ON/OFF switch. The contact points in the switch are also subject to oxidation. Try moving the switch very slowly from OFF to ON and back, looking for signs that the machine is trying to turn on. If there's still no response try putting slight pressure into the switch while moving it back and forth, to help try to wipe away some of the oxide. With machines that have a shaft on the switch that is long enough to hold with your fingertip, you may also want to try slightly pulling on the switch while moving it back and forth. You may find that the machine starts working with the switch part way between OFF and ON but not when all the way to ON - you may have to live with this but at least the machine works!
Another area of weakness exists on some machines that have a jack for external power. In some of these jacks there is a spring contact that connects the battery when no external power supply is plugged in. This is generally the case only with DC external supplies used with replaceable-battery machines and not with AC transformers used on many rechargeable-battery machines. This jack can fail due to corrosion or due to the contact losing its springiness. Unfortunately the calculator has to be opened up to try to fix a problem with the jack, which you may or may not want to do.
If you decide to try to fix this, open the calculator carefully and examine the part of the jack on the inside of the machine. You want to look carefully at the jack to see if it has three wires going to terminals on the jack and a springy metal contact that is supposed to short two of the terminals together when the external power plug is not inserted.
If you see this arrangement, you want to check that the shorting spring is making good contact. If not, then try to restore the springiness with an appropriate tool. Also, make sure that the contact points are free of oxide. I've done this by gently scraping the contact points with a small knife blade or a nail file.
Another thing to look for while you have the machine open is to see if it uses connectors between two or more parts of the electronics - oxide can develop on the connectors and cause the machine to appear dead or erratic. Sometimes the connectors may be near the battery compartment and corrosive materials from leaky batteries can cause problems with those connectors - I've seen this on several early Rockwell machines. If you're adventurous you may want to pull the connectorized boards apart, gently clean the connector pins, and put the parts back together - but if you do so be sure to take precautions against static electricity, otherwise the electronics could be damaged.
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