In the early days of electronic calculators, one common IC was the MM5737, which supported four functions and eight digits. Its big brother, the MM5738, was slightly more advanced, since it included a single memory, a constant function, a percent function, and a battery-saving feature that would turn off the display after about 60 seconds.
What gave early hardware hackers (long before the term was invented) something to do was the fact that many calculator manufacturers stocked only the MM5738, even though some of their calculators didn’t make use of the extra functions. The cost of the chips was about the same; they simply didn’t wire in all of the functions on the less expensive “four banger” calculators.
Someone at Popular Electronics noticed this, and figured out how to add the hidden features to the less expensive models, which was revealed in the September 1975 issue. The first step was to determine which chip was contained inside, and this could be done from the keyboard, without even opening up the case. This was because the MM5738 had the ability to peform repeated squares. From the keyboard, you simply had to enter 3, x, =, =. If the display said 81, then there was an MM5738 inside. If the display said 9, then the calculator used the more basic MM5737, and no modification was possible.
The author acknowledged that there would be no economy in trying to find a keypad with the added buttons. Instead, he proposed mounting four pushbuttons on the side of the case, to serve as the MS (memory store), MR (memory recall), K (constant), and % (percent) keys. He performed the modification on a Novus model 850.
I earlier wrote about the Novus model 650, an even more bare-bones model that lacked a decimal point. I suspect it used the same chip, and a similar procedure could have added it. The 650 had a retail price of $19.95 when it came out in 1974, but was down to $8.88 by December 1975.
Armed with the information in this article, owners of one of these very basic calculators could beat the system and save a couple of dollars by upgrading to a more advanced model by themselves.