As I have pointed out in a previous post, I am a calculator collector. As I was doing research about HP Calculators in order to replace the one used during my college years, I learned of the original handheld scientific calculator. This was invented by HP in the early 70s. At first the machine didn't even have a model name, it was just the HP Handheld Calculator, but later it was labelled as the HP 35 since it had 35 keys on it.
As of this writing in early 2025 prices for the original HP 35 are rising with good working examples going for several hundred dollars on eBay. These are much more repairable than modern calculators and there is at least one detailed tutorial video showing the process to take them apart and clean them. The repairability and information online made me feel comfortable enough to purchase a non-working unit on eBay and attempt a repair myself, but things are often more complicated than initially expected.
The unit arrived and I was able to take it apart as shown in the video. There were signs that this one had already been taken apart, which was disappointing since that was not shown in the eBay listing. I should have asked for pictures of the back. Cleaning the contacts did not resolve the dead keys on this calculator as it did in the YouTube video so it turns out that this calculator had something wrong with the electronics on the main circuit board. I reflowed the solder joints and poked around on the main board a bit, it isn't very complicated, but I'm guessing one of the custom logic ICs is damaged, so this project went on hold for a few weeks.
Shortly after I got an email from one of my saved searches on eBay. It was the cheapest HP35 on eBay in. quite a while. It was a loose calculator without a Power Supply. I talked to the seller and he had no way to power it up, so it was a gamble, but he also showed the back of the unit and this one had not been opened, so I took the chance and purchased it.
The second calculator was in really rough shape. When I got it on the workbench it had battery corrosion on the inside of the battery compartment and it wouldn't power up. I started disassembly, cleaning as I went. It was obvious that corrosion had gotten inside and affected the power circuits under the battery compartment, however, the design was modular and that was not a part of the main logic board that was bad on the other calculator, so I was still hopeful.
When I swapped the older logic board onto the newer HP35 it fired right up and worked! I was very happy to have a working calculator after buying two broken ones. As soon as I got one working I checked for the 2.02 bug. The working Logic Board is from the first year, 1972, but it does not have the bug, which is OK with me since it would have looked out of place in the newer case with the H P 35 logo on the front.
I also rebuilt the battery pack with 3 NiCd AA cells and it looks quite nice running without being plugged in...
So now that I have the first ever scientific calculator, the HP 35, and my favorite calculator, the HP48G that got me through college. These are the bookends for my collection and should not really need any more calculators in the collection. For now.
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