One day, I was browsing Goodwill, and something caught my eye from a bin of electronics. It was a Commodore Minuteman 3 calculator (Yep, same Commodore as the C64). With an awesome color scheme, a super cool looking retro LED bubble display, and Commodore branding, it was an ultimate goodwill find.

Of course it didn't work, but I was pretty sure it was a simple matter of getting it new batteries. So, I happily bought my find, and started delving into it once I got home.

Step 1: Evaluation

The first thing to do is figure out why it won't work. So, as I do with almost every device I get (especially the non-working ones), I opened it up. As I suspected, there was battery corrosion everywhere, and the batteries needed replacing.

At this point, you can test it, if you want. I tested mine by desoldering the power wires from the battery, and hooking them up to my Arduino Uno's +5V and GND pins acting as a power supply.

Step 2: Parts and Materials

You should have everything you need around the house already, besides the batteries.

Infinity * Cotton Swabs (Maybe not that many, but you'll need a lot)
Paper Towel
A screwdriver for taking it apart
Rubbing alcohol, AKA isopropyl alcohol
New batteries

Your calculator may use different batteries, but mine used 4 2/3 AA batteries. They're plentiful and cheap on eBay. Also, if your calculator has batteries in series- Make sure the ones you get have little tabs for soldering to. It will make constructing the new battery pack much easier.

Step 3: Cleaning It Up

Before cleaning, I took it apart as much as I could. I'd recommend doing the same, just don't remove anything glued, or otherwise somewhat permanently attached (besides the batteries).

To clean the case, I used paper towel and Cotton Swabs wetted with rubbing alcohol. I also rinsed it in water a few times, but you'll want to be careful of any labels. Another thing to note is that rubbing alcohol may do something to your plastic or make it less yellow (Which may or may not be desired). Test it carefully on a small area before cleaning everything with it.

The electronics and power jack I cleaned as best I could with some rubbing alcohol on numerous Cotton Swabs. A annoying thing about cotton swabs and electronics is how the cotton will catch on any leads poking through the board, so you could also use a rag or paper towel dampened with rubbing alcohol.

Note: Make sure to take a picture of what wires went where before desoldering the batteries- My calculator has the negative side of a capacitor hooked to the power, for some reason, and if I hadn't had a picture to check on I would have hooked it up to the ground side.

Step 4: Making the New Battery Pack

The old batteries in my calculator were arranged in series to provide 4.8V. I arranged the new batteries the same way, and arranged the tabs as shown in the pictures. Then I soldered the tabs connecting the batteries together (Make sure to get a proper solder joint!) and folded them over.

Step 5: Reassembly

Now you can put everything back together!

It might help to take pictures during the disassembly process so that you can see what went where.

Since the old batteries had multiple wires going into one tab of the battery pack, I had to widen the holes on the new tabs some to fit all the wires. I did this by twisting a tiny Phillips head screwdriver around in them. Be careful not to twist the tab too much, though!

One other thing to be aware of, if your old batteries leaked very much- The wires that connected to the batteries can get corroded and not be making any sort of electrical or solder connection. You'll need to cut the ends off and strip some more insulation off before resoldering them, if that's the case.

Step 6: Finish!

If everything goes well, at this point you should have a working, retro calculator. Now you can show it off to people, and use it!

I brought it to my Robotics club a few times and people thought it was super cool (One of the coaches said she was waiting for George Jetson to appear XD). As for charging the batteries, as there's absolutely no way to tell when it's fully charged, I just leave it on the charger I have for about a day and then it seems to last a few weeks with moderate use.