Has anyone retrofitted their older calcs?


Hello all,

As I admire keeping an older 35, 45, 65, Series E, HP-67, 21, etc. in its pristine condition, has anyone retrofitted their calc insofar as to re-engineering the battery to accept store-bought AAs or some other easily accessible battery size? yes, yes, I know it hampers up the vintage/pristine condition of the unit but, replacing the original batteries doesn't seem possible as no one makes spares for HP's 35+ year old masterpieces.


I modified my HP-25 to run on alkaline batteries but it was not a pretty mod on the inside. I ended up soldering small copper plates to the battery terminals so the positive alkaline battery post would have a nice contact surface. I also cut the battery box spring in the middle and inserted a silicon diode to reduce the 3 volt battery voltage to about 2.4 volts. The modification still works 25 years later although the battery box is a tight fit.


Although I do have a similar retrofitted 25 I bought here some years ago (at least 10), ironically, the battery door is still practically impossible to open. Although I did manage to press down, push forward and get the door open once and fit the plastic 'I' leg (don't know what to call it) back in place after the battery switch, that's the only time I ever changed the batteries. I can't seem to get the door open anymore. But, at least someone else may be able to help.


When you push the battery door down actually you are moving the batteries against the springs in the back. If the batteries are not freely moving back and forth inside the battery box, it will make it very difficult to open.


So, what do you recommend I do?


My recommendation is, if you can remove the battery pack, make sure batteries are freely moving back and forth before placing it back into the calculator; if you are unable to remove the battery pack, I would open up the calculator very carefully which then the terminals pushing against the batteries should relieve the pressure allowing you to remove the battery pack.


Only modified the battery pack with modern Flat-Top NiCd batteries which I can remove and recharge on a wall charger then reinsert into the calculator. On 33E, attached a thin double sticky foam with a piece of copper tape inside the battery compartment across from the battery terminals to allow series contact of the batteries with each other. There is not enough room for botton top batteries.


Actually, several people make replacement battery packs for the old HP calculators. Go to eBay and search for "HP calculator battery". Any of these are much better solution than anything you'd come up with on your own.


On the Woodstocks, you can use a razor blade to cut out the little plastic piece that runs lengthwise down the middle of the two batteries in the stock battery pack. Then you can just pop out the old ni-cad batteries, stick in some AAs, and you're good to go without any further modification (battery door should still open as designed). I haven't had any problems running my 25C at 3V, without a diode to reduce the voltage.


On a 33E with corroded terminals, I chose the "easy way" - I just removed the battery pack, and put in its place a holder for two AAA batteries. There's plenty of space inside for it, because the oroginal pack uses two AA cells. I then just soldered the terminals from the holder to the PCB, and that's it! :) It looks perfect from the outside, and inside the battery compartment, you can simply find the battery holder, and the batteries are easily accessible...



IMHO also the best method for a calculator which you're only using occasionally for fun. Which I believe is the vast majority of LED calculators out there. Most are properties of some collector who want to use it occasionally.

While it's intriguing to replace the cells with some low self discharge NiMH to get it as near original condition as is possible and granted, it will work perfect without any further modification and their capacity and charge retention would have made people back then green of envy it's not the best choice for such a light usage pattern. Suddenly you'll forget about the cells the calculator will be stored for a few years and next time you remember it and want to take a look at it has suddenly suffered same fate as the a vast part of the cheap calculators out there sold by non technical people - leaked cells.

For calculators with standard power jacks like TI SR-50 or random 4 bangers with rechargeable pack I just empty the pack for cells. When powering them it can easily be done via the charge jack but remember that the original charger is most likely a high internal resistance one ("constant" current with high open circuit voltage) relying on the battery pack to do the voltage regulation. So when powering them all you have do to is supplying them via a regulated universal wall plug or similar.
Or as I do in the SR-50/51 case use an external 4 AA (4 as it contains an internal diode) holder with NiMH as supply. Have thought about possibilities on how use the original pack shell a base for 3 AAA cells instead. Yeah even AAAs are better these days than AA back then ;) But the big trouble is that I want to preserve the way the calc makes connection to the pack.
Similar with a SR-51-II of the older revision where the step up converter is in the calculator itself rather in the pack. There a holder for 2 AAA will fit nicely where the original AA flattops once where - with a cutout for easy changing.


I use a 3 x AAA plastic battery holder and glue/melt to the bottom two strips of cooper that serve as the battery pack contacts. Then just add a some strps of flooring protector, stick-on, carpet-like cushion material and you have a replaceable battery pack that can be filled with alkaline (or Energizer lithium) primary cells. I came up with this idea year ago when I got tired of recharging the battery packs for a collection of mostly unused machines.


Katie's invention has worked great for me, too. I used self-adhesive copper tape (from a guitar supply outfit) for the contacts.


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