HP-35: once working, now only one dim segment on display


Greetings all!

I rescued a old HP-35 calculator not too long ago. ($1!) It worked perfectly, but I still disassembled and carefully cleaned the internals, with no problems.

Today, I got a new aftermarket 2000mAh rechargeable battery pack, to replace the original leaked and dead battery pack. I installed the battery, plugged in the HP-35, and all was well. I then turned off the calculator.

A few minutes later, I turned the calculator back on. There's no familiar "0" in the first LED; instead, there will be one lone, very dim segment displayed in the fourth LED from the left. The displayed segment changes when I turn the calculator on and off. Occasionally, it will display two very dim decimal points in the fourth and fifth LED. It doesn't matter whether I use only the battery, the power supply without the battery, or both together - the result is the same.

Any thoughts?


Bump. Anyone? Or are you all just waiting for it to come up for sale on eBay for parts ...


Maybe one of the truly expert members here will chime in. We're not ignoring you, just don't have a good idea what specifically it could be. Normally when displays stop working properly, some electronic component has gone bad in which case you won't be able to repair it unless you are able to get your hands on a donor calculator. Since you only paid $1 for it, just put it in a closet or toss it.


An idea: ask Randy Sloyer at FixThatCalc.com if he could use your 35 for spare parts. He's done good work for me, at fair prices, so I've been happy to have found some old, inoperative calcs to send him.


I know this may sound trivial, but are you sure the new battery pack has any charge in it?And did you try running the calculator on a charger?
Joel Setton


Perhaps you received a bad battery? Low voltage can cause the symptom you see. Anything is possible, I've seen both bad cells and reversed cells in after-market packs.

As the 35 will work safely from the charger alone, remove the battery and try with just the ac adapter. If no joy, check the output of the charger. The center pin is common, one side is regulated ~4.5 volts dc and the other side is a 55 ma constant current source that charges the battery that when open circuit, should measure approximately 16 volts dc. A schematic is available in the archives:

Classic charger schematic


I think I have an extra '35 charger, which I would be glad to donate to a worthy machine. (It might even have a NASA sticker on it!!)


I'm trying to get my father's old HP35 working. The power supply is broken (the transformer has become disconnected from the circuit board - all four traces are broken) and I can't find anywhere to buy a replacement. Is your offer of a charger still open?



Yeah - my offer is still open, but now I need to find my other (my original!) charger first. Let's go off-forum to continue (shaffer (at) alumni.caltech.edu ).

As I was looking, I came across a 82041A charger (for Woodstocks, I guess). Somebody has one listed on TAS for $120! Is that reasonable?


As I was looking, I came across a 82041A charger (for Woodstocks, I guess). Somebody has one listed on TAS for $120! Is that reasonable?

That's beyond absurd.


The wires are often broken on the charger near the plug. Try to move it slightly and see if it helps.

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