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HP-21 Battery Charging Problems



#11

My dad found his old HP-21 and gave it to me! Nice calculator, only I find the rechargable batteries very annoying. When I charge them, sometimes they'll keep a charge and other times they don't accept any power at all! I've checked the contacts, the charger gets warm, the calculator stays cold. Any suggestions? Any place to buy batteries?
One last thing...being new to this calculator and not having a manual, how the heck do you properly use the memory recall arithmatic? Thanks.


#12

Jonathan,

If the calculator stays cold, there's a bad contact somewhere. There are two likely possibilities. Perhaps the charger cable is broken near the wall wart or the connector. More likely in my experience is a bad contact between the battery terminals and the connectors inside the battery bay. This problem is troublesome (those tin-plated contacts tend to oxidize, hence the problem) and it's also dangerous, because if you ever turn on the calculator while the charger is hooked up and the battery is not connected properly (and cannot perform its power regulation function) you can destroy the calculator in a matter of seconds.


Viktor

#13

Viktor is likely on to the solution as the battery pack springs weaken with age as well. Go to Radio Shack and buy a small package of the flat "matchlight" solder. taking two strips, fold each in half individually, and slide in front of the battery contact in the pack just past the plastic housing so it will stay in. Reinsert pack and test. Try two strips if needed but can make the pack hard to get out.

#14

Do not use the AC adapter if the batteries are not in good condition or with good contacts. You may ruin your nice machine in a few (milli)seconds. See article by Katie Wassermann about rebuilding batter packs, at the Articles Forum here at the MoHPC.


#15

take all that good advice seriously. i didn't know and killed a 22 a few years ago; a calculator that could have lived another 20 years. i still feel dumb.

#16

I'd even charge the battery pack externally to avoid any damage. I always have a risky feeling, when I'm charging such batteries inside a 20 series calc.

Besides that, the charging circuit seems to be not much more than a diode and a resistor (can anybody confirm that ?) which I consider a very bad design practice, even when it was state of the art in the 70s.

The batteries could live much longer when you don't "grill" them using that charging circuit.

Using the battery pack to smoothen the rectified power is a crime from the view of the electronics engineer, especially when the contacts simply can't hold forever. Most HP-2x users surely didn't ever know, what killed their calculator...


#17

> Besides that, the charging circuit seems to be not much
> more than a diode and a resistor (can anybody confirm that ?)

I certainly can. I was inside a couple of these last night and I confirm that

#18

The half-wave rectifier (one diode) produced a pulsating current that was supposed to help charging the batteries in a shorter time than a steady current does. I remember I read this at the time, I cannot quote the source.

In "defense" of HP 2x, keep in mind that simple ICs as voltage regulators or charging controllers were not available at the time. And I admit I have a soft spot for designs a mere mortal can understand. But some more protection, like a zener diode should be there, indeed.


#19

I wonder if there's something to be said for retrofitting
with a zener?

I have an HP-29C which is one of my favourite machines.
Fortunately I had the foresight at the time to get the
external reserve power pack charging unit (82028A). So
that, along with a spare battery means I never have to plug
it in directly. Nevertheless, it would be nice to be able
to plug it in ... (without being worried sick) <g>

And many people, probably most, don't have the external
unit. Maybe there's room for a project to build some for
those people that need them. (They're pretty simple. I
haven't openned mine up, but I suspect that the most
difficult part would be to fashion a connector which worked.)

Cheers!

#20

Thanks for all of your advice and education guys. My calculator is doing something really strange now. I haven't charged it since before I posted the original message, but when I turn the calculator on, the calculator LEDs light up for about 5 seconds and then die off. The same thing happens again when I turn it on about 20 seconds later (over, and over...). Does this show something faulty in the calculator's circuitry? Something tells me this may turn out to be a good lab project for me next quarter in school (I'm taking a course in electricity & magnetism).


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