HP21 on charger -- flashing LEDs


Hello all,

I have an HP-21 that works with its rebuilt battery pack. I recently purchased an HP 82026a charger (appropriate for the HP-21 according to this site). However, when I plug the caclulator (with batteries) into the charger and turn it on, the display flashes over most segments and the right-most digit lights up almost completely. I have measured the output from the 82026a and find it to be 10.28 Volts, which appears in line with published values. If I leave the calculator off but plugged in to the charger its right side becomes warm to the touch after 15 minutes.

Q: Does the flashing display indicate dangerous operation or some other problem, or does it just mean that the battery pack needs charging? Is this abnormal? Could I be damaging the calculator?

Any explanations would be helpful.



Flashing display indicates that too much AC is reaching the calc electronics. (The machine depends upon the nicads to limit the voltage) You say you rebuilt the pack, did the machine work afterwards? Does the machine work properly if you remove the charger? What I would do is: Clean the battery contacts on the PCB and the battery pack. Verify that the contacts on the batteries make contact with the rivets. Another favorite is that dry joints appear between the rivets and the PCB track. Let us know?


The story is more complex than I first let on. I have two HP-21 calculators, one rebuilt partially charged battery, and one charger.

There is still sufficient residual (or initial?) charge in the rebuilt battery pack to power up the calculators. Both calculators appear to calculate without issues, although I have tried to limit the use of the battery pack until I have fully charged it. In short: the calculators work fine on the batteries when the charger is not connected.

If I connect the charger to either HP21 (with battery pack in place) and turn it on, I see the flashing segments I described in the first post.

Based upon the fact that both calculators operate on the battery alone, I don't think I have dirty terminals. As I said, the AC output of the charger appears to be in line with specs (10.2 volts). Does this give us something further to go on?



Something furthur to go on - yes...I stand by my earlier observation - poor contacts (despite it working off charge) but I'll add poor state of batteries. Charge the batteries for 10 hours or so how long does the machine operate? (What voltage do the batteries read when theyve been used for 5 minutes?) Two other possibilities: The diode on the charging circuit is marginal and the Charge limit resistor is the wrong value. (Should be Grey Red Gold Gold; for long term use on charge I would change this to double or triple the value). As Viktor observes the side of the casing *will* get hot.


Thanks for sticking with me on this.

I have never fully charged this battery pack; when I bought the calculators they came with dead batteries and I had no charger. I had the battery pack rebuilt and upon arrival it had some unkown level of partial charge. I have since purchased the charger and was on my way to charging the batteries when I noticed the flashing segment issue. I will check the contacts but I doubt that both of my HP-21 calculators have wrong resistor values and bad diodes (I guess it's possible though).

Is it safe to assume that charging the batteries without turning on the calculator should not harm the calculator?

Do I understand correctly that you are suggesting I charge the batteries for 10 hours or so and than take a look.

I sure feel ignorant. Thanks for the help, Dave.


If you're even slightly suspicious of the nicad pack then I would measure the voltage across each of the cells when not under charge, and again when under charge.

I would also measure the voltage across the charge current limiting resistor too, and this will determine the currtent actually flowing into the pack.

Each cell should measure about the same voltage.

Sure signs of problems are:

Very small (or even reversed) voltage on one or more cells when not under charge

Unusually large voltage when under charge.

If voltages appear OK, but are uneven, it may be a good idea to discharge the cells individualy until they hit 1V, and then recharge the pack. Hopefully they will all come up to about the same voltage.

Typically only a single cell in a pack fails, but you should always replace ALL cells.

To determine the charge current, use the formula i = v/r where i is in amps, v in volts (voltage drop across the charge resistor) and R in ohms.

grey red gold gold is 8.2 ohms by my calculation, but you may wish to check it (or my memory) by directly measuring it.

at this value, with a 9V charger (under load), and 3 nicads, you'd expect a voltage drop of around 5 volts and therefore a current of 600 mA and a power dissapation of 3 watts!

This is completely absurd, so clearly my calculation of the resistor value is way off (could it be 82 ohms grey-red-black-gold?) or the loaded voltage of the charger is lower (5v?).

But the point of this is that the charge current is about 0.1C (i.e. 60mA for 600mA/hr nicads). Any significant deviation above or below this may indicate problems.

If the current is low, and the nicad pack voltage is unusually high it may indicate that one or more of the cells is not accepting a charge.

If the current is high, and you tend to leave the charger on for extended periods, then one day you'll find the previous problem will occur :-)


Dave - to summarise other correspondents replies - check the nicad cell voltages if their about even then just charge for 10 hours and review the situation. If the cells are NOT even replace both. Until the nicads are sorted little else can be done.


Thanks everyone for your help.

Both HP-21s had dirty contacts in the battery compartment. I cleaned them off and charged the battery pack last night and all appears well. The calculators now perform correctly with or without the charger connected. There are no flashing segments anymore and the batteries are charged.

Once again, thanks for all the help.



I measured the voltage on one of my Woodstock chargers, and it's 10.2V AC just as it is on yours. And since you say you have two calculators that exhibit this behavior, the problem is likely not with the calculators either.

Which leaves the battery pack. You say you had it rebuilt. Exactly what kind of batteries were used for the rebuild?



What's the voltage on the charger when it's charging the nicads?


Sorry, but I don't know -- I haven't opened up the charger while it's charging. I'll check tonight and let you know.

I was somewhat surprised when I first opened up the charger -- there's nothing there but a voltage selector switch connected to a dual wound primary (for 110 and 220) transformer. For some reason I expected rectification and filtering circuitry to get a nice smoothed DC output, but I was wrong.




I had Batteries Plus rebuild the pack so I don't know exactly what cells they used. However, I found that the contacts in the battery compartment of both calculators were dirty. Once they were cleaned I was able to charge the battery pack and both calculators work on and off the charger.

Thanks for the help,




Yes, dirty contacts explain the problem alright, although it's quite unusual to have the same symptoms on two calculators at the same time.

In response to your question in another message, the charger is just an AC transformer; the rectifier is in the calculator itself, and as for smoothing the current, that's done by the battery pack. This is why it's important never to operate the calculator without a properly connected battery pack. AC charges are used by many vintage calculators, by the way.

Incidentally, many use a large capacitor (perhaps in parallel with a small value resistor of suitable wattage) as a battery eliminator, so that they can safely operate their calculators with no NiCd batteries inside.



Since a capacitor will smooth the pulsating voltage, but it will not be regulated, the voltage received by the calculator will be higher than normal, and may damage the calculator. A 3.2 or 3.4 Volt, DC **regulated** supply, applied to the AC adapter pins may work. If the polarity is wrong, the series diode will protect the calculator, but it will not work (so swap the terminals!). Again, please take this only as a suggestion and be careful, no warranties !!



You're absolutely right about that. One "solution" that I mentioned in my message is the use of a resistor in parallel with the capacitor, to "simulate" the extra load that the batteries represent. However, even this could be risky (besides, who wants to experiment with a vintage HP calculator?) so indeed, a regulated power supply may be the safest solution.

Speaking of overvoltage... does anybody know which (if any) HP calculators can be damaged by the use of alkalines in place of NiCds, and what the symptoms are of such damage?




I have used my HP 25 (now stolen) with two alkalines, without problems, it may be the safest way to go without nicads. I think battery life will be about 6-8 hours of use... WARNING: NO AC ADAPTER ALLOWED WHEN ALKALINES ARE CONNECTED!!



Thanks, that's good to know. I'm working on an old Woodstock calculator that I am hoping to repair; however, I was concerned that my attempts to run it from two alkalines may have made a bad situation worse.

Yes, I am aware that attempting to "charge" alkalines can have spectacular, but rather unpleasant, results...




Warmness to the touch on the right side is normal; that's where the big resistor is that limits the charging current on the battery. The flashing display is definitely abnormal, and I'd recommend that you don't operate the calculator this way. Short of recommending that you keep the batteries inside (which you say you do anyway) I have no idea what might be the cause.


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