I have got a HP-21 in mint condition but the charger is missing.

Could anybody give me any information of the output voltage of the transformer? is it AC or DC?

I have the charger of the hp-33E. Does anybody know if it works with the hp-21 (changing the connector, of course)?

Thanks in advance




AC adapters for the HP3x (Spice) series are rated ~9VAC, .8W output and 3W input. I remember I had an original HP25 AC adapter and I compared specs and measured the output voltage (in the 80's, I did not take notes...), but I remember the specs for both adapters were compatible.

I use an original Spice AC adapter in my HP25 for more than 9 years and it works fine, charges fine. But you mentioned a charger: do you mean external charger or AC adapter?

One suggestion: if you are going to use the HP3x series AC adapter, do not cut the connector off. Instead, open the AC adapter, remone orginal cord and connector, keep it and solder another cord. If later, by any reason, you find an original Woodstock (HP2x series) that fits in the HP25, you can trade it for your "still" original Spice adapter, provided you will restore its original cord and connector. Or even sale it.



Dear Vieira:

Thank´s for your fast answer.

I´m afraid my english is not good enough so I will try to clarify my question:

I need charging the NiCd battery inside the HP-21 and I only have a HP-33E adapter 82090B(9VAC 0.9VA). I think I can connect it to the HP-21 plug just changing the connector. I have visited the HPmuseum main page and both calculators are supposed to use the same adapter, but they say both are 10VAC 1.8VA, and as I said I have the original one for the HP-33E and it is 9VAC 0.9VA...

I don´t want to destroy my "new calculator". It works great with no rechargable batteries




I think I understood you in your first post. What you wrote now makes me believe I understood you alright. Maybe I was the one who did not write well... ;-)

I use the Brazilian AC adapter that used to come with the HP3x series sold here (HP31, 32, 33, 34, 38). They all share the same AC adapter model. Mine has ID # 82119B and has a 115 - 230 Vac input select switch and delivers 9.1 Vac (I measured it right now) with a maximum of .9 Watts output power (written on it). I've been using this AC adapter in my HP25 for more than 9 years and the calculator works fine. The HP25 originally uses the same AC adapter that comes with the HP21, so I concluded the HP3x series' AC adapter would work with the HP21 as well.

About the connector: my suggestion was just to keep the original cord for future restoring.

I remember I measured the output voltage from an original HP2x AC adapter and it looked fine for me using the HP3x series adapter. Maybe I thought, at that time, that the 1 Vac diference from both adapters would not be representative for the internal HP2x (woodstock) charger circuitry.

Is it O.K. for you? My HP25 still charges the batteries (they get a bit warm when charging) and works till they are exhausted. Also, the AC adapter itself works smoothly, gets a little bit warm.

Hope it helps. Anyway, if you still feel you should have more answers (and the best policy is waiting for other posts), wait a little more or post again. I'd do that; this little babe demands caution, it's a rare, reliable and well-featured tool.

Best regards and success.


You should try to get a real Woodstock adapter. They show up on Ebay quite regularly ($10-$25). The Woodstock charge circuit is much cruder than the Spice one. The Spice adpater puts out a higher voltage than the Woodstock and could possibly damage it.


Hi there, did you read my post, i have the broken printer and you have a board. The version i have is the chromed timing wheel the same side of the strap.

thank you :)


Actually, the internal charger circuit (inside the calculator, between the plug and the NiCd battery) is the same for both the Woodstock and Spice models (it consists of a single diode and a 8.2 Ohm resistor in series).
The batteries are also electrically the same (2 AA NiCd cells in series), which leads me to suspect that the same AC adpater (charger) can be used on both series.
I have done electrical tests on a variety of HP chargers (measuring the output voltage with different load resistors), and the characteristics of the Woodstock and Spice chargers are much the same -- About 10V open-circuit, and an internal resistance of around 13 ohms.
(Incidentlaly, I made these measurements using a 3421 data logger -- the digital output of the 3421 drove some relays to switch in a resistor chain, and the 3421 measured the voltage. All controlled by one of my HP71s...)
Fianlly, I charged my Spice calculators using a modifed (changed cable) Woodstock charger for a time, before I got a 240V input Spice charger.
My tests, therefore, suggest that these 2 families of chargers are electrically interchangeable.

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