My HP21 is sleeping... Help me wake it!


I have an HP21 that has had somebody install alkaline AA cells in the battery pack.

Nothing happens when I turn it on.

Q1: Will the HP21 be damaged by alkaline cells being used in place of the original nicads?

Q2: Looking at the back of the calculator with the charging port to the top and the battery terminals to the bottom, the terminals are marked - (left) and + (right). Is this correct?

Q3: Are the screws under the rubber feet like nany other models?

Q4: Assuming there is nothing obvious wrong if I need to open the calculator, what are common problems.

Some preliminary fault finding that shows a change in resistance across the battery terminals when the calculator is turned on tends to suggest it's not the switch :-(


HP 21 is now awake and working beutifully.

The fact that the two alkaline cells were flat didn't really help.

At least I'm cautious.

A1: Alkalines are OK (there's a thread on this)

A2: Yes! I traced the circuit from the charger -- from the same thread as above, and knowing it's an AC charger...

A3: Yep! under the top two, thr feet are like rubber stoppers (cute!)

A4: Flat batteries (blush)

Even more preliminary fault finding would have indicated that the potential across the battery pack was lower than for a nominally discharged nicad pack.


Forgive my mindless chatter, but I can't figure out the serial number of this HP21.

It's 1508S17472

This tells me that it was produced in the first week of 1975, about 4 weeks before they were released, but the number 17472 seems WAY too high for this.

Does this have a different serial number scheme than the familiar YYWW where YY is added to 1960 and WW is week + 8?

p.s. I promise to stop talking to myself... soon.


"the number 17472 seems WAY too high for this. "

This number doesn't seem unusually high. Some of my 21s are in the 3xxxx range.


My early HP-21 has a serial number of 1507A95363.


Gee, they really churned them out!

The YYWW part of the serial number, does 1507 refer to the 7th week of 1975, or is it earlier. A reference I have suggests that this WW number had a fudge factor built in to allow for time from manufacture to market.


FWIW, there are at least two HP21 prefixes before 1507. I have a database of hundreds of HP calc serial numbers that I've gleaned mainly from public sales (some other collectors have contributed, too), and there are quite a few HP21s with 1504A and 1506A serial numbers. I've seen 1504As in the 50xxx-55xxx range, and 1506As in the 70xxx-75xxx range. For the 1507A range, they've gone from 09xxx-95xxx.

I, for one, don't believe that there is any exact correlation between serial number prefix and the actual week of manufacture, for Classic and Woodstock machines. As my database of serial numbers grows, one of these days I'll write an article for my web site to explain why I believe that. One easy clue is that I've opened up quite a few machines, and the date codes of the ICs inside are almost always later than the date code stamped on the serial number.



Thanks, that is very interesting information.

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