HP 21 - How to troubleshoot?


Hello, everyone. Someone has offered to sell me a HP 21 calculator, but claims that "it has an erratic display probably because of corrosion from one of the old battery cells". I'm aware that powering one of these units without a good battery pack installed can damage it. Is there a way to determine whether or not said damage has occurred? The seller's description has me worried, but I'm hopeful that it's not terminal (pun unintended). Thanks in advance!


Erratic display seems like a good description for the symptoms of a fried chip caused by overvoltage from the charger due to battery failure/removal. Also often described as "random zeroes" flickering across the display. I've seen a lot of these on sale lately with nice photos of the flickering zeroes.

Edited: 30 Sept 2013, 10:05 p.m.


Steve nailed it.

The ONLY problem I have seen with a working calc with corroded terminals is intermittent BUT functioning display. That is the calc works when it works. In fact I purchased an HP 29c as only working sometimes, the key word is 'working' as in adding and subtract, etc. but that the functioning was interspersed with no power to the display.

That turned out to be intermittent contact ( cold solder joint) on one of the battery contact rivets.


Thanks for the responses. I have heard further from the seller, and received two photos of the calculator in operation. In the 'before' picture, '8's can be seen flickering across the display. After powering it using two 1.2V rechargeable batteries taped together, the display shows what looks to be a normal set of digits. Does this mean I'm in the clear, or could there still be something wrong? I can post the photos, if necessary. Once again, thanks!


Firstly, you mention the flickering eights, is this with the power adapter plugged in?

Secondly, when you say normal set of digits, the normal power on of the 21 with everything functioning should be 0.00. Nothing else is normal.

If the battery pack produces this 0.00 consistently with power off/power on cycles as well as some simple math functions such as {1 ENTER 1 +} resulting in a display of 2.00 then it is okay or at least restorable.


Let him type 12345678 - if that works (and a picture proves it) it probably works.


You may read an old and very good article written by Katie Wasserman about some Woodstocks problems that she diagnosed and solved with the addition of some components, so to correct a timing issue. It may help.

Just my 0.2 AR$

Best regards


Not sure if that reply should have been a reply to my post - but a burnt out chip is beyond being fixable and erratic symbols are probably not a sign of some fixable timing issues.


Frank, it was not a reply to your post, just a suggestion for the original poster. I recall vaguely that some problems may be soved by fixing the timing issues, but of course there are many other problems around (and damaged chips are out of luck, indeed).

Disclaimer: I apologize if my imperfect command of the English language prevented my message to convey its intended friendly meaning.


I'd be carefull about getting him to do things. At least make sure he doesn't plug in the charger anymore. You don't want him to fry the ACT if it has survived so far.


Let him type 12345678 - if that works (and a picture proves it) it probably works.

Yes, the 'after' picture indeed did show that (well, '9876543210'). Also, I had the seller (who was unfamiliar with RPN before this, interestingly enough) punch in a simple calculation, and [2] [ENTER] [2] [+] brought back 4. Should I take the plunge? Yet again, thanks!


From what you said the calculator showed a sequence of numbers (9 to 0) and performed a simple calculation correctly, I would assume that this calculator is fully functional. However you won't know the condition of the keyboard and power switches until you actually have it in your possession. The other issue is if any of the LED segments are damaged (an unlikely failure). Likely that the batteries will need to be replaced with new Ni-cads, NiMH or alkaline batteries.


Hmmm... IIRC, the HP-21 can not show 10 digits, i.e. 9876543219.


Hmmm... IIRC, the HP-21 can not show 10 digits, i.e. 9876543219.

Here's what the HP 21 page on this site says: "To reduce the size, the number of digits in the display was reduced from 15 digits to 12 and the exponent shared space with the mantissa. Another reduction in size was realized by combining the decimal point with a digit display rather than giving it a separate position of its own."

So less total digits for the display, but likely still able to show the usual 10, since things were compressed to make up for it.

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