I think I may havee literally blown it

Hi all.

Today, I just got an Excellent condition 28S. In installing new batteries into the calc, I tried Enercell 23A batteries. They looked N size so, I though, with the way batteries are labelled nowadays, I thought 23A was Enercell's size name for N cells. What did I know?

Anyway, yes, I did install these in the 28S and tried to turn the calc on. But, no go.

I then called RS, Radio Shack, to ask if 23A was the same as N. OH No they're not and N cells are called just that. Afterwards, I looked and each 23A cell is 12V. Could I have fried the 28S accidentally? Can my 28S be fixed?

In all honesty, with the 15C, I became introduced to the fact the same battery is given different size names from various manufacturers. This idea has stuck with me for over 25 years. So, I thought that Enercell's 23A battery was their size/voltage name for N cells. So, perhaps it's my naiivete which contributed to harming my 28S.

Please help.

Edited: 21 May 2012, 9:36 p.m. after one or more responses were posted


So have you tried with the corrct LR1 afterwards?

The LR23 batteries have a different size compared to the LR1.

LR1 1.5V - AM5 4001, LR1 MN9100 810, E90, 910A, KN, UM5, Size N, Lady, L1129 11.5 x 29.0

LR23 12V 23A LRV08 VA23GA MN21 MS21, VR23, 1811A, A23, L1028 10.0 x 28.0

So the LR23 are 1.5mm thinner and 1mm shorter, even thinner than the micro size batts for the HP-48;-)




I'm afraid you may have inadvertently fried at least some of the calculator's circuitry. I'm not sure whether it's feasible to dismantle the 28S to see how much damage has been done as I believe it's one of the HP models that's not repairable.


Yes. I was so distraught that, after I called RS, I went down there and picked up N cells. Even after installation, the 28S was DOA.


This Memories Forum post might have helped:


Well, let's hope it can be fixed.


It seems possible.

A few years ago, I landed an excellent condition 9114 drive. The previous owner had modernized it with a DC jack to accommodate a standardized plug in addition to the jack we see in the Topcats, certain printers, and the HP41 rechargeable pack. I plugged in an adapter that fit and fired the thing up. It was a 12V adapter and 9114 needs, at most, 4.5 or 6V. Perfectly working drive was cooked in seconds. Fortunately, the original owner took the drive back and sold it off for components and parts. and I made up the difference, so my folly cost me a lot less, but I learned a bitter lesson.

I fried a 6 volt device with 12V. You subjected a 4.5V device to 36V. So sorry, Matt, but I think you did some irreparable damage :(

Your predicament makes me feel grateful for my flawed 28S. My otherwise mint unit has a battery cover that is fraying around the edges and ready to go any day now--so much so that I don't use the calc, which I have as a curiosity more than anything, and keep carefully stored without batteries. I understand that someone out there has actually come up with a solution for the 28S's infamous battery cover weakness. Alas, I don't think a voltage surge has such an easy fix.



Well yes 36 staggering volts would do it. And sadly, yes, a bitter lesson learned. But, let me ask, could any of the 28S be salvaged for parts?

And yes, between my folly in not realising that the age-old lettered sizes (AA, D, C, etc.) are just that, standardised and my naiivete for not logically concluding that a 28S would NEVER need 36V of power, it is a lesson learned a rather hard way.

Edited: 21 May 2012, 9:37 p.m.


Well, that makes me feel a lot better AND a lot more cautious. Not certain why I didn't think of it before--the 41 uses N cells too! Why didn't I think to grab a 41 and just put those batteries in? Farthest thing from my mind. Thanks for the testimonial.

Now I don't feel like I should beat myself up for such absent-mindedness.

Edited: 21 May 2012, 9:34 p.m.


Alas, I don't think a voltage surge has such an easy fix.

Unsure if/where this may fit in the housing. But if I follow the
above, a 5.1V 1W zener diode across the power rail downstream of
an added surface mount fuse should avoid such a failure.

edit: typo

Edited: 22 May 2012, 12:27 a.m. after one or more responses were posted


Sorry, but I can't help with '28 innards information. I had to leave the one HP issued me with them when I left in 1989...

Regarding overvoltage and HP calculators, I recall one of the PPC Journals (or CHHU Chronicles or HPX Exchanges or ???) where someone found 1/2N cells - just like N cells but half length. They put 8 (!) of them in their HP-41 and found it could be overclocked to 3x speed, unlike most that petered out at 1.5 or maybe 2x.

Those old coarser geometry chips were rugged! Of course, 8 cells = 12V, not 36V so the likelihood of the 28 surviving is really low.

My condolences!


someone found 1/2N cells - just like N cells but half length. They put 8 (!) of them in their HP-41 and found it could be overclocked to 3x speed, unlike most that petered out at 1.5 or maybe 2x.

I don't believe that. 12V into a 41C is almost certain to damage it. I wouldn't recommend that anyone try it.


Thank you. Much appreciated.


I think I may havee literally blown it

Yes, I think you probably have. Unfortunately the one HP you chose to do it to is probably the most difficult to open. From descriptions and pictures posted here in the past it requires removing the plastic stick on decals/faceplates above the screen and on the right keyboard, and drilling out the stakes. Re-attaching the stakes afterward is another problem. And then of course it's not guaranteed to be repairable.

From our repair expert:
HP 28S: shucking the clamshell

If you are interested in trying to open it I have a working logic board/display assembly from one I have that has a broken hinge that I would be willing to part with, email me through the forum if you are interested.


For what it's worth, there are several 28S calculators available on TAS, some of them well under $100.



I once saw an owner doing exactly the same thing, in 1999. As we agree with, the HP28S circuitry - along with all other calculators with the same feature - is fed at the moment the batteries are placed in, I mean, no 'switch key', just a ON/OFF toggle key. So, the mere fact of inserting the batteries and locking the battery compartment is enough having the circuits fed with 36Vcc.

Fact is that after the owner tried to turn it ON several times she gave up and returned to the store where she bought the batteries. They tested them and told her they were fine. When she told me that - about a couple of days later - and showed me the batteries I feared the worse. I had my HP28S at hand and offered to test hers with the batteries installed in mine. Believe me, the little 'toasted' HP28S was so too happy with the batteries ('She's alive!' I said) that I actually gave them to the also happy owner. She also gave me the three 12Vcc batteries and I used them with remotes till they expired.

Of course I cannot tell the batteries orientation when they were inside the calculator, she had them in her hands when talking to me, so there is no way to make sure the full 36Vcc actually fed the mainboard.

It actually happened with me, not with a friend of a friend of mine.

Hope for the best of yours. Give it a couple of days, who knows...


Edited: 22 May 2012, 12:35 p.m.


When I read your post, my stomach dropped. Give it a couple of days and see if the 28s will awaken. Good Luck.


Come to think of it, the 28S has a capacitor to prevent memory loss while changing the batteries. Perhaps that needs to discharge itself to an acceptable voltage for the calculator to work again? (just speculating, but who knows? Maybe if I have a 28C/S to take apart one day I will experiment).


I might be interested in a good-looking, non-working HP 28x; so please let me know by email if you plan to discard your damaged unit. Thanks in advance.

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