HP-41CX that gets hot


Hi, I just got an HP-41CX off ebay. When I put in N-cells, the area on the back of the calculator below the battery area gets hot. I cannot even get this thing to turn on. Anyone have any guesses as to the problem? Thanks!


HI; (I´m back, folks...)

If yours is a halfnut HP41CX, then the area below the the batteries in the HP41CX has both the buzzer and the additional, small board with the time module hardware.

If the buzzer or the time module is shorted (dammaged), chances are that you can remove any/both of them and have the 'remaining' calculator working fine. Replacing the buzzer is easier (any other HP41 and many HP calculators have it), but you´d need an original HP41CX time module to replace another, though.

If it is a fullnut HP41CX, I´m clueless.

Hope you succeed, anyway.

Luiz (Brazil)


Thanks, looking at the display, it's not a halfnut; it's an older fullnut. However, the SN begins with 2345S, which may make it a fullnut?


The display is definitive. There is no way to have a fullnut display with halfnut electronics, or vice versa.

The serial number is NOT definitive.


I once got a fullnut HP41CX with the same problems (batteries went hot, and it didn't turn on). I opened it. It had been opened again, and the plastic rolls that hold the screws and the PCB in place were broken. The PCB was slightly removed from its original position, so I guess the zebra connector has shorted some traces or has applied the wrong signal to te contacts. Anyway, I could not repair it anymore. After assembling it correctly, it always displayed '@@@@@@@@@@@' and emitted a high pitch.

Hopefully you have more luck!


Make sure you have the correct type of batteries. I once bought batteries that fit in the battery holder but were of the wrong type and these batteries got very hot and the calculator did not turn on!!!



What do you mean? I use N-cells and they work in another CX that I own.


Disposable 1.5V "N" cells are correct. There is also a rechargeable pack made for the HP-41, and other rechargeable packs have been fabricated using "AAA" cells.

There is a certain 12V cell that is very similar in size to the "N" cells. They are used in remote controls for garage-door openers and other devices. Their strong IR signal is probably the reason for their high ouptut voltage.

Don't use 'em in an HP-41!

I concur with the diagnosis of a short circuit, with the calc getting hot, and not turning on, probably due to the voltage drop caused by abnormal current flow.

If you can't find an obvious cause of the short circuit, schematics are available from a site that one can find by Googling "Warren Furlow".

-- KS

Edited: 12 July 2006, 11:08 p.m.


I would return it to the seller and hope for a refund. Your HP-41CX looks like DOA.

Now if you have the technical knowledge and equipment and want to spend a lot of time playing with it, you can try to see whether you have a clock signal on the I/O bus (pins fi1 and fi2.

here is a picture of the I/O bus as you look into the I/O bays
from the back of the calc:


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