HP 41CX locked in program mode


I have a friend who gave me his mint-looking HP41CX. He couldn't get it to work any more. These handhelds aren't my specialty but I had a look. The batteries were badly drained. Some so low they measured 60 mV of reverse polarity. I got some new N cells, put them in, and the calculator came up with "memory lost." That solves that, I thought.

Not so, in seconds, the 41CX went into program mode and became unresponsive to most keys. The run/stop key produces a display with one "0" on the left and two zeroes on the right, for as long as I hold down the run/stop key. I've taken out the batteries, shorted the battery terminals, opened the calculator, shorted the electrolytics, put it together again, run the resent sequences, tapped it lightly to reseat anything loose, and rattled my beads.

For a brief moment, I could enter a short sequence (2 enter 2 x) and got the right answer (4) and then the 41CX popped back into program mode. I'm suspecting a short in the keyboard. Probably the PRGM key.

Any clues as to what's happening and what's to be done? I've seen nothing like this in the archived forum stuff, but it may be too far back.


I'm suspecting a short in the keyboard. Probably the PRGM key
No, a stuck PRGM key would have the unit come up in program mode and show 00 REG 219.

Fullnut or halfnut?


I don't know whether the reverse polarity did any damage to the hardware,

however the locked kbd could have it's cause in a bad contact,

either between the I/O connector and/or the CPU-to-kbd pcb connector.

Another typical error cause are broken screw posts.

Is it a halfnut model (rounded LCD frame)?


Randy and Raymond, it's got a square LCD bezel.

Edited: 2 Oct 2009, 9:22 p.m.


Hi, Steve;

now the next reasonable question should be: is it ACTUALLY a CX? If the LCD bezel has squared corners, then the unit is a fullnut and the mainboard is separated from the keyboard. Would it be possible to compare the mainboard with the pictures here and tell us which one compares OK with yours? The fullnut type may produce some unexpected behavior when the mainboard and the LCD driver do not establish good communication (bad contact with the contact strip under the mainboard)


Luiz (Brazil)

Edited: 2 Oct 2009, 10:23 p.m. after one or more responses were posted


It looks like the one marked:

HP-41C/CV/CX Common Board (CX version)

It's got the three electrolytic capacitors on the left, the doubled up piggy-backed ROMs along the top, and the assorted bent-over resistors along the bottom.


Sorry, I added an extra line in my previous follow up exactly when you were posting yours...


Luiz (Brazil)


Any crud/flux on the logic board around the rom area is from a sloppy repair can cause problems. So, remove the processor and clean both sides well with isopropyl. Allow to dry for several hours.

Check the logic to keyboard connector for any corrosion. Clean with isopropyl if needed. If damage is severe, it will need to be replaced.

Reassemble and test. If the problem remains, it is most likely a bad rom. The only way to test further would be with the 5061-7221 service module.

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