HP 11C Requiem for an old friend


I regretfully noticed a couple of days ago that my HP 11C seems to have failed. I initially thought its batteries were drained, but replacing them has had no effect. The calculator will display "0.0000" and will not respond to any buttons (including the ON button).

It has been a great calculator; I bought it in the summer of 1983. It's seen me through undergraduate & graduate engineering school, 4 moves, and 4 engineering jobs. It's never failed until now. It is scratched, dented, beat-up, missing its little rubber pads on the back, and pretty dirty, but it has been a great tool and has faithfully given me the right answers to thousands of calculations over the last 18 years. My hat goes off to the engineers, technicians, and management that created the 11C- it was a great accomplishment.


Have you tried the recovery procedures described in the owner's manual? The 0.0000 display is the default display (after "Pr Error") following a power loss, so there may still be hope. If you don't have the manual any more, it says to try the following procedures on a calculator that doesn't respond to keystrokes:

Press the "ON" and "Y**X" keys simultaneously (that's Y to the X power, 4th from the left in the top row), then release.

If that doesn't work, remove and reinsert the batteries (you've probably already tried this).

If the above doesn't work, momentarily short the battery terminals to each other while the batteries are still in the calculator. (A paper clip seems to do best.)

My own 15C gave me a scare once when it seemed to just die, and I had to resort to the shorting procedure to get it to wake up again. It still acted a bit strange, and on closer inspection I found that after 19 years the battery terminals (this is an early unit with the flat contacts, and no spring) had tarnished and flattened out to the point that the batteries were making intermittent contact. Cleaning with an eraser and bending them back slightly has the unit working reliably (I hope!) again.


Refer to page 234 of your manual. Try shorting out the
battery terminals.


If the fixes suggested by others (and mentioned in the user manual) don't help, then it still might be possible to repair it.
Take the back conver off (screws under the feet). If it's one of the early models with the separate logic assembly, then
clean the zebrastrip connector between the logic PCB and the keyboard PCB.
If that doesn't help (or it's a newer type, all on one PCB), then, if you have experience resoldering surface-mount components, try resoldering the 2 chips to the PCB. I've had problems with dry joints in voyagers.


Yes, it might be not that dead !

And I would add in the recovery procedure (only if the ones describes above are not successfull) to inverse polarity of the batteries for a short while...

And if successful, to care about it as it will be difficult to find such a good calc, or at high prices... the nearest best calc is the 32SII that can still be found at some stores, but hurry ! RPN is almost dead at hp's !


Another very common cause of an HP calculator not responding to keystrokes is a key that is stuck closed or otherwise shorted. Check that all your keys are working properly... there is a chance that one can be shorted closed but still feel normal though. If you can check the key matrix with an ohm meter and look for a stuck key.

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