EXIT key reassigned on 42s


Here is an interesting problem: I recently was given a 42s that did not function. I installed 357 batteries (none were in the calculator) and with the help of this site was able to get it to turn on, but not in the normal way. I had to activate the self test or reset modes from the keyboard. I turned my 42s off by executing the "off" function. Shorting the battery terminals together for an entire day has not fixed the problem. The other day, by pure luck, I discovered that simultaneously pressing the EXIT and "summation+" keys turned the calculator on. Similarly shift then simultaneously EXIT and SUMMATION+ turns the 42s off. It seems to me that the EXIT key has been reassigned. I am aware that this can be done, but I would have thought that doing a memory clear would have undone the reassignment. Anyone have suggestions on how to fix this?

Many thanks,


That seems to me to more likely be a hardware fault.



Seems quite like a resistive "short-circuit" in the keyboard. Pioneers are prone to suffer from this desease after heavy (repetitive) use of a key.

Alas, lack of real steel dome keyboard... polycarbonate instead? :-(

Sorry for the bad news (wish I was wrong).




Could be a bad connection of the keyboard connector onto the PCB. It is just under the LCD. If you press just under the screen, it will probably work too.

To see the internals of a 42s see:
Lyuka's 42s page


In the HP-42 (very different from the HP-41 in this aspect) keys are not reassignable. ASSIGN works only with the six top-row keys, and by means of the CUSTOM menu (three virtual rows of six keys each). So ASSIGN cannot cause this behavior. There are reset and diagnostics sequences involving the EXIT key and some keys from the top row.

The keys can be also polled in the context of a running program by means of GETKEY function, but your description does not suggest any connection with this function.


You're forgetting the "programmable menu", where you can reassign the top row of the keyboard, *and* the up, down, and EXIT keys.

Having said that, Charles' description does sound like a hardware problem. A key assignment wouldn't survive a cold start, and it sounds like the calculator had been without batteries for a prolonged period of time.


It appears that I have a hardware problem. All of the keys on this 42s are tight and springy with no wobbles, and it appears to be in very good overall condition. Therefore I will send it in to FixThatCalc so they can try to repair it. Thank you all for your expert and prompt advice!



if the calculator is otherwise fully functional, and passes the keyboard test, then why have it fixed? all you need to do is remember the slightly 'odd' on/off keyboard sequences.


Does EXIT work after the calculator is turned on? I mean, for instance, if it works to exit from a menu.

If it works (and what follows is just a guess), then the issue may have to do with a hardware interrupt signal, specifically used for the power-on function. The problem may be related to the keyboard to mainboard connection.

If it doesn't work, then it looks more like a keyboard, keyswitch or keyboard connection to mainboard issue.

To turn the calculator off, you can use the OFF function (XEQ "OFF"). Keep in mind that the ON function merely disables the timer which turns the calculator off after some 10 minutes of inactivity.


Hi Charles

I've ever experienced that kind of problem on a heavily used 42s.

The 'PWRON' pin of the processor seems to have relatively heavy pull-down resistance
or requires relatively high current to activate the processor.

The nominal resistance of the 'ON' key is about 430 [Ohm] (at new) to 470 [Ohm] (after moderately used) by my measurement.

That resistance can go as high as 1K [Ohm] on heavily used - worn carbon contact - keypad

which is not low enough to rise the voltage to cross the threshold to turn-on the processor.

At the end of life of the contact, pressing several keys might barely work to assert the active high signal at 'PWRON'

by some internal voltage glitch caused by common impedance or so.

I did membrane switch transplantation to fix that problem.
But it's a kind of difficult thing to do.



Edited: 18 Feb 2012, 10:17 a.m.


I've had a chance to play with my 42s as many have requested. Here is what I found:
1.With the calculator on, I can only use the exit key together with the SQRT and LOG keys to do a reset or self test but nothing else. These were also the methods I used to activate the calculator when it was off. (i.e. before I found the trick of pushing EXIT then SUM+. The EXIT key does not return from a menu, but EXIT-SUM+ does. I've not been able to change the brightness of the display, though.
2.No other two-key sequence turns the calculator on.
3.As Bart (UK) suggested, I was able to turn the calculator on and off by pushing just below the LCD screen in conjuction with using EXIT. This method worked as well and reliably as the EXIT-SUM+ method.
4.As Lyuka suggested, I was able to turn the calculator on (but not off) by hitting a group of keys together with EXIT. For example 7,8,9,DIV. This method seems to work better with the numbered keys than with the function keys. It also does not work very reliably.
My decision to send the calculator in will depend on whether I feel my 42s will work reliably as is. If Lyuka's diagnosis is correct, the 42s may have a limited life span, plus the problem may be difficult to fix. If Bart is correct, then the fix might be simpler. I enjoy tinkering with electronics, but drilling on the back of a thin plastic case is beyond my comfort level.

Thanks again,



Looking at pictures of the carbon button contacts attached by Lyuka reminds me of the contacts used in remotes (TV, CD, etc). I have been able to revive dead keys by using a product called Cramolin made by CAIG labs. This material goes by the names of Cramolin R-5, DeoxIT and ProGold G5. You want the stuff that is the cleaner not the protectant, though the ProGold is both. One can get it as a concentrated liquid or spray. Here is how I rejuvinate the controls: 1. First clean both services with 91%+ isoprolyl. 2. Then moisten a q-tip with Cramolin. 3. Sparingly apply the substance to both sides of the contact. Do not oversaturate to the point of making a mess. 4. Leave it sit for about an hour. Then with a clean q-tip slightly mointened with alcohol wipe off the contacts. The q-tip may get black. 5. Then apply a second light coat of Cramolyn to the contacts. Again be sure not to oversaturate. Wipe off the excess if you use too much with alcohol and start again. 6. Then reassemble. Some of the revived remots have lasted for a year or two. Some have lasted much longer before needing another treatment.

Hope this will be of use.


3.As Bart (UK) suggested, I was able to turn the calculator on and off by pushing just below the LCD screen in conjuction with using EXIT. This method worked as well and reliably as the EXIT-SUM+ method.

Sounds like you've found your problem. The carbon ink traces
on the key matrix flex interconnect mate to bottom pcb fingers
under pressure via a foamed plastic/rubber/? strip. Either
that strip has relaxed or the frame twist lock isn't creating
pressure in the first place. If you pop the unit open
(admittedly involved on the service-hostile 42s) you
might have a look at the lower middle frame twist tab and
take up on it a bit to see if doing so restores contact.
Otherwise undoing all tabs, removing the pcb and either shimming
the existing foam strip (or better yet replacing it) should solve
the problem.

The sqrt key may possibly be only a mechanical factor as
pressing in that area will cause the frame retaining the
compression strip to move slightly toward the pcb and
potentially restore electrical contact to pins. If this
is indeed the case I'd expect pressing the 1/x key to accomplish
the same. But intermittents IME can at times have puzzling

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