HP-15C locks up when certain keys are pressed


Hello,a ll-

I have a 15C that locks up whenever the CHS, 7, 8, 9 or divide by keys are pressed (top row, right half of keys). I inherited this unit, it has the REV D circuit board. It's otherwise functional and in great shape.

Any suggestions or fixes would be welcome - I'd hate to have to shelf this unit.




I am not sure about it, but when you mention REV D circuit board, do you mean the revision with a single PCB or the one with the flex circuit board with the LDC and electronics? I cannot remember...

In any case, the keyboard matrix is the same, i.e., left and right half of it share the same columns (5) and each half has its own four rows.

First (maybe silly) question: [ON]/[×] self-test returns O.K.?

Keyboard test ([ON]/[÷]) maybe out of question because the [÷] key seems troubled, but...

I'd check if there is any conductive 'thingy' connecting copper trails of the processor's terminals, mostly in the terminal connected to the common row these keys share.

Hope this helps a bit.


Luiz (Brazil)


Hello, Luiz-

It is the single piece PCBoard. The self test shows no problem. I cannot run a keyboard test.

Is there a way to remove the board from the upper housing? All I see are plastic pegs every centimeter melted over holding the board in place.

Thanks for the suggestions.

Greg Gosian


Is there a way to remove the board from the upper housing? All I see are plastic pegs every centimeter melted over holding the board in place.

This is not directly related to your request but I hope it gives you some insight:


You can notice I didn't do a good job when resoldering the plastic rivets. I hope you're more skillful than I am :-)




Thanks, Gerson


Removing the keyboard will not help, it will only compound your problem. The *only* reason you would need to do that would be to replace the LCD.

You most likely need to replace the keyboard scanner chip, 1LF5 or 1LM2, it depends upon which version hardware you have. You'll find the same part in a similar vintage 12C.

If your 15C is one of the last versions produced, it will have only one IC, in which case you're out of luck as it can only be found in another identical 15C.



Thanks for letting me know - I won't try venturing underneath. I have a 1LM1 chip (small square), and 1LQ9 chip (larger, rectangular) accessible.

Any idea which is the one I am looking for and where I can find a replacement?

Thanks for the reply


Ugh, a two chip variant. It should be the small 1LM1 but I don't know for sure, Eric Smith might know.

I've got a bad feeling that the only source is going to be another two chip 15C :(


I was afraid of something like that.

Thanks, everyone, for all of the advice.


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