How do I put my HP19BII Calculator back into RPN mode?

I take it you mean a 12C Platinum.

Press f ALG. ALG is the f-shifted function of the EEX key, just above the double-size ENTER key. This will switch the 12C Platinum from RPN mode to Algebraic mode, and ALG indicator will light up instead of the RPN indicator.

To switch back to RPN, press f RPN. RPN is the f-shifted function of the CHS key just above the EEX key.

I checked the manual, the image of the face of the calculator and there is no way to change the entry mode from RPN to Algebraic. Your calculator is still in RPN, it is the only mode it has. Reset it.

I checked the manual, the image of the face of the calculator and there is no way to change the entry mode from RPN to Algebraic. Your calculator is still in RPN, it is the only mode it has. Reset it.

Press f ALG to switch to the Algebraic mode.

ALG is the shifted function of the EEX key.

To switch to the algebraic mode press f [ALG] (the EEX key).

Assuming you have a 12C Platinum, press f [RPN] to switch from the algebraic to the RPN mode. [RPN] is the shifted function of the CHS key on the top row of the keyboard.

I presume you're on a Platinum since the non-Platinum 12Cs don't have an algebraic mode.

To switch back to RPN, press f [RPN]. That's the yellow "f" key followed by the "CHS" key.

The HP 12c is a purely RPN calculator; it does not have an algebraic mode.

To switch the HP 12c Platinum to algebraic, press f [ALG].

Try this, hook up new battery and go to MODE and darken and lighten the screen.. do it ten to fifteen times. if that doesn't help, try hook up most left and most right's battery terminal to a DC supply at 4.5V. dont go above 4.5 it will fry your calc (it is design to handle 3V). now you are forcing extra juice into the dead LCD screen lines... and reapeat the MODE + darken trick. I fixed mine like that! Works! and remeber to leave it completely black(darkest) for a while to keep that dead line alive!

Has it been dropped recently? The screen may have cracked from impact. Press the Reset button on the back using a ball point pen.