HP HP-67

Display Control 51
Press Display
e
All those operations that cause an error condition are listed in
appendix C.
Low Power Display
When you are operating the HP-67 from battery power, a red lamp
inside the display will glow to warn you that the battery is close to
discharge.
6.02 23 Low Power Display
You must then connect the ac adapter/recharger to the calculator and
operate from ac power, or you must substitute a fully charged battery
pack for the one that is in the calculator. Refer to appendix B for
descriptions of these operations.