Try this ...from the manual:
■ Auto power OFF
The transceiver can be set to automatically turn OFF after a
specified period with a beep when no switch is pushed.
120 min., 90 min., 60 min., 30 min. and OFF can be specified.
The specified period is retained even when the transceiver
is turned OFF by the auto power-off function. To cancel
the function, select “OFF” in the auto power-off item in set
mode.
This can be selected in set mode. (p. 59)
73,
John AC4JK