B&K Precision 879B - Manual - Manual (Page 27)

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brightness for 15 seconds. After another 15 seconds
(30 seconds total from the time of turning on), the
back light will automatically turn off to conserve
battery power.
When Using External Power
When the meter is powered using an external AC
adapter, the back light display will turn on upon
holding down the button for 2 seconds. It will
stay at maximum brightness continuously until the
user presses and holds down the button for 2
seconds again.
Note: If a battery is installed while using an AC
adapter simultaneously, unplugging the AC adapter
will automatically turn off the back light after it has
been lit for 30 seconds.