Pentax ETH-310 - Manual (Page 18)

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2.3.3 [HOLD] key
Press the [HOLD] key twice in succession to hold the displayed
horizontal angle and retain it. (Press once, the key buzzer will sound,
then press again during the sound.)
Press the key once more to release.
To prevent operation error, the buzzer sounds for three seconds
when the [HOLD] key is pressed.
HOLD is not initiated unless the key is pressed during
the buzzer sound.
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