Sharp OZ-640 - Manual (Page 92)

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5. Insert the two new batteries on top of the ribbon tab, taking
care to position them correctly according to the plus (+)
and minus (-) terminals, as shown below.
6. Replace the cover.
7. Set the battery replacement switch to NORMAL
OPERATION.
If nothing happens when you press [ON]:
* Setting the battery replacement switch to REPLACE
BATTERIES shuts off power to the unit. Check that the
battery replacement switch is set to NORMAL
OPERATION.
* Repeat the above battery replacement procedure step by
step.
* After following the above battery replacement procedure,
the LCD contrast setting which is set by user will be back
to its default mode.