Panasonic KX-FLB758RU Service Manual page 167

7.3.2. RTC BACKUP CIRCUIT
1. Function
This unit has a lithium battery (BAT600) which works for the Real Time Clock IC (RTC: inside IC604).
The RTC continues to work, backed up by a lithium battery even when the power switch is OFF.
2. RTC Inside (IC604) Backup Circuit Operation
When the power switch is turned ON, power is supplied to the RTC (inside IC604). At this time, the voltage at pin F17 of the
IC604 is +3.3V. When the power switch is turned OFF, the BAT600 supplies power to RTC through D602. The voltage at pin
F17 of IC604 is about +2.5V. When the power switch is OFF and the voltage of +3.3V decreases, pin F17 of RTC (IC604)
becomes roughly the same voltage as the battery voltage. RTC goes into the backup mode, in which the power consumption
is lower.
Points to notice when replacing the battery
When the backup battery becomes low or the backup function is turned off for repairing, recover the backup power supply
following the procedure described below.
(1) Replace the battery and IC. J600 should be removed beforehand.
(2) Short the both ends of C762. (Ex. Pinch them with tweezers.)
(3) Connect terminals of J600 with solder.
(4) Short the both ends of C762 again. (Ex. Pinch them with tweezers.)
(5) Make sure that the voltage of 3.3V/BATT test land is more than 2.7 V.
Digital Unit component view
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KX-FLB758RU