45
Service Manual
7-6
When the power switch is first turned on, a pulse through C15 turns on Q13, which then
turns on Q3. With the Q3 collector voltage now near its emitter voltage and the FET gate
near the battery voltage, the FET switch is on. At the same time, Q12 is turned on; its
collector pulls down the base of Q3, latching the circuit on.
When the voltage between the base of Q12 and the anode of VR4 drops below 6.9 volts
(battery voltage of about 7.0 volts), Q12 cannot be maintained on due to the 6.2 V zener
diode in its emitter circuit, and the circuit unlatches itself. The gate of Q3 then goes to
zero and turns off, disconnecting the battery pack. If the battery pack charge is low, this
circuit does not latch, and the battery pack is protected from being deeply discharged.
7-9. Constant Voltage Trickle Charger
The Constant Voltage Trickle Charger uses a voltage regulator set at 9.25 volts. The
output voltage is set by the values of R38, R40, and the setting of R39. The 30-volt
supply on the Main Circuit Assembly provides the source for the regulator. Trickle
charging occurs only when the meter is operated on line power.
If trickle charging occurs when the battery pack is low, R2 limits charging current to
about 15 mA maximum.
When the meter is operated solely on battery pack power, the charger is disconnected
from the battery pack. A logic level signal (ACON*) from the Main Circuit Assembly
(J1-5) is maintained high, and the output of comparator U2-1 goes low, pulling U1-1 to
common and setting the U1-2 output at 1.25 volts. CR4 is thus reversed biased,
disconnecting the charger from the battery pack.
7-10. Other Circuits
During the various modes of operation, switching diodes provide the required battery
pack connections. During battery pack operation of the meter, CR3 connects the battery
pack to the switching supply on the Main Circuit Assembly. During charging (line power
connected), CR5 connects the battery pack to the charging circuit. If line power is
disconnected, CR9 disconnects the battery pack from the charging circuit.
The 5-amp battery pack fuse opens if either the battery pack is connected with the wrong
polarity or if the battery pack output is shorted on the battery pack circuit board.
7-11. General Maintenance
7-12. Removal
Use the following instructions to remove the Battery Pack Option from the meter:
1. Make sure the meter is turned off and unplugged from the power outlet.
2. Remove the screw from the bottom of the case and the two screws from the rear
bezel (as shown in Figure 7-2 detail A). While holding the front panel, slide the case
and rear bezel off the chassis. (See Figure 7-2 detail B.) At this point, the rear bezel
is not secured to the case.
3. Locate the flat white battery pack option cable connector at the Main PCA (see
Figure 7-3.) Use needle nose pliers to detach this connection; alternately pull up on
each end of this connector.
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