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Transistor Q5205 and op amp U5202B provide regulation of the
-12 VDC supply, which comes from bridge rectifier BR5201. One
input to the op amp is from the regulated +12 VDC, which allows
the -12 VDC to track the +12 VDC. Capacitors C5208 and C5209
filter the supply.
C. BATTERY CHARGER CIRCUIT
Op amp U5202A and transistors Q5212 and Q5215 form the battery
charger voltage regulator. Trimpot R5241 allows adjustment of
the voltage. Transistor Q5213 limits the current. When the
+12 VDC regulator is operating (Power Switch in LINE position),
+12 VDC is applied to the base of transistor Q5214, which
reduces voltage on the base of transistor Q5213. This drops
the charging rate from 1.4 amps to .9 amp. With the switch in
BATT, op amp U5202A is disabled, shutting down the battery
charger circuit. However
J
an external DC source can be used to
charge the battery directly.
D. TRANSFORMER DRIVER CIRCUIT
Multivibrator U5203 and its timing capacitor C5240 and resistor
R5230 produce a 400 Hz squarewave to drive transistors Q5210
and Q5211. These two transistors drive power transistors Q5207
and Q5208 respectively, each of which feeds one leg of the 24 V
tap of the 30 VCT secondary winding of the transformer. The
emitters of driver transistors Q5210 and Q5211 are returned to
the +5 V supply to reduce heat dissipation in resistors R5224
and R5225 and to increase the efficiency of the power supply.
Transistor Q5206 supplies power to counter U5204, which counts
output pulses from multivibrator U5203 and to the base and
emitter of transistor Q5209. The counter normally grounds the
base of Q5209, allowing it to conduct. After approximately 10½
minutes, the counter output to Q5209 goes high, turning the
transistor off, and shutting down the multivibrator. Since
transistor Q5206 continues to conduct, counter U5204 is not
reset until the Power Switch is turned "OFF". As long as tran
sistor Q5209 is conducting, it supplies power to op amp U5202
in the battery charger circuit, which shuts down the battery
charger to conserve battery power.
7-2-2 PREPARATION FOR TESTING
A. REMOVAL AND DISASSEMBLY
Remove Battery from within the FM/AM-500A chassis. If repair
or replacement is required, refer to Rear Panel Removal and
Disassembly paragraphs in Section 6 for procedures.
7-7
Summary of Contents for FM/AM-500
Page 152: ...DETAIL A 11 Figure 6 6 Dual vco 6 14 01 Assembl Y ...
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