TM 11-6625-486-14&P
The wires connected to the crystal oscillator and
If transistor Q1 (fig. FO-2 (2) or FO-3 (2) must be
modulator circuit board are long enough to permit
replaced, potentiometer R4 must be readjusted (para 6-
repairs to be made without unsoldering any wires. The
circuit board is under the frequency meter chassis and is
secured to the chassis rear plate by five screws that
factory-sealed unit and, if trouble has been traced to this
extend into threaded bushings mounted on the board.
unit or if the humidity indicator turns pink, the coilbox
Remove these screws and tip the board upward. When
must be replaced with a new unit.
repairs are made, remount the board and replace the
c. To remove any of the front panel control knobs,
five screws.
use an Allen wrench.
6-16. Removal and Replacement of Audio Amplifier
d. If either the FUNCTION or the RANGE SW
Circuit Board
needs to be replaced, use the wiring diagrams (fig. FO-
6 and FO-7) and carefully mark or tag each wire as it is
The wires connected to the audio amplifier circuit board
removed to avoid misconnection when the new switch is
are long enough to permit repairs to be made without
installed. Note the wire colors. For wiring on the
unsoldering any wires. The circuit board is in a shielded
boards, refer to figures 6-25 through 6-33.
compartment under the chassis. The board is held in
CAUTION
position by a screw at each corner of the board.
Do not use a soldering gun; it may
Remove the four screws and lift the board from the
include damaging ac signals in the
shielded compartment.
When repairs are made,
transistors.
remount the board and replace the four screws.
6-17. Removal and Replacement of Power Supply
with a 25-watt maximum capacity. When soldering the
Circuit Board
transistor, semiconductor diode, or other part leads to
the circuit board, solder quickly. To avoid transistor or
The wires connected to the power supply circuit board
semiconductor diode damage, use a heat sink (such as
are long enough to permit repairs to be made without
a longnosed pliers) between the soldered joint and the
unsoldering the leads. The board is held in a vertical
transistor of the diode.
position by four screws that extend into two brackets
f. To view the printed wiring from the parts side of
mounted on the chassis. Remove the four screws and
the circuit board, hold the board in front of a light
gently pull the board and cabling away from the fuse
source.
posts. After making repairs, remount the board and
g. Repair damaged circuit board printed wiring by
replace the four screws.
bridging the gap with a piece of single conductor hookup
wire.
Board
h. In an emergency, or during battle conditions, a
The wires on this circuit board are too short to permit
defective circuit board resistor or capacitor can be
repairs to be made without removal of the board
replaced without removing the circuit board. With a pair
entirely.
of cutter pliers, cut the part in the center and use the old
leads for mounting the new part.
carefully unsolder the bare wire connected from terminal
i. When repair work has been completed, the
9 of the mixer circuit board to the ANT. jack; then (in
frequency meter should then be given a final test to be
AN/USM-159 only), unsolder the 10K resistor (R308,
sure that it operates with its original accuracy.
FO-6 or FO-7) connected from terminal 7 to RANGE
SW S703. Unsolder the resistor at the switch terminal
6-15. Removal and Replacement of Crystal
only.
Oscillator and Modulator Circuit Board
main unit with two screws that extend through the front
CAUTION
panel (right side) and six screws through the side panel.
When removing the crystal oscillator
Three of the screws are on a horizontal line across the
and modulator board, do not let the
side panel and the other three are in a vertical line from
terminals come in contact with the
top to bottom. Remove the eight screws.
hot terminal (purple wires) on filter
capacitor C704.
6-20
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