Manual Chapter 3

500-Watt VHF Low Band Transmitter Chapter 3, Circuit Descriptions
325A, Rev. 0 3-27
The AGC control board contains two
peak-detector networks that provide
detected outputs that are used for front
panel and remote meter indications of
forward and reflected output power
levels, the AGC detector voltage level,
and the VSWR cutback protection if the
reflected power level increases above the
preset level.
Two voltages, +48 VDC from the internal
switching power supply and +12 VDC
from the exciter control panel, are
needed for the operation of the tray. The
+12 VDC connects to J3-7 and J3-8 on
the rear of the tray; these are wired to
J8, pins 4 and 1, on (A13) the AGC
control board. The +12 VDC is connected
to U8, a +5 VDC regulator IC that
supplies the +5 VDC needed for the
operation of the front panel-mounted
LEDs.
The (A10) +48 VDC switching power
supply provides the +48 VDC to (A8) the
current metering board (1198-1609). The
current metering board distributes the
voltages through fuses to the amplifier
devices on the filter/amplifier, the low-
band driver board, the low-band amplifier
board, and the three final low-band
amplifier boards.
The fuses F1, F2 and F3 are 10-amp
fuses; F4 is an 8-amp fuse; and F6 and
F7 are 1-amp fuses. F5 is not used in this
configuration. There are two spare fuses,
one 1 amp and one 10 amp, located on
the top, right-hand side of the tray.
The 10 amp fuse F1 protects (A3-A1) one
of the low-band final amplifier board; 10
amp fuse F2 protects (A3-A2) another
low-band final amplifier board and the 10
amp fuse F3 protects (A3-A3) the last
low-band final amplifier board. The 8
amp fuse F4 protects (A2-A1) the low-
band VHF amplifier board. The fuse F5 is
not used in this configuration. The 1
amp fuse F6 protects (A1-A2) the
filter/amplifier board. The 1 amp Fuse F7
supplies +48 VDC to J8, pin 2, on the
AGC control board. The +48 VDC is
connected to regulator IC U7 that takes
the +48 VDC and provides a +12 VDC
output. The +12 VDC is used for the
operation of the AGC control board. The
+12 VDC is also connected through the
current metering board, jumpered from
TB1-5 to TB1-6, to the phase shifter
board, the filter/amplifier board, and the
overdrive protection board.
The current metering board also supplies
sample outputs of the operating currents
of the amplifier devices in the tray to the
front panel current meter. The meter in
the (I
1
) position reads the current for the
(A3-A1) low-band final amplifier board;
the meter in the (I
2
) position reads the
current for (A3-A2) the low-band final
amplifier board and the meter in the (I
3
)
position reads the current for the (A3-A3)
low-band final board. The meter in the
(I
4
) position reads the current for the
(A2-A1) low-band VHF amplifier board.
To read the desired current, place switch
S2 in the proper position, checking that
S1 is in the Current position. These
current readings can be used when
setting up the idling currents, no RF drive
applied, for the devices. (I
1
, I
2
, and I
3
)
are each set for 2 amps, while (I
4)
is set
for 3 amps, when the tray is a visual
amplifier, or a visual + aural amplifier,
and they are set for 1 amp when the tray
is an aural amplifier.
In the tray, the 230 VAC is applied
through jack J4 to terminal block TB1.
When CB1, the 15-amp, front panel-
mounted circuit breaker, is switched on,
the 230 VAC is distributed from TB1 to
(A11 and A12) two cooling fans, which
will begin to operate, and to (A10) the
switching power supply. There are two
surge suppressors, VR1 and VR2,
mounted on TB1 that provide protection
from transients or surges on the input AC
line. There are two other surge
suppressors, VR3 and VR4, mounted at
the input to the switching power supply
from each AC line to ground, that provide
protection from transients or surges on
the AC line.