Manual Chapter 3

500-Watt VHF Low Band Transmitter Chapter 3, Circuit Descriptions
325A, Rev. 0 3-28
The switching power supply only
operates when the power supply enable
control line, jack J3, pins 9 and 10, on
the rear of the tray, is shorted. The
enable is generated by the control panel
when the amplifier array is switched to
Operate. The enable is applied to (A5)
the AGC control board (1142-1601)
which, if there is no thermal fault,
connects the enable from J10, pins 6 and
7and then to J1-6 and J1-8 located on
the switching power supply assembly.
The green Enable LED DS2 on the front
panel will light, indicating that an enable
is present. If the amplifier array is in
Standby, or if a thermal fault occurs, the
AGC control board will not enable the
switching power supply. As a result, the
+48 VDC will be removed from the
amplifier modules and the front panel
Enable and Module Status LEDs will not
be lit.
The front panel meter (A9) uses the front
panel Selector switch S1 to monitor the
AGC Voltage, % Output Reflected Power,
% Forward Power, and the Switching
Power Supply Voltage (+48 VDC). The
meter in the AGC position will read
anywhere from .5 volts to 3 volts. The
meter is calibrated in the Power Supply
position using R86 on the AGC control
board. The % Output Power is calibrated
using R44 and the % Reflected Power is
calibrated using R53 on the AGC control
board. With S1 in the Current position,
S2 can be switched to read the idling
currents, no RF drive applied, of the
high-band amplifier boards. Typical
readings are an idling current of 2 amps
visual, or visual + aural, or 1 amp aural
in the amplifier assembly I
1
, I
2
, and I
3
,
positions and 3 amps in the I
4
position.
The reflected power sample from the 3-
way combiner board through the low
pass filter board is fed back to the AGC
control board at J5. On the board, the
reflected sample is connected through
the detector circuit to VSWR cutback
circuit U13C. If the reflected power
increases above 20%, the output power
of the tray, as set by R60 (the VSWR
cutback on the AGC control board), will
be cut back to maintain a 20% reflected
output level. The red VSWR Cutback LED
DS4 on the front panel will remain lit
until the reflected level drops below 20%.
There are three thermal switches in the
tray for overtemperature protection. Two
of the thermal switches (A13 and A14)
are mounted on the rear of (A3) the
heatsink for the low-band final amplifier
boards and the third thermal switch
(A15) is mounted on the heatsink for
(A4-A1) the 3-way combiner board. The
thermal switches close when the heatsink
on which they are mounted reach a
temperature of 175° F. The closed
thermal switch causes the AGC control
board to remove the enable to the
switching power supply. This eliminates
the +48 VDC and lights the red
Overtemperature LED DS5 on the front
panel. The AGC control board will
extinguish the Module Status LED DS3.
3.3 (A9) Bandpass Filter Assembly
(Appendix C)
The RF input connects to the (A9)
constant impedance bandpass filter
assembly at jack J1 of (A1) a
splitter/combiner board (1092-1092).
The splitter/combiner board divides the
combined RF signal into two signals
before feeding it to jack J1 of (A2 and
A3) two 7-section bandpass filters with
traps. These filters screen the 6 MHz-
wide signal and attenuate the –4.5-MHz
and +9-MHz out-of-band products. These
signals connect to jacks J2 and J3 on
(A4) the other splitter/combiner board
(1092-1092), that recombines the two
signals before sending them on to jack J1
of (A5) the directional coupler module
(1092-1308). The directional coupler
module provides forward and reflected
samples to the exciter tray for metering
purposes.
The RF Output of the Transmitter, 500
Watts, is at A5-J2 of the bandpass filter
assembly.