User's Manual

3300-Watt VHF Low Band Transmitter Chapter 4, Circuit Descriptions
335B, Rev. 0 4-29
J1 on the low band VHF amplifier board
(1198-1605) that amplifies the signal
approximately 22 dB.
The RF output of the low band VHF
amplifier board at J2 (+41.5 dBm)
connects to J4 of (A3-A2) the overdrive
protection board (1198-1601). The RF
signal is through connected directly to J5,
the RF output jack of the board. A
sample of the RF on the board is applied
to a diode-detector circuit that consists of
CR1 and U1A. The gain of amplifier U1D
is controlled by detector gain pot R11,
which is set to +.4 VDC as measured at
TP1. The set output of U1D is connected
to the comparator IC U1B. The trip point
for the comparator is adjusted by R12,
typically set to 110% output power, sync
only. When the signal reaches that level,
the overdrive protection board will cut
back the output power of the tray and
the red Overdrive LED DS1 located on
the board and the red Overdrive LED DS1
mounted on the front panel will be
illuminated. Typically, the output power
level will bounce down and then up and
continue bouncing until the output level
is lowered to the normal operating level
(100%). The red Overdrive LED DS1,
the green Module LED DS3, and the
Enable LED DS2 may blink on and off
during the bouncing of the output level;
this is a normal occurrence. The greater
the output level is above 110%, the
larger the bounce will be.
The RF output of the overdrive protection
board at J5 connects to J1 on (A3-A3)
the 3-way splitter board (1198-1608).
The splitter board takes the +41.3 dBm
input and provides three +36.3 dBm
outputs at J1, J2 and J3 of the (A3)
amplifier enclosure.
The three RF outputs connect to (A4) the
final amplifier enclosure. This enclosure
contains three (A4-A1, A4-A2 and A4-A3)
low-band amplifier pallets 1304348
(P400-VHF-L). The RF signals connect to
J1 on each of the low-band amplifier
pallets. Each amplifier pallet provides
approximately 18 dB of gain.
The RF signal inputs to the amplifier
pallets (+36.3 dBm) are amplified to
+54.3 dBm outputs at J2. These outputs
are connected to J1, J2 and J3 on (A5-
A1) a 3-way combiner board (1198-
1626). The 3-way combiner takes the
three +54.3 dBm inputs and combines
them to form the 750-watt RF output at
J5 of the combiner that connects to J2,
the RF output jack of the tray.
The (A5-A1) 3-way combiner board
provides a forward power sample at J6
and a reflected output power sample at
J7. The forward output power sample
connects to J4 on (A13) the AGC control
board (1142-1601). The reflected output
power sample connects to J5 on (A13)
the AGC control board (1142-1601). 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.
+30VDC and +12 VDC Voltages
Two voltages, +30 VDC from the internal
switching power supply and +12 VDC
from the VHF exciter tray, are needed for
the operation of the tray. The +12 VDC
connects to J3-7 and J3-8 on the rear of
the tray and 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) +30 VDC switching power
supply provides the +30 VDC to (A8) the
current metering board (1304362). The
current metering board distributes the
+30 VDC through fuses to the amplifier
devices on (A2-A2) the filter/amplifier
board, (A3-A1) the low-band amplifier
board, and A4-A1, A2 & A3) the three
final low-band amplifier pallets. +12
VDC from the AGC control board is
through connected to the (A2-A1) phase