Manual Chapter 3

500-Watt VHF Low Band Transmitter Chapter 3, Circuit Descriptions
325A, Rev. 0 3-26
circuit contains two amplifiers. The RF
connects through a pin-diode circuit to
amplifier IC U1. The amplitude of the RF
signal through the pin-diode attenuator
circuit is controlled by the voltage applied
to J4, the external control jack of the
board. Jumper W1 on J5 should be
between pins 2 and 3; these pins provide
external control, through J4, of the gain
of the board as well as the output level of
the tray. R9 is the manual gain pot that
is in the circuit when the jumper W1 is
between pins 1 & 2.
The front panel-mounted gain pot R3
connects to the AGC control board and is
used to adjust the AGC pin-attenuator
bias voltage that connects to J4 on the
filter/amplifier board. The RF signal, after
the pin-attenuator circuit, is amplified by
the second amplifier stage Q1 to about
+16 dBm; this signal is connected to the
output of the board at J2.
The RF output of the filter/amplifier
board connects to J2 of (A2) a RF
enclosure that contains the low band VHF
amplifier board, the overdrive protection
board and the 3-way splitter board. The
RF from J2 on the enclosure connects to
J1 on the low band VHF amplifier board
(1198-1601) that amplifies the signal 20
dB.
The RF output of the low band VHF
amplifier board at J2 (+34 dBm)
connects to J4 of (A2-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 comparator IC U1B. The trip point for
the comparator is adjusted by R12,
typically 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 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 (A2-A3)
the 3-way splitter board (1198-1607 or
1198-1608). The splitter board takes the
+34 dBm input and provides three +30
dBm outputs at J2, J3 and J4 of the (A2)
amplifier enclosure.
The three RF outputs connect to (A3) the
final amplifier enclosure. This enclosure
contains three (A3-A1, A3-A2 and A3-A3)
low-band amplifier boards (1198-1624 or
1198-1631). The RF signals connect to J1
on each of the low-band amplifier boards.
Each amplifier board provides
approximately 20 dB of gain.
The RF signal inputs to the amplifier
boards (+30 dBm) are amplified to +50
dBm outputs at J2. These outputs are
connected to J1, J2 and J3 on (A4-A1) a
3-way combiner board (1198-1625 or
1198-1626). The 3-way combiner takes
the three +50 dBm inputs and combines
them to form the 500-watt RF output at
J4 of the combiner that connects to J2,
the RF output jack of the tray.
The 3-way combiner board provides a
forward power sample at J5 and a
reflected output power sample at J8.
Both samples are fed through a low pass
filter board, low band (1198-1628)
before connecting to the AGC control
board. 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).