User's Manual

3300-Watt VHF Low Band Transmitter Chapter 4, Circuit Descriptions
335B, Rev. 0 4-28
assembly)
MAJOR ASSEMBLY
DESIGNATOR
BOARD/ASSEMBLY NAME DRAWING NUMBER
A3-A2
Overdrive protection board
(mounted in [A3] an RF enclosure
assembly)
1198-1601
A3-A3
3-way splitter board (mounted in
[A3] an RF enclosure assembly)
1198-1608
A4-A1, A4-A2 and A4-A3
Three low band VHF amplifier pallets
(mounted in [A4] an RF enclosure
assembly)
P400-VHF-L-18
1304348
A5-A1
3-way combiner board (mounted on
[A5] the combiner heatsink
assembly)
1198-1626
A13 AGC control board 1142-1601
A8 Current metering board 1304362
A10
+30 VDC switching power supply
assembly
PM3329B-5-1-R-2-E
1301504
The on-channel RF input signal (+7 dBm
Visual –3 dBm Aural) enters the rear of
the tray at the BNC jack J1 and is fed
through J1 of the (A2) enclosure
assembly to J1 of (A2-A1) the phase
shifter board (1198-1602). The board
provides a phase shifter adjustment of
the RF signal that is needed to provide
maximum output during the combining of
the three VHF amplifier trays in the 3
way combiner. The front panel mounted
phase shift potentiometer R2 (A7)
connects to J3 on the board and controls
the phase of the RF signal.
If the input signal level to the phase
shifter board falls below a preset level, a
high, which is an input fault, connects
from J5 of the board to J14 on the AGC
control board. When an input fault
occurs, the AGC control board generates
a fault output at J1, which is connected
to J4 on the filter/amplifier board. The
fault cuts back the RF signal level using
the pin-diode attenuator circuit on the
filter/amplifier board.
The phase-controlled output at J2 of the
phase shifter board (+7 dBm) is directed
to J7, the input jack of the (A2—A2) filter
amplifier board (1198-1606) that is made
up of two circuits. The first circuit is a
channel filter that is adjusted for the
desired channel frequency and
bandwidth. The filtered output (+5 dBm)
is connected to the second circuit that
contains two amplifiers. The RF connects
through a pin-diode circuit to the
amplifier IC U1. The voltage applied to
J4, which is the external control jack of
the board, controls the amplitude of the
RF signal through the pin-diode
attenuator circuit. Jumper W1 on J5
should be between pins 2 and 3, which
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
(A6) 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 +19.5 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 (A3) 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