Manual Chapter 3

500-Watt UHF Transmitter Chapter 3, Circuit Descriptions
425A, Rev. 0 3-26
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.
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
+14 dBm; this signal is connected to the
output of the board at J2. The RF output
of the filter/amplifier board connects to
J1 on (A2-A2) the high-band VHF driver
board (1219-1103). The board contains
an RF power FET that has a gain of 20
dB.
The RF output of the board at J2 (+34
dBm) connects to J2 of (A3) an RF
enclosure that contains the overdrive
protection board, the high-band amplifier
board, and the 4-way splitter board. The
signal is connected to J4 of (A3-A1) 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 (A3-A2)
the high-band amplifier board (1219-
1114) that amplifies the RF before it is
connected to J1 on (A3-A3) the 4-way
splitter board (1219-1101). The splitter
board takes the +34 dBm input and
provides four +30 dBm outputs at J1, J2,
J3, and J4 of the (A3) amplifier
enclosure.
The four RF outputs connect to (A4) the
final amplifier enclosure. This enclosure
contains four (A4-A1, A4-A2, A4-A3, and
A4-A4) high-band amplifier boards
(1218-1201). The RF signals connect to
J1 on each of the high-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
+49.5 dBm outputs at J2. These outputs
are connected to J1, J2, J3, and J4 on
(A5-A1) a 4-way combiner board (1219-
1102). The 4-way combiner takes the
four +49.5 dB inputs and combines them
to form the 300-watt RF output at J5 of
the combiner that connects to J2, the RF
output jack of the tray.
The 4-way combiner board provides a
forward power sample at J9 and a
reflected output power sample at J11.
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