Assembly Instructions Chapter 4

4-6 kW UHF Translator Chapter 4, Circuit Descriptions
837B, Rev. 0 4-45
monitoring the AGC, % Reflected
Power, % Output Power, and the power
supply voltage.
A –12-VDC enable is applied to the
board at J4-1 from the transmitter
control board in the UHF exciter tray.
The enable causes the J-FET Q5 to be
biased off, making the drain go high;
the high is applied to Q4, which is
biased on. The drain of Q4 goes low
and lights the green Enable LED DS4
on the front panel. The high at the
drain of Q5 is also applied through CR5
to J4, pin 7, which is a high enable, to
the +32-VDC switching power supply,
switching it on.
If there is an overtemperature fault,
which is a low applied to J4, pin 3, the
low connects through CR4 and
overrides the high enable, switching off
the switching power supply. As long as
the fault is present, the switching
power supply is off and the red
Overtemperature LED DS3 on the front
panel is lit.
4.7.15.1 Phase Control
The phase control pot on the front of
the tray connects to J8-1 (+32 VDC),
J8-3 (phase I/P), and J8-4 (RTN) on
the board. The phase-control output
connects from J7 on the board to the
input of the phase-control circuit on the
variable gain/phase board (1301549).
4.7.15.2 Automatic and Manual Gain
Control Circuits
The amplifier control board contains the
AGC function for the UHF amplifier tray
in which it is mounted. An AGC
reference-level input from the UHF
exciter tray is applied to J3 pin 1, and
is amplified by U3B. The output of U3B
is connected to J5, which is wired to
the front panel gain pot that sets the
output power level of the tray when the
AGC is in the Auto position. The voltage
at the arm of the front panel gain pot is
amplified by U2D and is compared to a
sample of the output power of the tray
in U2A. The error voltage from U2A is
sent through Auto/Manual switch S1 to
J10, which connects to the pin-diode
attenuator circuit on the variable
gain/phase board. A sample of the AGC
voltage level is connected to position 1
on the front panel meter switch. The
tray can also be operated in manual
gain by switching S1 to the Manual
position and adjusting R16 for the
desired output power level.
FETs Q1 and Q3 delay and slowly
reapply the AGC voltage to the variable
gain/phase board when the system is
switched on or when the board is
switched from Auto to Manual (or back)
to prevent the overdriving of the tray.
4.7.15.3 Overdrive Circuit
A sample of the output of the 40 Watt
amplifier assembly, from the coupler
board assembly connects to J11 on the
amplifier control board. The sample is
peak detected by CR7 and U3A and the
output is connected to U3D.
If the input drive level increases above
the overdrive threshold reference set
by R71, which is the drive level needed
to produce 110% output power, the
output of U3D goes high and is split
three ways. One of the highs is
connected through R38 and CR3 to
U3C, causing its output to go high and
lighting the red Overdrive LED DS2 on
the front panel. Another of the highs is
connected through R74 and R75 to Q6,
which is biased on, causing its output
to go low. The low is connected through
J10 to the variable gain/phase board
that cuts back its output level.
The final high from U3D is connected
through CR2 and R37 to U1D, which is
biased on, causing its output to go
high. The high is connected to U2A,
whose output decreases and its output
power is cut back. If this path is not
present, the AGC, because the forward
power decreases, will try to drive the
variable gain/phase board harder