User's Manual

2650 Watt VHF High Band Transmitter Chapter 4, Circuit Descriptions
435B, Rev. 0 4-29
The front panel Meter (A9), using (S1)
the Front Panel Selector Switch, monitors
the AGC Voltage, % Output Reflected
Power, % Forward Power and the
Switching Power Supply Voltage (+28
VDC). The Meter in the AGC position will
read anywhere from 1 Volts to 2 Volts.
The Meter is calibrated in the Power
Supply position using R86 located on the
AGC Control Board. The % Output Power
is calibrated using R44 and the %
Reflected Power is calibrated using R53,
located on the AGC Control Board. With
S1 in the Current Position, S2 is switched
to read the Idling Currents, no RF Drive
applied, of the High Band Output
Amplifier Boards.
Typical operating readings are an RF
Current of 12 Amps for the three final
P400 Amplifier Pallets I
1
, I
2
& I
3
positions
and 6 Amps for the P200 High Band
Amplifier pallet in the I
D
position. These
readings are with 100 % output power
for the transmitter with a black picture.
The Reflected Power Sample, from the 3
Way Combiner Board, is fed back to the
AGC Control Board at J5. On the board
the Reflected Sample is connected thru
the detector circuit to a VSWR Cutback
circuit (U13C). If the Reflected power
increases above 20 %, the output power
of the Tray, as set by R60 (VSWR
Cutback) on the AGC Control Board, will
be cutback to maintain a 20 % Reflected
Output level. The Red LED (DS4) VSWR
Cutback, located on the front panel, will
remain lit until the Reflected level drops
below 20 %.
There are three Thermal Switches in the
amplifier Tray for overtemperature
protection. Two of the Thermal
Switches (A4-A5 & A4-A6) are mounted
on the rear of (A4) the heatsink for the
High Band Amplifier Pallets and the third
Thermal Switch (A5-A2) is mounted on
the heatsink for (A5-A1) the 3 Way
Combiner Board. The Thermal Switches
close when the heatsink on which it is
mounted reaches a temperature of
175° F. The closed Thermal Switch
causes the AGC Control Board, to
remove the Enable to the Switching
Power Supply, eliminating the +28 VDC,
and also to light the Red LED Indicator,
Overtemperature (DS5), mounted on
the front panel. The AGC Control Board
extinguishes the Module Status LED
(DS3).
This completes the circuit descriptions for
the VHF amplifier tray, and also for the
other trays, assemblies, subassemblies
and boards that make up the 435B
Transmitter.