User's Manual
300-Watt Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT830A, Rev. 1 4-28
Table 4-1. Fuses, Idling Currents, and Voltage Settings for the Class AB Amplifier Devices
SWITCH
POSITION
AMPLIFIER
MODULE
TRANSISTOR
DEVICE
BIAS
ADJUST
POT
IDLING
CURRENT
VOLTAGE
SETTING
FUSE
I1 A4-A1 Q1 R6 *5 Amps 50 mV F1
I2 A4-A3 Q201 R206 600 mA 6.0 mV F2
I3 A4-A3 Q101 R106 600 mA 6.0 mV F3
I4 A5-A2 Q201 R206 300 mA 3.0 mV F4
I5 A5-A2 Q101 R106 300 mA 3.0 mV F5
I6 A5-A3 Q201 R206 300 mA 3.0 mV F6
I7 A5-A3 Q101 R106 300 mA 3.0 mV F7
I8 A5-A4 Q201 R206 300 mA 3.0 mV F8
I9 A5-A4 Q101 R106 300 mA 3.0 mV F9
I10 A5-A5 Q201 R206 300 mA 3.0 mV F10
I11 A5-A5 Q101 R106 300 mA 3.0 mV F11
*The A4-A1 transistor Q1 operates class A and is adjusted for 5 amps of operating
current.
The voltage drop across the selected
resistor using switch S1 is read with a
digital voltmeter (DVM) that is connected
from E1 to E2 on the board. This voltage
reading converts to the idling current,
with no RF drive applied, or the operating
current, with RF drive applied, of the
transistor.
4.3.11 (A8) Amplifier Control Board
(1265-1414; Appendix D)
The amplifier control board provides LED
fault and enable indications on the front
panel of the tray and also performs the
following functions: automatic gain
control (AGC); overdrive cutback, when
the drive level reaches the amount
needed to attain 110% output power;
and overtemperature, VSWR, and
overdrive faults. The board also provides
connections to the front panel meter for
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 to 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 +26-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.