User's Manual

Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT835A, Rev. 1 4-40
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 that uses 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.
4.8.12 (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 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