User's Manual

Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT835A, Rev. 1 4-22
amplifier U1A and 100-Ω resistor R87
to position 2 of switch S3. The ALC
sample connects to the board at J6-1
and is connected through buffer
amplifier U2C; R15, the ALC calibration
pot that adjusts the output of U2A, pin
1; through 100-Ω resistor R18; and to
position 1 on switch S3.
Typical readings on the meter are:
• % Reflected = <5%
• % Forward Power = 100%
• % Exciter = The level on the meter
needed to attain 100% output
power from the transmitter
Refer to the test specifications sheet for
the transmitter for the actual readings
for the items shown below:
• ALC = .8 VDC
U6 is a temperature sensor IC that
gives the operator the ability to
measure the temperature inside the
tray by measuring the voltage at TP1.
The sensor is set up for +10 mV equals
1° F (for example, 750 mV equals 75°
F).
4.3.10.6 Operational Voltages
The +12 VDC needed for the operation
of the board enters the board at jack
J4, pin 3. C28, L1, and L3 are for the
filtering and isolation of the +12 VDC
before it is split and applied to the rest
of the board. The -12 VDC needed for
the operation of the board enters at
jack J4, pin 5. C29 and L2 are for the
filtering and isolation of the -12 VDC
before it is split and applied to the rest
of the board.
When the +12 VDC is connected to the
board, it is split. Four of the +12 VDC
outputs are fed out of the board at J8-
16, J8-17, J8-18, and J8-19 through
diode CR7, CR8, CR9, or CR10 and
resistor R50, R51, R52, or R53 to any
external amplifier trays for use in their
logic circuits. The resistors are for
current limiting and the diodes are to
prevent voltage feedback from the
external amplifier trays.
4.3.11 (A3)+12V(4A)/-12V(1A)
Power Supply Board (1265-1312;
Appendix D)
The ±12-volt power supply board
consists of three separate power
supplies, two of which produce the +12
VDC and one for the -12 VDC needed
to power the circuit boards in the
exciter tray.
4.3.11.1 +12 VDC Power Supply
The 18 VAC from the external step-
down toroid in the tray connects to J1,
pins 7 and 8. The 18 VAC connects
through 7-amp fuse F2, for over-
current protection, to two full-wave
bridge rectifier circuits.
CR1, CR2, CR3, and CR4 rectify the AC
that is then filtered by C2 and
connected to U3 and U4; U3 and U4
are voltage-regulator ICs for the +12
VDC. The voltage-regulated and
current-limited +12-VDC output of U3
connects to J4, pins 1, 2, 3, and 4,
which is one of the +12-VDC outputs of
the board. The green LED DS3 will be
lit if +12 VDC is present to J4; C7 and
C8 are bypass capacitors. The +12 VDC
output of U4 connects to J5, pins 1, 2,
3, and 4, which is another one of the
+12-VDC outputs of the board. The
green LED DS4 will be lit if +12 VDC is
present to J5; C9 and C10 are bypass
capacitors.
CR5, CR6, CR7, and CR8 rectify the AC
that is then filtered by C1 and
connected to U1 and U2; U1 and U2
are voltage-regulator ICs for the +12
VDC. The voltage-regulated and
current-limited +12-VDC output of U1
connects to J6, pins 1, 2, 3, and 4,
which is one of the +12-VDC outputs of
the board. The green LED DS1 will be