Assembly Instructions Chapter 4

4-6 kW UHF Translator Chapter 4, Circuit Descriptions
837B, Rev. 0 4-6
Voltages for Operation of the Tray
The AC input to the Tray is 117 VAC or
230 VAC and is directed thru Jack J2, of
the (A1) Power Entry Module (1265-
1104), to the step down Toroid (A2).
The Power Entry Module contains an
On/Off Switch, a 4 Amp Slo-Blo Fuse
and three MOVs which protect the Tray
from transients or surges which may
occur on the AC Input Lines. When the
On/Off Switch is switched On, AC is
applied to the (A2) Toroid. The Toroid
steps down the voltage into two 16 VAC
outputs which are fed to (A3) the
+12V(3A)/-12V Power Supply Board
(1092-1206).
4.1.14 (A3) +12V(3A)/-12V Power
Supply Board (1092-1206) ;
Appendix D)
The 16 VAC Inputs are connected to
the two full wave bridge networks one
for +12 VDC and one for -12 VDC. The
output of the +12 VDC rectifier is fed to
three 7812 IC regulators (U1, U2 and
U3) and the output of the -12 VDC
rectifier is fed to one 7912 IC regulator
(U4). The ±12V Power Supply Board
provides the voltage regulated and
current limited +12 VDC and -12 VDC
to the rest of the boards in the Tray.
+12VDC for External Preamplifier
+12 VDC is also applied through a 1
Amp Fuse F1 to (A7) the input DC
Multiplexed UHF or VHF Filter. The +12
VDC is multiplexed in the Filter onto the
input coaxial cable that connects from
the (Optional) Remote Preamplifier Unit
to the Receiver Tray. This supplies the
Preamplifier with the +12 VDC needed
for operation. The Red LED DS1
mounted on the Terminal Block TB1 will
be lit if the +12 VDC is applied to the
coaxial cable. Note
: If the Red LED,
DS1, is lit, the +12 VDC may damage
Test Equipment that is connected to the
input of the Receiver Tray. If a
Preamplifier Assembly is not part of
your System, F1 should be removed,
therefore DS1 should not be lit and the
+12 VDC is not multiplexed onto the
input coaxial cable. A spare Fuse for F1
is supplied and stored near the fuse
holder for F1.
4.2 (A4) UHF Exciter (1245-1100;
Appendix C)
4.2.1 (A4) (Optional) Aural IF
Synthesizer Board, 4.5 MHz (1265-
1303; Appendix D)
The aural IF synthesizer board
amplifies each of the three possible
audio inputs and the amplifier circuits
that supply the single audio output. The
balanced audio or the composite audio
input is connected to the board while
the subcarrier audio (SCA) input can be
connected at the same time as either of
the other two inputs. The board has the
4.5-MHz voltage-controlled oscillator
(VCO) and the aural modulation
circuitry that produces the modulated
4.5-MHz output. The board also
contains a phase lock loop (PLL) circuit
that maintains the precise 4.5-MHz
separation between the aural (41.25
MHz) and the visual (45.75 MHz) IF
frequencies.
4.2.1.1 Balanced Audio Input
The first of the three possible baseband
inputs to the board is a 600-Ω,
balanced-audio input (+10 dBm) that
enters through jack J2, pins 1 (+), 2
(GND), and 3 (-), and is buffered by
U1B and U1C. Diodes CR1 to CR4
protect the input stages of U1B and
U1C if an excessive signal level is
present on the input leads of jack J2.
The outputs of U1B and U1C are
applied to differential amplifier U1A;
U1A eliminates the common mode
signals (hum) on its input leads. A
pre-emphasis of 75 ms is provided by
R11, C11, and R10 and can be
eliminated by removing jumper W5 on
J5. The signal is then applied to
amplifier U1D whose gain is controlled