User's Manual

3300-Watt VHF Low Band Transmitter Chapter 4, Circuit Descriptions
335B, Rev. 0 4-13
sample of the corrected IF is provided at
TP2. The IF output is normally connected
to an external IF phase corrector board.
4.1.5.10 Main IF Signal Path (Part 3 of 3)
After the IF signal passes through the
external IF phase corrector board, it
returns to the ALC board at IF input jack
J7 (0 dBm). The IF then passes through
a bandpass filter consisting of L20, C97,
C62, L21, L22, L23, C64, C99, and C63,
which is adjustable for best response.
This bandpass filter is intended to make
up for small errors in frequency response
that are incurred by the signal while
being processed through the linearity and
incidental phase correction circuits.
Following the bandpass filter, the signal
is split using L24, L25, and R89. The
signal passing through L24 is the main IF
path through the board. A sample of the
corrected main IF signal is split off and
connected to J10, the IF sample jack.
The main IF, whose level is controlled by
R99, connects to jacks J27 and J28.
These jacks control if a 6-dB pad is
included in the circuit by the positioning
of the jumpers W9 and W10. The 6-dB
pad is in when the jumpers W9 and W10
are connected between pins 2 and 3 on
J27 and J28. The 6-dB pad is out when
jumpers W9 and W10 are connected
between pins 1 and 2 on J27 and J28.
Normally, the pad is jumpered out. The
IF signal is then applied to a two-stage,
frequency-response corrector circuit that
is adjusted as needed. The variable
resistors R103 and R106 adjust the depth
and gain of the notches and the variable
capacitors C71 and C72 adjust the
frequency position of the notches. The
corrected IF signal is amplified by U13
and U14 before it is connected to J12,
the IF output jack of the board. The
output level is set for 0 dBm by R99.
The combined IF output of the ALC board
connects to (A11-A2) the filter/mixer
board. A sample of the IF is fed to J11 to
provide an IF sample point that can be
monitored without breaking the signal
path and gives an indication of the IF
signal after the linearity and the
frequency-response correction takes
place.
4.1.5.11 ALC Circuit
The other path of the corrected IF signal
is used in the ALC circuit. The IF is wired
out of the splitter through L25 and
connects to op-amp U12. The output of
U12 is wired to jacks J8 and J9 on which
jumpers W4 and W8 control the normal
or encoded operation of the ALC circuitry.
For normal operation, jumper W4 on J8
and jumper W8 on J9 are between pins 1
and 2. The IF signal is applied to the
transformer T5 that doubles the voltage
swing by means of a 1:4 impedance
transformation. The IF is then connected
to the ALC detector circuit on the board,
amplified by U10B and applied to jacks
J26 and J21.
For normal operation, jumper W7 on J26
and jumper W5 on J21 are between pins
1 and 2. The detected ALC voltage is
wired to U10A pin 2, where it is summed
with the front panel power control
setting. The output power adjustment
for the transmitter is controlled by the
screwdriver adjust pot R1, located on the
front panel of the VHF exciter tray. If the
(Optional) remote power raise/lower kit
is purchased, the power is set by
adjusting R75, a motor-driven pot
controlled by the switch S1 on the board.
An external power raise/lower switch can
be used by connecting it to jack J10, at
J10-11 power raise, J10-13 power
raise/lower return, and J10-12 power
lower, on the rear of the VHF exciter
tray. S1, or the remote switch, controls
relays K1 and K2, which control the
motor M1 that moves variable resistor
R75. If the (optional) remote power
raise/lower kit is not purchased, the ALC
voltage is controlled only by screwdriver
adjust pot R1 on the front panel of the
VHF exciter tray. The ALC voltage is set
for .8 VDC at TP4 with a 0 dBm output at
J12 of the board. A sample of the ALC at
J19 pin 2, is wired to the transmitter
control board where it is used on the
front panel meter and in the AGC circuits.