TM 11-6625-2781-14&P-2
produce a 200 MHz IF signal. This signal is amplified and
SERVICE SHEET 4
filtered prior to being applied to the Second Converter
Assembly A10.
It is assumed that the rf input signal, the first oscillator signal,
and dc operating voltages are present, and that the 200 MHz
The following components are factory selected values:
output signal of the bandpass filter is low or missing.
C2 in the A9A2 sub-assembly is selected to optimize
TROUBLESHOOTING PROCEDURE
mixer flatness.
When trouble has been isolated to the 200 MHz IF assembly
C8 in the A9A1 amplifier adjusts the IF amplifier gain to
it should be removed from the casting and installed in an
14 dB.
inverted position using the fasteners provided in the Service
Kit to provide easy access to all components or major sub-
assemblies. Both the first mixer and the 200 MHz Filter are
R7 in the A9A1 amplifier is selected, for maximum
sealed units; repairs are possible, but are not considered to be
flatness of the bandpass filter.
practical due to component matching and positioning
requirements.
The value of these components is not extremely critical. They
are selected at time of final assembly and test to ensure
NOTE: If repairs are required the adjustments
optimum compatibility with associated components.
specified in Paragraph 5-24 of Section V should
be performed.
2. FIRST CONVERTER SUB-ASSEMBLY
EQUIPMENT REQUIRED
The first converter is a double-balanced mixer driven by a 1V
Service Kit .......................................................HP 11592A
rms (200 to 310 MHz) signal from the first local oscillator. This
VHF Signal Generator ....................................... HP 608F
signal, applied to the mixer by the secondary of T2, drives the
50-Ohm Termination......................................HP 11593A
diodes of two adjacent arms of the bridge alternately into
Vector Voltmeter...............................................HP 8405A
Cable Assembly .............................................HP 10501A
is induced in the secondary of T3, with its polarity changed at
BNC Tee . .....................................................UG 274 B/U
the rate of the local oscillator frequency. This results in a
suppressed local oscillator signal with sidebands at plus and
CONTROL SETTINGS
minus the rf input signal frequency. The 200 MHz IF amplifier
then selects and amplifies the lower frequency sidebands.
CR1 and CR2 limit rf input signals to protect the mixer quad
INPUT ATTENUATION . ..........................................0 dB
from burn-out. C1 and C2 insure maximum flatness near 100
TUNING STABILIZER.................................................ON
MHz. Transformers T1 and T4 are baluns. T1 balances the
SCAN WIDTH ..........................................PER DIVISION
first local oscillator signal about ground, T4 unbalances the
FREQUENCY....................................................... 30 MHz
mixer output with respect to ground.
BANDWIDTH ...................................................... 300 kHz
SCAN WIDTH PER DIVISION .............................20 kHz
TEST PROCEDURE
1. 200 MHz IF ASSEMBLY A9 (General)
Since signals other than the desired 200 MHz IF are present
at the output of the mixer it is impractical to attempt evaluation
The mixer section of the 200 MHz IF assembly mixes the rf
of circuit performance by monitoring the mixer output directly
input signal from the 120 MHz low pass filter with the first local
with the Vector Voltmeter. A simple but conclusive test which
oscillator signal to
will isolate signal loss to a defective first mixer may be
conducted as follows: Set the analyzer controls to display the
first local oscillator feedthrough signal. SCAN WIDTH PER
DIVISION -10 kHz; BANDWIDTH -3 kHz; FREQUENCY -0
MHz. The signal displayed on the analyzer CRT should be
about -25 dBm. If the signal observed is -15 dBm or greater
the mixer is damaged (unbalanced) and repair or replacement
is
SERVICE SHEET 3.
RF Input, Attenuator Control & 120 MHz Low Pass Filter
Figure 8-27. Functional Block Diagram.
8-30
