TM 11-6625-2781-14&P-2
R19 is selected to provide the proper output level to
SERVICE SHEET 5
the rear panel output jack.
R28 is selected to compensate for differences in tank
It is assumed that the tuning ramp voltage from Voltage
circuit components.
Control Assembly A5 and dc operating voltages are present
and correct and that one or more of the output signals is
The three major circuits of the VTO Assembly (Oscillator,
missing or out of tolerance.
Power Amplifier, and Buffer Amplifiers) are discussed.
separately in steps, 1, 2 and 3.
TROUBLESHOOTING PROCEDURE
4. FIRST LOCAL OSCILLATOR
When trouble has been isolated to the Voltage Tuned
Oscillator Assembly, the assembly should be removed from
The principal resonant tank circuit components consist of L4,
the casting and reinstalled in an inverted position to provide
L5, and C6. The capacity of C6, which determines the
easy access to all components. Troubleshooting information
frequency at which oscillation occurs, is controlled by a
follows the technical discussions of individual circuits.
voltage ramp supplied from the Voltage Control Assembly.
It is suggested that the output levels specified in Step 3Test
The 180 degree phase shift across the resonant tank circuit is
Procedure, Table 1, be checked first. If any of the output
complemented by a 180 degree phase shift across the
levels are as specified, trouble may be in one or more of the
oscillator Q2/Q3. This is accomplished by operating Q2 and
buffer amplifiers, and steps 1 and 2 need not be performed.
Q3 past their beta cut-off frequencies to provide an apparent
phase shift of 90 degrees per stage. (Actually, the phase shift
EQUIPMENT REQUIRED
is 270 degrees per stage, but the end result is the same.)
Operating the transistors in this mode maintains a relatively
Vector Voltmeter .......................................... HP 8405A
constant output level across the operational range of 200 to
VHF Signal Generator ...................................HP 608F
310 MHz. CR1 and CR2 limit the input to Q3 which operates
Service Kit ................................................... HP 11592A
as a class B amplifier.
CONTROL SETTINGS:
TEST PROCEDURE
(Unless otherwise specified in individual tests).
SCAN WIDTH ....................................................ZERO
obtained, failure of the first local oscillator may be verified by
TUNING STABILIZER .............................................On
injecting a signal into the emitter circuit (TP D) of the Power
SCAN WIDTH PER DIVISION ........................20 kHz
Amplifier Q4.
FREQUENCY......................................................0 MHz
The input from the Voltage Control Assembly should be
200-310
MHz VOLTAGE
TUNED
OSCILLATOR
disconnected and a signal injected at TP D (Q4-e) from the
ASSEMBLY (General)
310 MHz may be used. Use the HP 8405A to verify the
The first local oscillator is a voltage tuned oscillator which,
presence or absence of signals at J1, J3 and J4. (The signal
when operated in the swept mode, provides an output of 200
levels for this test are unimportant and will vary with output
to 310 MHz or selected frequencies between these two limits.
from the signal generator.) If the output signals are not
The output of the oscillator is amplified and fed to three
present, proceed to step.2.
outputs through individual buffer amplifiers. In narrow scan
widths the local oscillator is not swept and when the analyzer
If the output signals are present with an input applied to TP D,
is operated with the TUNING STABILIZER on the first local
check Q2, Q3 and associated components.
oscillator output is sampled in the APC assembly by pulses
developed from the 100 kHz reference signal. These samples
2. POWER AMPLIFIER
are used to develop an error voltage which is processed in the
Voltage Control Assembly to phase lock the first local
Power Amplifier Q4 is a grounded base amplifier. C13 peaks
the gain of the amplifier at the first local oscillator frequencies,
prevents the stage from oscillating, and by-passes high order
The following components are factory selected values:
SERVICE SHEET 4
R2 is selected to provide the proper adjustment
First Converter and 200 MHz IF Amplifier.
range of R3.
R17 is selected to optimize the VTO output flatness.
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