Model 8640B TM 9-4935-601-14-7&P Model 8640B TM 9-4935-601-14-7&P
SERVICE SHEET 7
PRINCIPLES OF OPERATION
General
The FM shaping networks, in conjunction with the FM amplifier (Service
Sheet 6) and the peak deviation attenuator, condition the modulation signal
to drive the RF Oscillator's varactor diodes which electrically tune the
oscillator. The tuning sensitivity of the oscillator with respect to the
modulation input decreases as the tuning voltage becomes more positive.
The shaping networks compensate for the non-linear tuning characteristic.
In addition, the varactor diodes can be tuned by the phase lock circuits to
synchronize the RF oscillator with an accurate and stable reference
oscillator.
FM Enable (A7)
The varactor diode cathodes are switched by FM Enable reed relay K1
either to R39 when the FM is disabled or to the amplifier output when the
FM is enabled. The relay is energized only when the FM switch is not OFF
and when the PEAK DEVIATION and RANGE switches are set to an
allowable combination. The maximum peak FM deviation possible is 1% of
the output frequency at the low end of a range (e.g., 2.56 MHz deviation on
the 256 512 MHz range). The PEAK DEVIATION and RANGE switches,
however, can be set to combinations that exceed this (e.g., 2.56 MHz
deviation on the 2 4 MHz range). For such unallowable combinations, the
FM amplifier is disabled (by A5K1 on Service Sheet 6), the varactor diode
cathodes are grounded (by A7K1), the meter input is opened (by A5K1),
and the REDUCE PEAK DEVIATION annunciator lamp A6DS2 is turned on
(see Service Sheet 8). The interaction of the PEAK DEVIATION switch
and the RANGE switch is accomplished by differential gearing between the
two switches.
Positive and Negative Shaping (A7)
The Positive Shaping network presents an increasingly lower impedance to
the input as the input voltage increases. Resistors R11, R12, and R13 set
the base voltage of transistor Q5, and Q5 sets the voltage supply to the
resistor-diode ladder. Transistor Q6 supplies most of the current.
Capacitor C5 keeps the base of Q5 at an ac ground potential. Diode CR9
protects Q6 in the event of a shorted +20V supply. The base-emitter
junction of Q5 temperature compensates the diodes of the ladder near it.
Transistor Q7 sets the voltage at the other end of the resistor-diode ladder
at one diode junction drop below ground; it also temperature-compensates
the diodes of the ladder near it. Transistor Q8 is a current sink. Capacitor
C6 frequency-stabilizes Q7 and Q8. The diode cathodes in the ladder
between Q7 and Q5 are at increasingly higher potentials. As the voltage at
the input to the ladder increases, the diodes turn on consecutively and the
FM Amplifiers (A3, A5, A9, A13)
SERVICE SHEET 6
SERVICE SHEET 7 (Cont'd)
impedance at the input lowers. The Negative Shaping network is
analogous to the Positive Shaping network except the polarity of all
voltages is reversed, the diodes are reversed, all transistors are
complemented, and the shaping characteristic is modified.
Phase Lock Loop Filter (A7)
The Phase Lock Loop Filter is a 17 Hz active elliptic-function low-pass filter
which filters the phase detector error voltage and drives the varactor diode
anodes (on Service Sheet 5). The phase lock input may vary from +5 to
+15V; the voltage at the varactor anodes varies from -13.6 to -16V with a
quiescent value adjusted by R19.
TROUBLESHOOTING
It is assumed that a problem has been isolated to the FM shaping circuits or
to the phase lock loop filter as a result of using the troubleshooting block
diagrams. Troubleshoot by using the test equipment listed below,
performing the initial test conditions and control settings, and following the
procedures outlined in the table.
Test Equipment
Digital Voltmeter............................................................... HP 3480B/3484A
Initial Test Conditions
Bottom cover removed (see Service Sheet F for removal
procedure). Extend A7 FM Shaping Assembly on extender
board.
Initial Control Settings
COUNTER MODE LOCK.....................................................OFF
Positive and Negative Shaping
A trouble in one of the shaping circuits will usually cause FM
sensitivity, distortion, and meter accuracy to be out of
specification and will also prevent FM linearity from being
correctly adjusted. The quickest way to troubleshoot the
shaping circuits is to use the ohms function of the DVM to check
the components.
Phase Lock Loop Filter
A trouble in the loop filter will either prevent the generator from
becoming phase-locked or prevent frequency modulation at low
modulation rates. Trouble might also cause an increase in SSB
noise or residual FM while phase-locked. A low voltage at
A7TP2 (VARACTOR ANODE) may indicate a faulty Phase Lock
Loop Filter or Varactor Assembly (shown on Service Sheet 5).
FM Shaping Circuits and Phase Lock Loop Filter Troubleshooting
Component Test Conditions and If Indication
or Circuit Control Settings Normal Indication is Abnormal
POSITIVE and Remove A7 Assembly Components check good Replace faulty compon-
NEGATIVE from chassis. Check com- ent
SHAPING (A7) ponent resistances with
DVM.
PHASE LOCK Initial conditions and Voltages check good Check counter phase
LOOP FILTER settings. Check voltages lock circuits and U1
(A7) shown on schematic.
Remove A7 Assembly Components check good Replace faulty compon-
from chassis. Check com- ent
ponent resistances with
DVM.
8-30
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