Model 8640B
TM 9-4935-601-14-7&P
SERVICE SHEET 6
PRINCIPLES OF OPERATION
General
The A5 FM Amplifier Assembly, in conjunction with the A9 Peak Deviation and
Range Switch and the A7 Shaping Assembly, conditions the modulation signal
to drive the varactor diodes which frequency modulate the RF oscillator.
Modulation signals may be do or ac coupled.
Input and Buffer Circuits (A5)
The FM modulating signal is applied to the Deviation Vernier which presents a
600Q impedance to the modulation source. Buffer Amplifier U1 is internally
connected as a voltage follower. The output of U1 drives the meter attenuator
(Service Sheet 8), the over deviation detector (Service Sheet 8), and the FM
Gain Compensation circuit through relay K1. When the FM switch is OFF, or if
the PEAK DEVIATION switch is set to an unallowable position, the relay is
de-energized and the signal path to the FM and meter circuits is opened. FM
gain compensation potentiometer A3R2 is geared to the FREQUENCY TUNE
control and adjusts the gain of the circuit. FM sensitivity is higher for higher RF
oscillator frequencies and the FM Gain Compensation circuit reduces the
modulation circuit drive at higher frequencies. The gain compensation
adjustment potentiometers (A3A4R2, R3, and R4) with unity gain isolation
amplifier A3A4U1, set the FM sensitivity at the frequency mid-point and
extremes, A3A4R5 adjusts the offset of A3A4U1. The output of the FM Gain
Compensation circuit drives the FM Amplifier input.
Amplifier (A5)
The FM Amplifier is a non-linear, feedback amplifier which drives the varactor
diodes in the RF oscillator. The amplifier and shaping circuits correct for the
non-linear tuning sensitivity of the RF oscillator by the varactor diodes. The
correction for the negative excursions of the modulation signal is provided by
the negative shaping circuit (Service Sheet 7 ) which follows the amplifier
output. Correction for positive excursions is provided by the positive shaping
circuit (Service Sheet 7) which is part of the amplifier feedback path.
Transistors Q1 through Q4 form a two-stage differential input amplifier. The
dual transistors Q1 and Q2 are connected in a Darlington configuration to
provide matched, high impedance inputs. Amplifier offset adjustment, R8
adjusts the do offset. The gain of the first stage is approximately one-half the
ratio R4/R3; gain for the second stage is approximately one-half the ratio
R5/R6.
Transistors Q5 through Q8 form an intermediate driver stage. The voltage gain
of the stage is approximately twice the ratio of the impedance across R27 to
that of R17.
The shaping circuits require more gain for large positive voltage excursions.
For low positive voltages, the resistor network R29 to R34 is in parallel with
R27. As the voltage increases' diodes CR10, 11, and 12 respectively switch off
and increase the impedance across R27 and thereby increase the amplifier's
gain.
Transistors Q9 through Q12 form the amplifier output stage. Transistors Q9
and Q10 are in a Darlington configuration and supply current to the load during
positive excursions Transistors Q11 and Q12 are in an inverted Darlington
configuration and sink load current during negative excursions.
SERVICE SHEET 6 (Cont'd)
Amplifier Configurations
The FM Amplifier is switched by the AS Peak Deviation and Range Switch into
three different configurations depending on the gain needed. For gains less
than 0 dB, the amplifier is in a unity gain configuration followed by the positive
shaping network (Service Sheet 7) which has little effect; an attenuator, which
determines the overall gain; and the negative shaping network (Service Sheet
7) which has only a small effect. The effect of the shaping networks is small
because voltage swings are small and the tuning characteristic of the varactor
diodes is fairly linear over the narrow range of operation. For 0 dB gain, the
amplifier is in a unity gain configuration, the positive shaping network and
attenuator have no effect, and the negative shaping network has a small
effect. For gains greater than 0 dB, the attenuator is in the feedback path and
the gain is inversely proportional to the feedback attenuation. The positive
shaping network is also in the feedback path and for large positive voltage
excursions it increases the feedback attenuation and hence increases the
amplifier gain. The negative shaping network is in the output path, and for
large negative voltage excursions, the output attenuation is increased and the
overall amplifier gain decreases.
Attenuator (A9)
Before entering the feedback path, the FM Amplifier output passes through an
attenuator formed by resistors R4 through R7 which reduces the open-loop
gain of the amplifier when only small closed-loop gain is needed. The gain
control attenuator used in the feedback or output of the amplifier is formed by
resistors R12 through. R22.
TROUBLESHOOTING
It is assumed that a problem has been isolated to the FM amplifier circuits 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 Me procedures outlined in the table.
Test Equipment
Digital Voltmeter .......................................................HP 3480B/3484A
Oscilloscope . ..................................................HE, 180A/1801A/1820C
NOTE
Use a 10 k
Ω
resistor, in series with the DVM probe tip, to
reduce spurious oscillations in the amplifier circuitry
while making dc measurements.
Initial Test Conditions
Bottom cover removed (see Service Sheet F for removal procedure). Extend
A5 FM Amplifier Assembly on extender board. Remove A7 FM Shaping
Assembly from chassis and disconnect cable A9W1 from A7J1.
SERVICE SHEET 6 (Cont'd)
Initial Control Settings
MODULATION FREQUENCY ..............................................400 Hz
FM ............................................................................................ INT
PEAK DEVIATION ................................................................. 5 kHz
PEAK DEVIATION Vernier .................................................. Full cw
RANGE .........................................................................0.5 - 1 MHz
FREQUENCY TUNE .........................................................Centered
(Four turns from stop)
RF ON/OFF ...............................................................................ON
FM Amplifier Troubleshooting
Component
or Circuit
Test Conditions
and Control
Settings
Normal
Indication
If Indication
is Abnormal
Buffer
Amplifier (A5)
Initial conditions and
settings. Adjust
PEAK DEVIATION
vernier for 2 Vp-p at
TP5(BUFFER IN).
2 Vp-p at TP6
(BUFFER OUT)
Check U1 and
associated
circuitry
FM Gain
Initial conditions
1 Vp-p at TP2
Check U1 and
Compensation
and settings. Ad-
(amplifier out-
associated
(A3A4)
just PEAK DEVI-
put)
circuitry
Circuit
ATION vernier for
1 Vp-p at U1 pin
3.
FM Amplifier
(A5)
Initial conditions and
settings. Adjust
PEAK DEVIATION
vernier for 2 Vp-p at
TP5(BUFFER IN).
Peak-to-peak
voltages at TP3
(+ INPUT) and
TP4 (— INPUT)
are the same
Set FM to
OFF and use
DVM to check
dc voltages
shown on
schematic
Switch RANGE
through all ranges
system and check
gain
Gain in
accordance with
FM gain table on
schematic.
Check
switching of
A9
RF
Oscillator (A3)
SERVICE SHEET 5
Figure 8-22. P/O A3A4 Connector Board Assembly Component
Locations
A13
Components Locations for A13 Assembly are on Service Sheet 25.
Figure 8-23. P/O A9 Peak Deviation and Range Switch Assembly
Component Locations (1 of 2)
8-28
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