
SERVICE SHEET 12
PRINCIPLES OF OPERATION
General
The A26A3 Modulator Assembly contains a current
controlled attenuator which varies the RF output level. The
A26A4 AGC Amplifier controls the drive to the modulator.
Attenuation is determined by the OUTPUT LEVEL vernier
and by the AM input signal when the AM switch is on or by
the pulse input signals when AM is set to PULSE.
Modulator (A26A3)
The RF signal from the binary dividers is amplified by
Modulator Preamplifier A26U2. The amplifier is a sealed
microcircuit that plugs into the rear of the modulator board.
The amplifier drive: the AUX RF OUT port through resistor
R1 and drives the modulator diodes. Diodes CR1 through
CR8 form a balanced resistive network in which the
resistance is controlled by the current biasing them.
Capacitors C3 through C6 improve the modulator balance at
high frequencies. The control current come, from the AGC
output amplifier through choke L2 and then split, between
R4, CR1 to CR4, and R3 or R5, CR5 to CR8, and R2. The
RF signal is coupled into the modulator through T1 and out
through T2. The modulator output drives the RF filters
(Service Sheet 10).
AGC Amplifier (A26A4)
The AGC Amplifier sums the negative detector output from
the A26A1 Detector Buffer Amplifier (Service Sheet 13) with
the positive AGC reference voltage from the OUTPUT
LEVEL vernier A1R1 (Service Sheet 16). The input to the
vernier is a 2 Vdc reference voltage upon which may be
superimposed an AM input signal (±2 Vpk for 100% AM).
When AM is set to PULSE, the amplifier's output (and
therefore, the modulator) is switched on and off by the input
pulses.
Summing Amplifier
Transistors Q1, Q2, and Q3 form a Summing Amplifier. The
output of Q3 is the amplified sum of the detector and
reference currents and represents the output level error.
Resistor R1 is adjusted to give the correct RF output voltage
corresponding to the AGC reference. When the internal
frequency doubler is used (512 to 1024 MHz RANGE in
Option 002 only), relay K1 switches summing resistors R2
and R5 into the summing line to adjust for the doubler
amplitude characteristic. Switch S1 allows the AGC circuits
to be tested in an open-loop condition.
SERVICE SHEET 12 (Cont'd)
Modulator Driver Amplifier
Transistors Q4, Q5, and Q6 form the Modulator Driver
Amplifier. R32 and C9 frequency-compensate the AGC
system. Capacitor C10 is switched in parallel with C9 in the
0.5-1 and 1-2 MHz ranges (called LO BAND 1) to give
added compensation. The LO BAND 1 line is grounded in
LO BAND 1 ranges and causes inverter U3B to go high and
inverter U4F (an open collector output gate) to go low which
switches in C10. In a similar manner capacitor C11 is
switched in parallel with C9 in the 2 4 and 4 8 MHz ranges
(called LO BAND 2). Transistor Q5 is a current source.
Transistor Q6 is a constant current sink. The difference
between the collector currents of Q5 and Q6 is the
modulator drive current.
In the pulse modulation mode of operation, Q5 is switched
on and off at the pulse repetition rate by transistor switch Q7
which is driven by the pulse Schmitt Trigger output of A26A2
(Service Sheet 13). When Q7 is on, Q5 and the modulator
are off, (i.e., when either the MOD PULSE line is low or
when the RF OFF line is low). Hot carrier diodes CR13 and
CR14 prevent saturation of Q7 and Q6 for rapid switching.
Capacitor C15 is switched in across the modulator drive line
by gates U3C and U4E to lower the rise and fall time of the
modulator in LO BAND 1 ranges to reduce RF ringing in the
filters following the modulator. Similarly, capacitor C16 is
switched in for LO BAND 2 ranges.
Pulse Overload Detector
In the pulse modulation mode, the peak detector in A26U1
(Service Sheet 13) samples the' RF output only when an
input pulse is present; when no pulse is present, the detector
output is stored on a capacitor. If the OUTPUT LEVEL
vernier is reduced while in the pulse mode, the error voltage
of the summing amplifier becomes very large and the
modulator is turned off. The detector storage capacitor then
discharges only during each pulse on period until the error is
zero. At low repetition rates and short on-periods, the
capacitor discharge time is very long. To correct for this,
Pulse Overload Detector U1B senses the condition of large
error (i.e., when the collector voltage of Q3 e0.4
Vdc) and switches a discharge resistor on to bring the
system to a near zero error condition.
Meter Amplifier
Amplifier U1A is an inverting amplifier with a gain of about
½ (adjusted by R12) which scales the detector output
voltage to drive the metering circuits. Capacitor C8 filters
any superimposed modulation signal on the detector output.
SERVICE SHEET 12 (Cont'd)
Modulator Overload Detector
If the OUTPUT LEVEL vernier setting or input modulation
signal requires the output to exceed its maximum capability,
Modulator Overload Detector U2 senses the condition and
lights the REDUCE PEAK POWER annunciator A6DS3
(Service Sheet 16). The reference voltage is set by
resistors R29, R30, and R31 and in addition by R28 when
the frequency doubler is used. When the output from the
OUTPUT LEVEL vernier exceeds the reference, the output
of U2 goes high and Q8 turns on which turns on Q9 and the
annunciator. Since the overload condition may be of short
duration, capacitor C13 holds the output of U2 high to keep
the annunciator lit for a longer period.
TROUBLESHOOTING
It is assumed that a problem has been isolated to the AGC
amplifier or the modulator 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
Oscilloscope. .........................................HP 180A/1801A/1820C
Initial Test Conditions
To test A26A4 AGC Amplifier Assembly, remove top cover
(see Service Sheet F for removal procedure), remove A26
AM/AGC and RF Amplifier Assembly casting top cover, and
remove A26A4 and extend for service (see Service Sheet E
for procedure).
To test A26A3 Modulator Assembly and A26U2 Modulator
Preamplifier, remove bottom cover (see Service Sheet F for
removal procedure) and remove A26 casting bottom cover
(see Service Sheet E for procedure).
Initial Control Settings
Meter Function ............................................................... LEVEL
COUNTER MODE:
EXPAND...........................................Off
LOCK ...............................................Off
Source ............................................. INT
AM....................................................................................... INT
MODULATION ................................................................. 100%
MODULATION FREQUENCY.......................................1000 Hz
RF Dividers (A10A2)
SERVICE SHEET 11
TM 9-4935-601-14-7&P
Model 8640B
SERVICE SHEET 12 (Cont’d)
FREQUENCY TUNE.................................................... 7.20 MHz
OUTPUT LEVEL ...........................................................+10 DbM
(switch one step ccw from full cw)
FM ........................................................................ OFF
OUTPUT LEVEL Vernier....................................... cw (+10 dBm)
RANGE ....................................................... 8 - 16 MHz
RF ON/OFF ...........................................................................ON
AGC Amplifiers and Amplitude Modulator Troubleshooting
Component
Test Conditions and
Normal Indication
If Indication
or Circuit
Control Settings
is Abnormal
SUMMING
Initial conditions and set-
≈
800 mVp-p sine wave
Check Q1, Q2, Q3 and
AMPLIFIER
tings. Set AGC switch, S1,
(1 kHz) at TP6 (CQ1)
associated circuitry
(A26A4)
to AGC off position.
MODULATOR
Initial conditions and set-
≈
2 Vp-p sine wave
Check Q4 and associated
DRIVER
tings. Set AGC switch, S1,
(1 kHz) at TP7 (DRVR)
circuitry
AMPLIFIER
to AGC off position.
≈
8 Vp-p sine wave
Check Q5, Q6, Q7 and
(A26A4)
(1 kHz) at TP8 (MOD)
associated circuitry
Set RANGE to 4 - 8 MHz
≈
7.2 Vp-p sine wave
Check U3A, U3D,
(1 kHz) at TP8 (MOD)
U4B, U4D, and associated
circuitry
Set RANGE to 1 - 2 MHz
≈
6 Vp-p sine wave
Check U3B, U3C, U4E,
(1 kHz) at TP8 (MOD)
U4F and associated
circuitry
MODULATOR
Initial conditions and set-
> -5 dBm (>125 mVrms
Check U2 and associated
PREAMPLIFIER
tings (AGC switch,
into 50
Ω
) at AUX RF
circuitry
(A26U2)
A26A4S1, set to AGC on
OUT jack on rear panel
position)
METER
Initial conditions and set-
Panel meter indicates
Check U1A and associated
AMPLIFIER
tings
+10 dBm (707 mV)
circuitry
(A26A4)
MODULATION
Initial conditions and set-
REDUCE PEAK POWER
Check U2, Q8, Q9 and
OVERLOAD
tings
annunciator unlit
associated circuitry
DETECTOR
Set OUTPUT LEVEL
REDUCE PEAK POWER
(A26A4)
switch one step cw
annunciator lit
NOTE
Annunciator should be off
at +19 dBm RF output and
on at +20 dBm output
PULSE OVERLOAD
Initial conditions and set-
≈
+9V at TP3 (OVLD)
Check U1B and associated
DETECTOR
tings except set AM to
circuitry
(A26A4)
OFF
CAUTION
Check that OUTPUT
LEVEL switch is set one
step ccw from full cw
Short TP5 (GND) to
0 Vdc at TP3 (OVLD)
TP4 (VERN)
8-42
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