TM 9-4935-601-14-7&P
SERVICE SHEET 13
PRINCIPLES OF OPERATION
General
The RF signal from the RF Filters is amplified by RF Output
Amplifier A26U1. The amplifier is a sealed microcircuit that
plugs into the rear of the AGC detector board. The amplifier
drives resistor U1R1 which sets the amplifier output
impedance. The output is ac coupled through U1C2 and
drives the Output Attenuator. The step attenuator consists
of resistive attenuator sections which are switched in and
out by cam driven microswitches. The attenuator steps are
10 dB with an impedance of 50
Ω
. Capacitor A1A2C1 ac
couples the RF output to the RF OUTPUT jack J1.
AGC Detector (A26A1)
The AGC detector (A26U1CR1) detects the negative peaks
of the RF signal from the Output Amplifier. The detector
output is summed with the positive AGC reference voltage
in the Summing Amplifier (Service Sheet 12). Detector
diode A26U1CR1 conducts whenever the RF amplifier
output is one diode junction voltage drop below the voltage
across U1C3. The capacitor is then negatively charged until
the amplifier voltage rises, at which time CR1 shuts off.
U1C3 then slowly discharges through resistor R3 and
transistor Q1 until another negative peak recharges it.
Transistor Q1 is normally on except in the pulse modulation
mode. In the pulse modulation mode, transistor Q1 turns off
between pulses, which prevents A26U1C5 from discharging.
(If A26U1C5 were to discharge between pulse bursts, the
Modulator would be driven for maximum output when the
next pulse arrived.)
Bandwidth Control (A26A1 and A26A2)
In the 2-4 and 4-8 MHz (or LO BAND 2) frequency ranges,
capacitor A26A1C7 is switched in parallel with A26U1C3 by
A26A1Q2. This reduces the amount of capacitor discharge
between RF voltage peaks on the lower frequency bands
and limits the AM bandwidth. Also capacitor A26A1C6 is
switched in for the 0.5 -1 and -12 MHz (or LO BAND 1)
ranges by A26A1Q3.
In the pulse modulation mode, capacitor A26A1C5 is
switched in by A26A1Q4. Switching of A26A1Q2, Q3, and
Q4 is multiplexed onto one line by transistors A26A2Q8 and
Q9. Q8 is a switchable current source. In LOW BAND 1,
Q8 generates just enough current to turn on the collector-
base junction of Q2. In LOW BAND 2, the current increases
enough to turn on both Q2 and Q3 (because the voltage
drop across A26A1R6 is enough to turn on Zener diode
A26A1VR2). When the PULSE CODE line is low, A26A2Q9
is on which turns on A26A1Q2, Q3, and Q4.
SERVICE SHEET 13 (Cont'd)
Detector Buffer Amplifier (A26A1)
Transistor Q5 and FET Q6 form a high impedance, unity
gain buffer amplifier. Diode CR3 and resistor R11 add a dc
offset which compensates for the junction voltage drop of
the detector diode.
Schmitt Trigger (A26A2)
A Schmitt Trigger formed by U1 and U2A converts the pulse
input voltage into 0 to 5V pulses. Resistors R20 and R21
set the trigger reference at about 0.5 Vdc. When the input
to U1 is above the reference, the output of U2A is low.
When the input goes below the reference, the output of U2A
goes high (+5V).
Resistor R23 adds a small amount of hysteresis to the
reference voltage. In the normal pulse modulation mode,
NAND gate U2C inverts the trigger output and switches
transistor A26A1Q1 on when the input pulse is high, or off
when the input pulse is low. Thus the charge on capacitor
A26A1C5 is stored between pulses, but is shunted by
A26A1R3 when an input pulse is present. Similarly, NAND
gate U2D inverts the trigger output and switches the
Modulator Driver Amplifier A26A4 (Service Sheet 12).
Rate Detector (A26A2)
Flip-flops U3A and U3B form a rate detector to turn off the
RF level drive to the meter circuits whenever the pulse
repetition rate falls below 20 Hz. Below 20 Hz rates, the
output leveling system cannot accurately control the output
amplitude. The flip-flops are arranged as retriggerable
monostable (one-shot) multivibrators with timing elements
R25 and C10, and R28 and C11. A low-going output from
U2A triggers U3A and the Q output of U3A goes low for 50
ms. If the repetition rate of the incoming pulses is higher
than 20 Hz, U3A retriggers and the Q output remains low. In
the absence of pulses from the Q output of U3A, the Q
output of U3B is low, transistor Q7 is off and the meter
operates normally. For pulse repetition rates less than 20
Hz, U3B is periodically triggered by the Q output of U3A.
The Q output of U3B goes high for 100 ms (or longer if U3B
is retriggered by U3A) and turns on Q7 which disables the
meter drive amplifier output, and the meter reads zero.
Thus the meter is turned off for low rate pulses. When not
in the pulse modulation mode, the output of inverter U2B is
low; the output of U2C is high and A26A1Q1 is held on; the
output of U2D is high and the modulator is held in its normal
on mode; and Q7 is held off.
TROUBLESHOOTING
It is assumed that a problem has been isolated to the power
amplifier and AGC detector or to the AM offset and pulse
switching 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
SERVICE SHEET 13 (Cont'd)
settings, and following the procedures outlined in the table.
Test Equipment
Digital Voltmeter ........................................... HP 3480B/3484A
Oscilloscope ........................................ HP 180A/1801A/1820C
Pulse Generator....................................................... HP 8003A
Power Meter .............................................................. HP 435A
Power Sensor .......................................................... HP 8481A
Initial Test Conditions
To test A26A2 AM Offset and Pulse Switching Assembly,
remove top cover (see Service Sheet F for removal
procedure), remove A26 AM/AGC and RF Amplifier
Assembly casting top cover, and remove A26A2 and extend
for service (see Service Sheet E for procedure).
To test A26U1 Output Amplifier and A26A1 AGC Detector
Assembly, also remove bottom cover (see Service Sheet F
for removal procedure) and remove A26 casting bottom
cover (see Service Sheet E for procedure).
Connect the pulse generator to AM INPUT. Set the pulse
generator for a repetition rate of 20 Hz, a pulse width of 25
ms, and an amplitude of 1V.
Initial Control Settings
Meter Function.............................................................. LEVEL
COUNTER MODE:
EXPAND .......................................... Off
LOCK ............................................... Off
Source............................................. INT
AM .................................................................................... OFF
MODULATION.............................................................. Full cw
MODULATION FREQUENCY ......................................... 1 kHz
FM .................................................................................... OFF
RANGE..................................................................... 8-16 MHz
FREQUENCY TUNE................................................. 7.20 MHz
OUTPUT LEVEL ........................................................+19 dBm
RF ON/OFF ........................................................................ON
NOTE
If pulse burst amplitude is too high for low duty-cycle pulses,
check all components connected between A26U1CR1 and
A26A1Q6 (G1) for dc current leakage.
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MODEL 8640B
RF Amplifier Pulse Switching and Step Attenuator Troubleshooting
Component
Test Conditions and
If Indication
or Circuit
Control Settings
Normal Indication
is Abnormal
OUTPUT
Initial conditions and
+19 dBm at RF OUT-
Check A26U1 and
AMPLIFIER
settings (+19 dBm
PUT
associated circuitry
(A26U1)
output). Connect
power meter and
sensor to RF OUTPUT.
Set AGC switch
≈
2 Vdc at TPA
(A26A4S1) to AGC
(A26A1Q6-G1)
off. Adjust OUTPUT
LEVEL vernier for
+13 dBm at RF OUTPUT.
DETECTOR
As above
≈
2 Vdc at TP1
Check Q5, Q6, and
BUFFER
(DET)
associated circuitry
AMPLIFIER
(A26A1)
SCHMITT
Initial conditions and
≈
5V pulse at TP6
Check A26A2U1,
TRIGGER
settings except set AM
(MOD PUL) and
U2 and associated
(A26A2)
to PULSE and AGC
4V pulse at
circuitry
switch, A26A4S1 to
TP5 (DET
AGC on position.
PUL)
RATE
Initial conditions and
Panel meter reads
Check A26A2U3,
DETECTOR
settings except set AM
normal (+19 dBm)
Q7 and associated
(A26A2)
to PULSE
circuitry
Set pulse generator
Panel meter reads
pulse repetition rate
approximately zero
to 15 Hz
BW CONTROL
Initial conditions and
<+1V at TP8 (BW)
Check A26A2Q8, Q9
(A26A2)
settings
and associated
Set RANGE to
≈
+5V at TP8 (BW)
circuitry
4 - 8 MHz
Set RANGE to
≈
+12V at TP8 (BW)
1- 2 MHz
Set AM to PULSE
≈
+19V at TP8 (BW)
BW CONTROL
Initial conditions and
Same signal level on
(A26A1)
settings except set
both sides of C7 and
AM to INT
C6
Set RANGE to
Signal level differs across
Check Q2, C7, and
4 - 8 MHz
C7 (i.e., no signal at Q2-e)
associated circuitry
Set RANGE to
Signal level differs across
Check Q3, C6, and
1 - 2 MHz
C6 (i.e., no signal at Q3-e)
associated circuitry
Set AM to PULSE
Signal level differs across
Check Q1, Q4, R3, C5
R3 and C5 (i.e., no signal
and associated circuitry
at Q4-e)
8-44
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