MODEL 8640B
TM 9-4935-601-14-7&P
SERVICE SHEET 11
PRINCIPLES OF OPERATION
RF Dividers (A10A2)
The A10A2 RF Divider Assembly frequency-divides the 256-512 MHz signal from the RF
oscillator to obtain lower output frequencies. (The overall operation of the A10 Divider/ Filter
Assembly is described on Service Sheet 10.) On the two highest frequency bands (256-512
MHz) and external doubler, the dividers are bypassed. On the other bands, the signal from the
oscillator is amplified and limited by buffer amplifier U11.
The outputs of the first three dividers drive complementary output OR gates (U10A, U13B and
U15B) which drive the next divider stage with one output and another complementary output OR
gate (U10B, U13A, and U15A) with the other. The latter gates drive output transformers T1, T2,
and T3 in push-pull, and are enabled by inverter transistors Q2, Q3, and Q4 respectively.
When an output OR gate is enabled, the next divider stage is disabled. (Note that ground is a
logical high and negative or open a logical low for EECL and ECL devices.) The Q and Q outputs
of the next two stages each drive NOR gates (U17B, U17C, U19B, and U19C) in push-pull which
in turn drive transformers T4 and T5 in push-pull. The final four divider stages operate in a
manner similar to the previous two stages. The NOR-gate outputs, however, drive a common
output transformer T6. The last NOR-gate output pair is enabled through diodes CR1, CR2, and
CR3 connected in a logical OR configuration.
All output transformers drive pi-network pads which are switched onto the line leading to the
modulator circuits. The attenuation of the first three pads (R6-8, R12-14, and R18-20) is set
(from 3 to 6 dB) to prevent excessive signal level from being applied to A26U2 (Service Sheet
12). The attenuation level is set by changing the values of the resistors.
Schmitt Trigger (A10A2)
Amplifier U1 is a Schmitt Trigger which senses when the voltage V
T
(proportional to the RF
oscillator frequency) reaches the value corresponding to the geometric mean of the frequency
band. The reference voltage is determined by resistors R55, R56, and R57; R58 adds a small
amount of hysteresis. Transistor Q1 complements the amplifier output. Inverter U6A activates
the low band relays A10A1K1 and K3 (Service Sheet 10); and U6B activates the high band
relays A10A1K2 and K4 (Service Sheet 10). The inverters are driven in complement except that
capacitors C18 and C19 hold both inverters on simultaneously for a few milliseconds during a
transition to provide a make-before-break action.
SERVICE SHEET 11 (Cont'd)
TROUBLESHOOTING
It is assumed that a problem has been isolated to the RF divider 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 the procedures outlined in
the table.
Test Equipment
Digital Voltmeter ................................................................................................ HP 3480B/3484A
NOTE
If problems occur only on the lower bands, an oscilloscope can be used to locate
the defective RF circuit. On the higher bands, either a high frequency
oscilloscope, a sampling oscilloscope, or a spectrum analyzer (with a high
impedance probe) can be used.
Initial Test Conditions
Top cover removed (see Service Sheet F for removal procedure) and A10 Divider/Filter
Assembly casting cover removed (see Service Sheet D for removal procedure).
Initial Control Settings
Meter Function ...............................................................................................................VOLTS
COUNTER MODE:
EXPAND ......................................................................................... Off
LOCK.............................................................................................. Off
Source ............................................................................................INT
AM...................................................................................................................................... OFF
FM...................................................................................................................................... OFF
RANGE .............................................................................................................. 256 - 512 MHz
FREQUENCY TUNE ................................................................................................... 550 MHz
OUTPUT LEVEL........................................................................................................... -10 dBm
RF ON/OFF.......................................................................................................................... ON
RF Divider Circuits
The quickest way to isolate a divider/filter problem is to use the front panel controls to set
various frequencies and frequency ranges while monitoring the output voltage meter. Usually a
problem will appear as shown in the following table.
RF Filters (A10A1)
SERVICE SHEET 10
SERVICE SHEET 11 (Cont'd)
Symptom
Probable Cause
Defective output circuit for one of the dividers,
No output on one band only
a filter, or a slide switch
Defective divider or 16 - 32 MHz low band filter
No output on one band and all bands below that band
or 0.5 to 8 MHz divider output transformer
Defective geometric mean switching (high band
Low power at highest end of bands (8 to 1024 MHz) only
filters not being switched in)
Overly high harmonics at lowest end of
Defective geometric mean switching (low band
bands (8 to 1024 MHz) only
filters not being switched in)
Intermittent power
Poor contact on slide switch
Changing bands does not change output frequency even
Loose coupler between RANGE switch and
though the counter may indicate a change
Divider/Filter switch assembly
The filters, slide-switches, and the relays driven by the Schmitt Trigger circuits are shown and discussed on Service Sheet 10.
NOTE
Check that the control inputs to the RF gates are correct before suspecting the gates themselves.
RF Divider Troubleshooting
Component
Test Conditions and
If Indication
or Circuit
Control Settings
Normal Indication
is Abnormal
SCHMITT TRIGGER
Initial conditions and
≈
+10V at TP1 (VT)
Check slide-switches
(A10A2)
settings
(Service Sheet 10) and
VT pot (Service Sheet 5)
≈
-3V at TP2
Check U1 and associated
circuitry
≈
- 0V at TP3
Check U6 and associated
circuitry
≈
- +5V at TP4
Check U6, Q1 and
associated circuitry
Set FREQUENCY
≈
0V at TP1 (V
T
)
Check V
T
pot (Service
TUNE to 230 MHz
Sheet 5)
≈
+5V at TP2
Check U1 and associated
circuitry
≈
+5V at TP3
Check U6 and associated
circuitry
≈
0V at TP4
Check U6, Q1 and
associated circuitry
RF DIVIDERS
Initial conditions and
-10 dBm on panel meter
Check appropriate
(A10A2)
settings then set RANGE
divider and associated
to each position
circuitry. Check that
following divider is off
8-40
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