572 Chapter11
Synthesizer Section
Frequency Span Accuracy Problems
j. If the voltage at U332 pin 2 is correct with A14J3 pins 9 and 10
shorted, but was incorrect with W10 connected, the YTO FM coil
is probably open; replace the A11 YTO.
k. Replace A14J23 jumper. Remove the jumper and resistor from
A14J3. Reconnect W10 to A14J3.
Fractional N Span Problems (LO Spans ≤2 MHz)
If the fractional N spans are inaccurate or non-existent, but the
fractional N PLL is locked to the correct frequency and other spans are
correct, there may be a problem with the HSCAN signal. Check that
HSCAN is present at the fractional divider, U113 pin 41 in function
block AS. HSCAN comes from the A3 interface assembly and goes to the
sweep generator circuitry in function block A and to fractional N.
1st LO Span Problems (All Spans)
1. Set the spectrum analyzer to the following settings:
Center frequency .................................................. 300MHz
Span ........................................................................ 20MHz
Resolution BW .......................................................... 1MHz
Video BW .................................................................. 1MHz
Sweep time .................................................................50ms
2. Check that there is 0 V to +10 V ramp of 50 ms duration at A14J15
pin 15.
3. If a scan ramp is not present, refer to "Sweep Generator" in this
chapter.
4. If there is a 0 to −10 V ramp at A14J15 pin 14, the fault is probably
in the Main/FM sweep switch. See function block H of A14 frequency
control schematic (sheet 2 of 5).
5. Check that there is a 0 V to +10 V ramp at U325 pin 1. The ADC of
the spectrum analyzer obtains information about the sweep from
this node.
Check span
attenuator(steps
6-13)
6. Continue with step 7 to check the span attenuator. See function
block L of A14 frequency control schematic (sheet 2 of 5).
7. With the spectrum analyzer set to the settings in step 1, monitor
A14U323 pin 6 with an oscilloscope. A 0 V to −10 V ramp should be
present.
8. Change the spectrum analyzer span to 10 MHz and checkfor a 0 V to
−5 V ramp at U323 pin 6.
9. Change the spectrum analyzer span to 2.01 MHz and check fora0V
to −1 V ramp at U323 pin 6.
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