A4 ALC TROUBLESHOOTING (CHANGE
8)
(Cont'd)
The modulator is checked simply by noting whether the actual RF attenuation is appropriate to
the modulation bias present.
NOTE
Turn off line power before removing or installing any assembly.
If low or no RF power is observed, remove all modulator bias current simply by removing
the A4 assembly from the Motherboard. With no bias current, the RF power should pass
through the modulator unhindered. If this is not the case, check the modulator diode as
follows:
1.
Select HP
83522A
[EXT]
ALC. Ground TPll. Enter
-
2
dBm RF power, and select
any CW frequency. Rotate the
EXT/MTR ALC CAL screw fully clockwise. This
should result in
-7
Vdc at TP5, essentially removing bias from the modulator.
Measure the voltlage across R49. It should be OV. If this is not the case, isolate the
modulator from its drive circuitry by applying a piece of cellophane tape to the pin
edge connection,
PI-44. If the voltage across R49 now measures OV, the modulator
diode is probably shorted. If the voltage across R49 still does not measure OV, suspect
the band blanking circuitry,
U8B and Q15. Remove the jumper from TPl1.
NOTE
Remove any tape applied to the pin edge connectors in the pre-
vious procedure.
If the modulator appears to
be
functioning properly, check the following RF levels with a
power meter or spectrum analyzer. When checking power levels internal to the RF signal
path, ensure that all
critical ports are terminated in 50 ohms.
2.
If power is low, check the RF level directly out of the YFO A12. Refer to the RF
Schematic Diagram at the end of Section VIII for the proper levels. Measure the RF
levels around the A17
Modulator/Mixer. With no modulation, approximately
+
13
dBm should be measured at the input of A17, with approximately
-
10 dBm at the
output. If no output is measured, make sure the Cavity Oscillator A16 is yielding at
least
+
8 dBm.
If maximum unleveled RF power is observed, attempt to achieve maximum attenuation
(minimum RF transmitted). Select HP
83522A
[EXT]
ALC. Ground TPll. Enter
-
2
dBm
RF power, and select a CW frequency. Turn the EXT/MTR ALC
CAL
screw fully
counterclockwise. The voltage level at TP5 should be
+
5 Vdc. Concurrently, the voltage
level at the output of the
~od Driver, P1-44, should be approximately
+
0.6
vdc
to
+
08
Vdc.
1.
If the voltage is significantly higher than this, the modulator diode is probably open.
2.
Check TP6 for approximately
+
2.0
Vdc. The difference between the test point and the
pin-edge connector gives an indication of how much current is flowing to the
modulator.
CHANGE
8
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