A4 TROUBLESHOOTING (CHANGE
8)
(Cont'd)
FULL UNLEVELED POWER (One or Both Bands)
Set the HP 83522A to sweep the full frequency range.
Attempt to level the power externally using the
HP
432A/B/C, 436A, or
438A
Power Meter
as shown in Figure 8-27. Select
MTR
leveling, and enter a slow (at least 30 seconds) sweep
time.
If
the RF power is now leveled, the failure is most likely in the detectors or the
Detector Selection Switch,
A4U6. Refer to the following paragraph. If this does not prove to
be
the case, the problem may be in the two analog switches U3B and U6A. It may be
necessary to perform the ALC adjustments in Section V of this manual.
SWEEP
AF
OSClLLATOR
PLU64N
mwER
METER
THERMISTOR
w.mb
Figure
8-2
7.
Power Meter
Leveling
Set
up
Check thd Detector Selection Switch
by
entering a CW frequency within the band or
leveling mode in question and trace the detector voltage through
U6B. If the input to be
selected does not match the output, check the MUX
A0 and MUX A1 lines (see Table 8-10).
Also check U12 and U13 as described under Digital Control.
Check the voltage at TP5. If it is greater than or equal to
+
5
Vdc, suspect the Mod Drivers
or Modulators.
If
it is below -2 Vdc, suspect the Detectors and Detector Leg.
NOTE
Turn off
LINE switch before removing or installing any assembly.
With the ALC assembly removed from the plug-in, 27.8 kHz
squarewave modulation from the Model
8350A/B is not available.
However, the HP
8756A
27.8
kHz
squarewave can be connected to
the
rear panel
PULSE
IN connector to maintain
HP
8756A
compatibility.
To check the RF components, remove the A4 ALC assembly from its socket. This removes
all bias from the modulator, and should allow maximum power through the RF path
in
all
bands. If full power (over
+
15
dBm) is then detected, the RF Amplifier A14, the Cavity
Oscillator, A16, and the DC Return A15 are verified. Suspect primarily the detector. Also
inspect the modulator, as well as the A4 PIN Mod Driver and Detector Selection Switch.
CHANGE
8
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