HP 83522A Service Notes page 246

Service Model 83522A
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 r'emoving 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 83522A
EXT
ALC. Enter -2 dBm RF power, and select a CW
frequency at any point in the band. Ground A4TP11. Rotate the
EXTIMTR ALC
CAL
knob fully clockwise to set TP6 to -0.6 Vdc,
essentially removing bias from the modulator. Check TP8 for -10
Vdc. If this is not the case, isolate the modulator from the drive
circuitry by applying a piece of cellophane tape to the pin edge
connection (PI-44). If TP8 now measures -10 Vdc, the modulator
diode is probably shorted. If the test point still does not achieve -10
Vdc, suspect the blanking circuitry, U9B and Q9.
NOTE
Remove any tape applied to the pin edge connector in the
previous procedure.
If the modulator appears to be functioning properly, check the following R F
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 A12 YO. Refer to the
RF Schematic Diagram at the end of Section VIII for the proper levels.
Measure the RF levels around A17 ModIMixer. 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 A16 Cavity Oscillator output is at least +8 dBm.
If maximum unleveled R F power is observed, attempt to achieve maximum
attenuation (minimum R F transmitted). Select 83522A
EXT
ALC. Enter
-2 dBm RF power, and select a CW frequency anywhere in the band.
Ground A4TP11. Rotate the EXTIMTR ALC
CAL
knob fully counter-
clockwise. The voltage level at TP6 should be +7.5 Vdc. The voltage levels
at the output of the PIN Mod 0 Drivers (Pl-44) should be approximately
+0.6 Vdc to +0.8 Vdc.
1.
If the voltages are significantly higher than this, the modulator diode
is probably open.
2. Check TP8 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.