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Service
HP 8481H/82H
Figure 4. Operational Amplifier
The Auto Zero Feedback circuit is coupled to the Power Sensor from
the Power Meter. The dc voltage used to set the zero level is applied
to the input of FET A2U1Q1 by using A2R1 and the series resistance
of the thermocouple AlTCl as a voltage divider.
When the Power Sensor is used with the HP 436A Power Meter,
the resistance of R1 indicates the type of power sensor in use. As a
result, the power meter automatically selects the proper measurement
range. The 3.48K ohm resistance of R1 causes selection of the —15
to +35 dBm range. With the HP 435A Power Meter, R1 serves no
function.
The troubleshooting information which follows is intended to isolate
a problem to a stage. The defective component can then be identified
by voltage and resistance checks. The field-effect transistors (FET's)
in A2U1 are slightly light sensitive. As a result, dc levels are shifted
slightly when the FET's are exposed.
Troubleshooting
Caution
Be extremely careful when measuring accross tbe gold wires. They
are delicate and can be damaged easily.
The A1 bulkhead assembly normally supplies +3.79 ±1 mV when
the RF input is 3W. Measure this voltage at A2U1 pin 1. This dc
voltage will vary somewhat if the A2 input amplifier is inoperative,
or if the bulkhead assembly is disconnected from the input amplifier.
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