2. Service
Top-Level Diagnostic Tree (Diagnostic Section 1)
56
1.3.8 RF-Level Control-Current Test
The amplitude of the microwave probe signal is controlled by an amplitude
modulator in A15 driven with current generated from a DAC on A7. If control
current is absent, amplitude modulation is lost.
1 Remove power and disconnect the cable from A1J19 to A15J1.
2 Using a suitable SMA adapter, connect the DMM to the cable from A1J19
and set the DMM to measure at least 16 mA dc.
3 Disconnect the CBT from A2 at A2J2. Cycle instrument power and wait for it
to enter the Fatal Error mode.
4 Observe and record the current measured at A1J19.
5 Remove power and reconnect the CBT cable to A2J2.
6 Reapply power to the instrument and verify that LCD displays Warming up.
7 Quickly observe and record the average current at A1J19.
8 Remove power and reconnect the cable to A1J19.
QUESTION: Was the Fatal Error current 15.8 0.3 mA, and the
Warming up current 1.58 0.03 mA?
If Yes: The control current circuit is operational.
If No: Remove power, replace A7, and retest.
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