HP 70908A, Service Manual

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If Hardware Error Messages (7000{7999) Occur
7074 Discriminator unlock
This hardware error occurs when the power level supplied from the multiplier to the
discriminator (dierence between the LO and preselector frequencies) is greater than
the discriminator loop lock range . The A10 power supply/driver reports the error
when the voltage at A10TP1-5 exceeds the 010.4 V to +10.4 V range .
T o solve this problem:
1. Isolate the problem to the HP 70900B local oscillator source or the HP 70908A
RF section .
Since the HP 70900B local oscillator source could cause frequency error , it is the
rst place to start troubleshooting.
Checking for HP 70900B local oscillator source failures:
Run the HP 70900B Tune + Span oset adjustment procedure for the A8
frequency control assembly .
Note the voltage displayed on the DVM and subtract it from 4.5 V before
making any adjustment (keep track of the sign of the dierence).
Make the rst adjustment. When nished, press
NNNNNNNNNNNNNN
DONE to get to the other
adjustment.
Again, note the voltage displayed on the DVM before making any adjustment.
A dd the dierence from the previous calculation to the DVM reading, and then
subtract it from 4.5 V.
If either of the dierences is greater than 11 mV, there is a problem with the
HP 70900B local oscillator source . Refer to the adjustment procedure failures
section in the HP 70900B Service Guide.
If there is no problem found using the HP 70900B Tune + Span oset adjustment
procedure , run the YTO linearity verication test in the HP 70900B Service
Guide.
If the YTO linearity test fails , refer to the HP 70900B frequency control
reference voltage adjustments and the YTO frequency endpoints adjustment.
Then rerun the YTO linearity test.
If it still fails , refer to the YTO linearity adjustment troubleshooting section of
the HP 70900B Service Guide.
Checking for HP 70908A RF section failures:
If both of the above procedures pass , the problem is caused by the HP 70908A
RF section . Refer to \Troubleshooting the A3 SYTFMD " and \Troubleshooting the
A10 P ower Supply/Driver ".
2. If necessary , obtain service from Hewlett-P ackard. (Refer to \If Y ou W ant
Hewlett-P ackard to Service Y our RF Section " in Chapter 1.)
7076 Multiplier unleveled
This hardware error occurs when a low output is detected from the
A10 power supply/driver . The error is reported when the voltage at A13U1 pin14
(A13 ALC/bias) is less than +0.65 V . This voltage is sent to the
A10 power supply/driver where it is translated to TTL (TTL low = unleveled). The
TTL information is sent to the A12 controller where the error condition is reported to
the system controller .
T o solve this problem:
1. Troubleshoot the A2 LO multiplier . (Refer to \Troubleshooting the
A2 LO Multiplier ".)
2. If necessary , obtain service from Hewlett-P ackard. (Refer to \If Y ou W ant
Hewlett-P ackard to Service Y our RF Section " in Chapter 1.)
T roubleshooting 4-7