Agilent Technologies Infiniium 90000 A Service Manual page 67

Chapter 4: Troubleshooting
Over-Temperature Fault
Apply AC power and turn the instrument on if it does not start up on its own. Observe whether
the trigger ICs fan is turning. If it is not, replace it and redo this part of the diagnostic test. If it
is turning and the instrument turns off after four seconds or more then consider the following.
If any of the case fans were turning and the trigger ICs fan was turning, the problem is most
likely that the rotation sensor on the ICs fan is defective. Replace the trigger ICs fan and redo
this part of the diagnostic test. If the instrument stays on after you reset the shutdown logic (by
removing AC power for thirty seconds) and powered it back on, then you have fixed the
shutdown issue. If the unit shuts down again after you have replaced the trigger ICs fan then
the problem could be either that the rotation sensor for all the fans located on the backplane
assembly failed or the rotation sensor for the case fans have all failed. This last reason is not
very likely, so the first recommendation is to replace the backplane assembly and then redo
this part of the diagnostic test.
If, after replacing the trigger ICs fan and the backplane assembly, the unit does not shut down
then you have fixed the problem. Simply replace any case fans that were not working and the
problem is fixed.
W A R N I N G INJURY CAN RESULT!
The following procedure has you touch fan hubs with objects or requires the use of objects to
stop fan blades from moving. Use caution when working around the cooling fans with the cover
removed from the oscilloscope. The cooling fan blades are exposed on one side and can be
hazardous. Be careful not to touch the moving blades with any part of your body.
If the unit still shuts down, however, then replace all the case fans and confirm proper operation.
To verify proper operation, stop the trigger ICs fan from rotating (simply reach down and touch
the hub the fan blades are attached to with some object - be careful not to touch the fan blades
with your fingers). The unit will shut down. Reset the logic (by removing AC power for thirty
seconds) and make the case fans so they all cannot rotate (for example: either by two people
holding the four fan hubs or by inserting an object in between the blades of each of the four
fans). Power the unit back on. The oscilloscope should shut down in about four seconds. This
confirms proper operation of the fan sensing and shutdown function.
If the unit shuts down and the TEMP FAIL indicator LED is on (refer to the circled area in Figure
4-12 for the location of this LED on the backplane assembly) then one of the scope channels is
getting too hot or the temperature sensing circuitry has failed. Normally, you would have to
turn the instrument on and leave it on for a while before you would see a shutdown because of
temperature. If all the fans are turning, it is highly unlikely that the unit would shut down due
to temperature unless the airflow is being restricted or the input air is already too hot to cool
the insides of the oscilloscope. If one or more of the case fans has failed, the unit would overheat
and shut down, but the oscilloscope would alert the user that one or more case fans has failed
and the unit needs to be repaired.
It is a possibility that all the case fans and the trigger ICs fan are working, but the unit still shuts
down because of temperature due to one or more of the channels malfunctioning and drawing
too much current. In this case, you would need to replace the acquisition assemblies one at a
time until you find which assembly is defective. It is also possible for one of the heat sinks on
one of the acquisition assemblies' critical parts to become detached and cause that part to over
heat and shut down the instrument.
If the oscilloscope still will not power on after you have followed the required steps in this
Power Supply Trouble Isolation section (and assuming you have looked through the other two
sections included in this Power Supply Trouble Isolation section to ensure your problem cannot
be fixed there), refer to the Motherboard Verification section in this chapter for additional
diagnosis suggestions.
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