Over-Temperature Fault - Agilent Technologies Infiniium 90000 A Service Manual

Over-Temperature Fault

If the oscilloscope turns off without you pressing the front panel power button, the first step is
to unplug the oscilloscope so the +5V standby supply can fully discharge (this takes around 30
seconds). Then plug the oscilloscope back in. Press the front panel power button and count
how many seconds it takes for the oscilloscope to lose power again (once the oscilloscope loses
power, keep it plugged in). If it takes four seconds or more then you likely have an over-
temperature fault or a fan fault.
Once you have determined that it takes four seconds or more for the oscilloscope to lose power,
the next step is to remove the handle, the outer oscilloscope cover, and the inner top cover.
This will expose the mother board cavity and the top edges of the backplane assembly.
There are four fans on the left side of the instrument that are used to cool the instrument. There
is also a single fan attached to the backplane assembly that cools the trigger ICs. In order for
the instrument to shut down because of a cooling system fault, one of the following condition
must exist:
• The circuitry that generates the reference potential for all the fans (the four case fans and the
• The fan that cools the trigger ICs failed or the high side drive circuitry for the trigger ICs fan
• All four case fans do not turn. It is highly unlikely that all four fans would become faulty
To diagnose a fan failure, first reset the sensing and shutdown logic by removing AC power
until the green LED on the motherboard goes out (or just wait 30 seconds). Move around to the
left side of the instrument so you can see the case fans (Figure 4-11).
Figure 4-11
trigger ICs fan) would have to fail. This would result in none of the fans turning and the
instrument would shut down.
failed. It could also be that the rotation sensor for the trigger ICs fan failed. This sensor lets
the instrument know that the fan is indeed turning. If the sensor is faulty, the instrument may
think the fan is not working when in fact it is.
simultaneously. The more likely reason for all four fans not turning is that the high side drive
circuitry for these case fans failed.
Chapter 4: Troubleshooting
Over-Temperature Fault
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