Keysight Technologies Infiniium 90000A Series, Service Manual

Page: 59 / 136

Troubleshooting
4
90000 A-Series Service Guide 59
Over-Temperature Fault
If the oscilloscope takes 4 seconds or more to lose power as described earlier,
then the problem is a cooling system failure.
Four case fans are located on the left side of the oscilloscope to cool it. In addition,
a single fan is attached to the backplane assembly to cool the trigger ICs. For the
oscilloscope to shut down because of a cooling system fault, one of the following
conditions must exist:
• The circuitry that generates the reference potential for all five fans may have
failed. This circuitry failure would cause the fans to stop turning and the
oscilloscope to shut down.
• The fan that cools the trigger ICs failed or the high side drive circuitry for the
trigger ICs fan failed. It could also be that the rotation sensor for the trigger ICs
fan failed. This sensor lets the oscilloscope know that the fan is turning.
• All four case fans do not turn. It is unlikely that all four case fans would become
faulty simultaneously and stop turning. It is more likely that the high side drive
circuitry for the case fans failed.
Follow these steps to diagnose a case fan failure.
1
Reset the sensing and shutdown logic by removing the AC power until the
green LED on the motherboard goes out (or just wait 30 seconds).
2
Move to the left side of the oscilloscope so you can see the case fans.
Case fans
3
Apply AC power to the oscilloscope and turn it on if it does not start up
automatically. Visually check to see if any of the fans are not turning. If none of
the fans are turning, it is likely a problem with the high side drive circuitry for
the case fans. In that case, replace the backplane assembly.