Keysight 9000 Series Service Manual page 63

Chapter 4: Troubleshooting
Sequencer 3
Input Supply
1 -12V Bulk
2 -6 V
3 -5.2 V
4 -3.8 V
When a fault occurs on a given sequencer, it notifies the other two using the Fault signal. The other two
sequencers report this as an External Fault as well. For example, if the first sequencer has an over-voltage
on Channel 1, it will report an External Fault with Channel 1 as the source. The other two sequencers
will report it as an External Fault with the source as Channel 4. Similarly, if the first sequencer has an
under-voltage failure on +12V, it will report a Reset Fault on Channel 1 and the other two sequencers
will report External Faults on Channel 4. As you can see, any fault on one of the sequencers always
reports an External Fault on Channel 4 on the other two sequencers. Look for the fault that is not an
External Fault on Channel 4 as the cause of the shutdown. If all three sequencers report an External
Fault on channel 4, it will take some work to determine the real source. It could be on any of the Channel
4 sources (+2.5V, +1.2V, -3.8V), an over-temperature fault (look for the over-temperature LED to light),
or an FPGA issued fault.
Sequence Fault: A Sequence Fault happens when a supply does not come up in time during a power-
on sequence.
Reset Fault: A Reset Fault happens if the oscilloscope shuts down during the monitor phase because a
supply went under voltage. This is probably due to the supply failing to an off state or the load becoming
too much for the supply.
Command Fault: A Command Fault happens when the ON sequence to one of the sequencers goes
HIGH to initiate a turn-on, but then goes LOW before the turn-on sequencing finishes. Similarly, if the
ON signal goes HIGH during sequencing off, a Command Fault occurs. This kind of fault indicates
something is going wrong with the ON signal. Since the second two sequencers are fed by7 other
sequencers, this will most likely happen on the first sequencer. The ON signal is driven by the minimum
on-time limiter. There could be a problem here as well.
External Fault: An External Fault can be caused by several failures. If a supply goes over-voltage, an
external fault is issued. If the over-temperature sensor or FPGA issues a fault, it will be displayed as an
external fault on channel 4. This present an ambiguous case as an External Fault on channel four could
occur for multiple reasons. To help determine the cause, the over-temperature LED will light during an
over-temperature fault. However, this LED does not latch to failure. Once the oscilloscope cools to
below the over-temperature condition, the LED will turn off. The main reason for an over-temperature
fault is a stopped fan or blocked air inlet. Check for these and power the oscilloscope back up. If it
continues to shut down, it could be a bad over-temperature sensor or comparator circuit.
Assembly Level Repair
How to Use the Power
Fault LEDs
63