Keysight Technologies Infiniium 90000A Series, Service Manual

Page: 55 / 136

Troubleshooting
4
90000 A-Series Service Guide 55
If the FP SUPPLY FAIL LED is lit
If the FP SUPPLY FAIL LED is the only one lit, there might be a problem with one of
the supplies that goes to the front panel assembly, or a problem on the front panel
assembly. Unplug the front panel power connector (see cable circled below) and
turn the oscilloscope on again. If no other under-voltage indicator LEDs on the
backplane assembly turn on, the fault is on the front panel assembly and it should
be replaced. If either the BP POL FAIL or NEG SUPPLOES FAIL LEDs turn on, then
replace the backplane assembly and confirm that the fault goes away with the
front panel power reconnected to the backplane assembly.
Front panel power cable connector
If the oscilloscope turns off again after two seconds and no under-voltage LEDs
are lit, the bulk +12 V supply may be defective and may need to be replaced.
Before replacing the supply, be sure to verify that the +5 V standby potential from
the bulk supply is good by confirming the green LED on the motherboard is lit. If
this LED is not lit when AC power is applied to the instrument, the bulk +12 V
supply might need to be replaced.
Also be sure to verify the operation of the AC OK and DC OK signals before
replacing the supply. The AC OK signal indicates that the applied AC signal is
within the specified input range (TTL high logic level indicates AC input is good). It
is possible for this signal to indicate the AC input is not within the requested range
even though it is correct. The DC OK signal indicates whether the main output
from the supply (+12.6 V) is good (TTL high logic level indicates the DC output is
good). It is possible for this output to be false even though the supply output is
correct.
Both of these outputs are open collectors and would require a pull-up resistor to
some external potential to confirm that the output logic level is high, or a simple
resistance measurement to ground would indicate whether the open collector
output transistors are turned off, meaning both outputs would be in the logic high
state.