Agilent Technologies Infiniium 90000 A Service Manual page 63

If there is still a short circuit, then both acquisition boards need to be removed. Once the
acquisition assemblies have been removed and unplugged from the backplane assembly,
measure each assembly individually to see if there is a short between the +12V net and ground
(see the figure below). The resistance you should see on a good acquisition assembly between
the +12V net and ground is about 2000 ohms. Make sure you make the measurement using a
DVM with the red lead plugged into the HI input and the black lead plugged into the LO or
COMMON inputted the DVM. If there is a short between these two nets, the reading you will
get will be very low (near zero ohms). Measure both acquisition assemblies and replace as
required.
Figure 4-9
This figure shows how to measure the acquisition assembly resistance between +12V and ground on the back side of the
board on the right end of the power connector.
If no shorts are found on the bulk 12V supply net or if you have fixed all the shorts and the unit
still will not turn on then a series of steps need to be followed which includes replacing parts
with "known good" ones until the problem has been discovered.
Start by replacing the bulk 12V power supply with a known good supply. If the oscilloscope still
does not power on, replace the motherboard with a known good one. If the oscilloscope still
does not power on, replace the front panel ON/OFF switch with a known good one. If the
oscilloscope still does not power on then it can only be the backplane assembly causing the
problem and it needs to be replaced. Once the faulty board or switch is discovered, all previous
parts removed from the instrument should be reinstalled as they were not the cause of the
problem.
Chapter 4: Troubleshooting
Under-Voltage Fault
63