Agilent Technologies Infiniium 90000 A Service Manual page 62

Chapter 4: Troubleshooting
Under-Voltage Fault
oscilloscope. On the backplane assembly, measure the resistance across the FET (Q1409)
between +12V and ground as shown in Figure 4-10 (Q1409 is located in the upper right hand
corner of the backplane) or measure the resistance across the bulk supply input pins on the
backplane assembly. Make sure you have the red lead plugged into the HI input and the black
lead plugged into the LO or COMMON input of the DVM. On a good backplane assembly, you
should measure about 700 ohms. If there is a short on this board, you would measure something
very near zero ohms. If there is a short, disconnect the power cable to the front panel (see Figure
4-6) and redo the resistance measurement. If the short is gone, replace the front panel assembly
and verify the instrument will now power on. If there is still a short circuit, disconnect the two
large connectors that provide power to the motherboard (J1201 and J1202) from the backplane
assembly (see the circled cable connections in Figure 4-8) and redo the resistance
measurements across the FET (Q1409) or the bulk supply input pins on the backplane assembly.
If the short is gone, replace the motherboard assembly and confirm the unit will power on.
Figure 4-7
Figure 4-8
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