150 Chapter 7
Troubleshooting the Processor, Power Supply, and Display
Verifying the A26 CPU Assembly
The message “LAN timeout; External loopback test failed”
indicates the instrument didn’t sense a LAN connection.
A gray screen appears for about 15 seconds. Next a screen appears with
the instrument model number and firmware revision information. This
screen may be displayed for 2 minutes.
If the boot process hangs up here, reload the PSA firmware.
Once the boot process is complete, the instrument runs the auto-align
procedure documented on page 49.
If the instrument does not boot, look to see if the LEDs on the CPU
assembly are flashing. When the instrument power is turned on, you
can observe the processor LEDs from the rear panel access holes by the
LAN connector. At power on, the CPU DIAG LEDs behave as follows:
— At power on, all four LED’s turn on for a second.
— The left most LED turns off leaving the other three LED’s on for
approximately 15 seconds.
— The three LED’s go off and the left LED turns on.
— The LED’s go through a flashing sequence.
— All four turn on for approximately 10 seconds.
— All CPU diagnostic LED’s turn off after the boot-up is complete.
At the end of the sequence, all of the green LEDs should be off. If one or
more of the green LEDs remains on, suspect the CPU assembly as being
defective. If a LAN connection is present, the yellow “LAN CO LI” LED
remains on. The yellow “RX” LED will blink.
If the screen remains blank (dark) and the green LEDs never blink, the
instrument was not able to boot from bootrom. The bootrom on the CPU
assembly could be bad, or the instrument clock signals could be
missing. The bootrom is not a replaceable part.
If the 30 MHz sample clock from the A8 Analog IF assembly to the A7
Digital IF assembly is not present, the display signal processor chip on
the digital IF assembly will not reset, causing the PCI bus to hang up
and the instrument will not boot.
To check the 30 MHz sample clock:
• Put the A7 Digital IF assembly on an extender board (E4406-60021).
• Check A7J4 for a 30 MHz signal using an oscilloscope, as shown in
Figure 6-1 on page 142.
• If the signal is not present, the cause could be the A8 Analog IF
assembly, or the A11 Reference assembly. Go to "A11 Reference
Assembly Description" on page 118 to check the reference assembly.
If the boot-up process stops after the message Processor starting
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