AT&T 3B2 Maintenance Reference Manual page 125
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Also See for 3B2:
- Off-line diagnostic manual (739 pages) ,
- Maintenance reference manual (422 pages) ,
- Operation manual (415 pages)
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Trouble Analysis
System Will Not Power-up
The fan and LED indicators should show that the 3B2 computer is receiving line voltage. You
may want to check the line voltage to ensure that the required 110 V AC (220 V AC) is present
and available as input to the power supply.
All power supply models in the 3B2 computer line operate on the same basic principle. When
the power switch is thrown to the ON position, power is supplied to the load for about a half
second. During this time, the system board comes on line and supplies +5VOC on the
PWRON1 lead, running back to the power supply. This signal latches the power supply to the
on position. As long as this signal is present, the power will remain on (unless there is a
voltage regulation or thermal problem).
If
the system board does not supply +SVOC on the
PWRONl lead before the supply times out, power will be removed. Should this occur, the
problem could be either a power supply voltage problem, or a bad system board.
The power supply voltages can be checked by the following procedure:
1" Power down the 3B2 computer and remove the cover.
2" Remove the power supply header connector from the system board (Models 300,
310, and 400) or the backplane (Models 500, 600, 700, and 1000).
3" The PWRONl lead must be jumped to +5VDC. On Models 300, 310, and 400, use
any of pins 12-15 on the same connector. On Models 500, 600, 700, and 1000, use
any of the pins on either of the backplane power connectors.
4. Tum the system on.
If
the power latches on, the voltages can be checked (see
proper subassembly chapter for pinouts). If not, replace the power supply.
NOTE: Using this procedure, the fans and thermal sensor must be connected on the 3B2
Models 600, 700, and 1000. If not, the power will be dropped after 1 minute. There must also
be a load connected to the supply or the voltages cannot be realistically measured.
LED Announced Sanity Failures
Sanity failures that occur during system initialization are normally indicated through a flashing
amber (red on older models) LED on the front panel of the computer.
It
is possible that a
processor sanity error will prevent the LED from flashing, the LED may indicate an error
condition that really does not exist, or the system may be incapable of giving an LED
indication.
System Troubleshooting
5-17
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