Different boot problem...

[DoN Nichols]

In article <12880 at helios.TAMU.EDU> n138ct at tamuts.tamu.edu (Brent Burton) writes:
>Well, I shutdown the system again (have you heard this phrase from me or what?)
>and reassembled it.  Turned it on and no boot.  We then quickly ripped the
>top off to measure the power supply levels and we found a problem.
>
>The voltages were the same as I mentioned in my last posting.
>EXCEPT the -12V (pin 18 on the PS-->MB cable) was +0.79V.  This is
>a bad sign.  What hints are there?

	Either the -12V is shorted somewhere, or the regulator chip is
defective/damaged.  I mentioned the location in my previous article, but who
knows which will reach you first.  At the end of the power supply board,
near the output connector, is a small heat-sink labled on the board U101.
It is a -12V regulator chip, probably a 7912 or moral equivalent.

	This regulator/heat-sink combination can be fairly easily hit by the
case when re-installing, especially if the hard drive is not present, and if
it bends in one direction, it can break the leads, or pull them out by the
roots.  Try gently pressing the top of this heat-sink one way and the
other.  If this kills/restores your -12V, you'll need to unsolder the heat
sink and the chip, replace the chip, and re-solder.  (I had this happen, but
the leads broke at the pc-board, so I was able to wrap buss-wire around the
terminals, and make new leads.  It works fine, now.  The failure was
catastrophic, rather than intermittent, so the remedy was more apparent.)

	Hope that this cures your problems.
		DoN.
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