24 Mar
2007
24 Mar
'07
7:30 p.m.
Further thought and I realized that slots 16 and 21
are for the mos/bipolar
memory controllers. I don't have mos/bipolar memory. So I was wondering what
else comes off that regulator board on the bottom 742. From recent memory I
believe I recall the LTC comes from there. But LTC being fried wouldn't
I thought that ACLO, DCLO, and LTC all came from the top H742, if only
because that's the one where the 'ground' rail of the PSU board is
connected to system ground.
cause the cpu to fail to come up would it? And since
I'm not actually using
mos/bipolar I'm left wondering what else is coming off that board. AC lo or
DC lo signal perhaps? I seem to remember that something odd was done to the
regulator board in that lower 742 to make the output -15v instead of 15v
like the top 742.
It's n example of one of VOnada's laws 'There is no such thing as
ground'.
What they did was connect the rail called '+15V' on the PSU prints to
system ground.. That means the rail called 'ground' on the PSU prints is
now -15V wrt suystem ground (think about it!). And that's what they
called the -15V output.
But this also means the ACLO, DCLO and LTC outputs of the lower H742 are
references to the -15V rail. Hence the 'Do Not Connect' warnings on the
power harness prints. I don't think they're used for anything.
I rememebr the system clock generator needs a -15V supply (that's the CPU
clock, not the line time clock or antthing like that). I can't remember
if that comes from the lower H742, or from the -15V 'brick'.
In any case, I obviously SHOULD fix the regulator in the bottom 742 so I get
that -15v back but I am left curious as to if that is what is causing the
cpu to suddenly no longer come up and run. Am I on the right track here?
Well, my advice is to correct any problem that you find, whether it's
directly related to the fualt or not. This machine was supposed to have a
-15V supply coming from the lower H742, so let's get that working again.
There may of course be other faults to trace too.
-tony