9 Mar
2006
9 Mar
'06
4:06 p.m.
Well, I can tell you I am throughly confused when it comes to measuring this
particular power supply system with a multimeter. I *did* do some measuring
a few months ago on this, and could not for the life of me get a reading off
of the backplane, nor the power distribution board. However, the only
response I got was the light going out on the H7441 when I tried to measure
the +15V line that was, AFAIK, working properly.
I even traced down which black was the ground for what, if it mattered, and
made sure I used that to measure.
I'll repeat the measuring process this weekend, and write up a very detailed
status report here of my findings.
-----Original Message-----
From: cctalk-bounces at classiccmp.org
[mailto:cctalk-bounces at classiccmp.org] On Behalf Of Tony Duell
Sent: Thursday, March 09, 2006 3:09 PM
To: cctalk at classiccmp.org
Subject: Re: 11/34a problems continue
If something "blew", there must be a major 'problem'
somewhere
else!
Did you find out why the board "blew"?
I've not even started my normal rant about board-swapping (rather than
repair) yet....
Anyway, I do feel that it is somewhat foolish to replace a
part -- any part -- withoug a clear idea of what the problem
is. You might be looking in totally the wrong area,. or the
part might be being damaged by a fault elsewhere...
The initial fault, IIRC, was that the lamp on the -15V
regulator brick wasn't lit. Now that doesn't even mean that
the -15V rail is missing (the lampe might have blown). I can
think of many reasons why that lamp might not light, including :
The lamp itself has burnt out
The -15V rail is missing due to a fault in the reuglator brick
The -15V rail is missing due to a shrot-circuit in on of the
loads on that rail (e.g. a serial card)
The input to the -15V regulator is missing due to a bad
connection, open-circuit transformer secondary winding, whatever
The +15V supply to the -15V regualtor is missing due to a
fault on that regulator board
The +15V input is missing due to a bad connection/broken wire
between the
+15V board and the -15V brick
The +15V rail is missng due to a short in one of the loads on
that rail (e.g. a serial card)
The +15V rail is missing because the input to that regulator
is missing (brocken connection, open-circuit transformer
secondary, etc)
It is pointless, IMHO, to go any further unless you know
where to look, and that means doing tests to eliminate some
or all of those posibilities. When you know where the fault
is (e.g. you know the +15V rail is misisng, you know the
inputs to that board are OK, and the load doesn't seem to be
a dead short), then you can look at the next level down and
find, e.g. an open-circuit power transistor.
The best advice I was ever given for faultfinding (and it
applies to everything, not just classic computers) was
'Measure, think, then repair'. Make measrurements and gather
evidence. Think about what those measurements are telling you
(it may then be necessary to make more measurements..). Then,
and only them, start replacing components.
This may not be the modern way to repair things, but it's not
let me down yet (which is more than I can say about some
other 'methods').
-tony