11/34a problems continue

Tony Duell ard at p850ug1.demon.co.uk
Fri Mar 10 18:54:11 CST 2006


> 
> 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

Right... Alas I am a lot better at diagnosing electronic faults than 
mindreading :-)...

Let me again start with a general comment. If you get what seem to be 
crazy readings, then it's time to investigate. Maybe you've made a 
mistake. There's nothing wrong in making mistakes, I do it all the time 
(as does every other engineer, hacker, whatever that I know). By findout 
out what you're doing wrong, you learn a lot.

Or maybe there's a real fault that needs to be traced and cured. In which 
case those seemingly odd readings will help in finding it. 


> response I got was the light going out on the H7441 when I tried to measure
> the +15V line that was, AFAIK, working properly.

Hang on a sec...

Do you mean H754 (the -15V regulator) here?

I assume you do mean the +15V line. Now, IIRC the +15V regulator is a 
very simple circuit without much overload protection other than a fuse on 
the regulator PCB. If you managed to short the +15V lien to ground, the 
most likely result would be to blow that fuse. That would kill the +15V 
line, and with it the -15V line.

One very easy way to do that is to accidentally plug your meter leads 
into the current sockets on the multimeter. Some meters (including my 
Fluke) beep if oyu set them to a voltage range with the leads in the 
current soockets to warn you about this. Most don't.


> 
> I even traced down which black was the ground for what, if it mattered, and
> made sure I used that to measure.

The ground is commnn throught the entire system, it's linked between the 
backplanes. The other power rails are not linked -- there might be, for 
example,. several +5V lines in the system (there are 2 in just the 11/34 
CPU box) that are not linked. They both should be at about +5V wrt the 
common ground, but there might be slight differences. This should all be 
shown in the power distribution PCB schematics.

-tony




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