PDP11 training troubleshooting - test-drb@ccmp.vtda.org

3 Nov 1998

re: PDP11 troubleshooting class (AKA PDP11 Systems new hire)

...
  From: ard(a)p850ug1.demon.co.uk (Tony Duell)
 To: classiccmp(a)u.washington.edu
 Subject: Re: PDP-11/44 boot prompt

 [Sticking tape on an edge connector to introduce a fault]

  It wasn't a repair procedure... it was a call
it broken and try to 
 find/fix it class exercise.  Sometimes when a bad board was a available 
 they would be used.  
 Hmmm.. This actually wouldn't have worked for me, if I'd been allowed to 
 do what I liked to the machine. 
Well, the class wasn't 8-(...
You worked with scope, diags, front panel lights and were not allowed
to pull the boards to look at the fingers.

A couple of instructors began to request real bad boards from
the in-house maintenance techs who maintained the machines
at Crosby Drive.  (I wonder if Jack Whitford at DEC's reading this one
now.  Thanks for the PDP11 new hire course Jack -- it got me 
to where I am now...)  The Vax course wasn't even this good.  The 8600
was too fast and to scope.  The 11/780 too large.  Those were
microdiag courses... Run the micro's and let 'em call out the errors.
Not too useful when the A B C cables to the Unibus are swapped
by a bad installer or if AC/LO and DC/LO from the BA11 cause
micros to fail.  I remember seeing folks choke on that stuff in the
field.

You would scope out problems with the 465 scope, get diagnostic 
errors, figure which signal line was wrong and recommend which
board needed swapping.  The instructor would say yes or now
to the board.  Sometimes you got it... some times the signal was
floating due to other causes -- even when he pulled the tape.

These were the worst maintained most beaten to death 11's at DEC.
They were machines left over from in-house duties elsewhere and
often they were old prototypes.  11/40's with the old power
distribution and such.  Machines that would delight in driving a 
student nuts.

You see, the tape often would rub off a board during class and stick into 
the backplane connectors at random locations.  Often the machines
would develop intermittants due to massive amounts of board pulls
and reseats.

...
 > 
> You see, one thing that's always taught in books of fault-finding is to 
> use a logical approach, make tests, etc. And yes that's an _extremely_ 
> valuable skill to learn. 
Yup.
...

 However, in the real world I'm more likely to start with a 'lucky dip'. 
 Unplug all connectors/boards, clean contacts, look for bent pins. Check 
 all fuses. Power up and check the power rails with a voltmeter. 
 True... if
you know someone's been playing inside.  But in Field Service
with machines under maintenance most folks keep their hands out of
them. (except for printers and terminals)  This means something broke.
Check power and work from there.

...

 By doing that I've probably found 75% of common faults (bent pins on 
 those 0.1" header plugs like you find on 3.5" floppy drives, IDE drives, 
 SCSI drives, etc) are very common).  
 I've never seen an IDE drive or
SCSI drive with bent pins where 
no one ever played with the cable.

...
 > Then, if there's still a fault I use the
logical approach. Get out the 
> 'scope and analyser, sit down with the printset, and start tracing the fault.
> 
> So, of course I'd have spotted a taped over pin very early on. 
Not if you couldn't remove the boards from the backplane.

...

 Allison

-tony 

Bill
Proud survivor of the company formerly known as Digital

ex-DEC Field Service 1981-86
Somerset, NJ, Piscataway, NJ, Princeton, NJ
DEC Branch 79J
Former District Installation Team 1981-82
DEC Branch 7HT

+---------------------------------------------------------------------------+
| Bill and/or Carolyn Pechter    |        pechter(a)shell.monmouth.com        |
|   Bill Gates is a Persian cat and a monocle away from being a villain in  |
|  a James Bond movie              -- Dennis Miller                         | 
+---------------------------------------------------------------------------+