I'm so stupid... Was: Head Cleaners

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Tue Jun 29 18:32:48 2004

> I've had an inclination for some time to start a thread along these
> lines: tales told at one's own expense. Of course, one can't make such a
> suggestion unless one is willing to put up one's own tale. So here's one
> at mine (I'm willing to follow up with another if this catches on at all):

Oh, where to start? I've got dozens of such stories where I've been
silly. Perhaps this will adversely affect my reputation here, on the
other hand I am man enough to admit I'm an idiot, so here goes...

I once spend hours at school figuring out why a tape recorder appeared
dead. No motor, no valve heater glow, etc. No outputs from the mains
transformer. I can't remember how many tests I did before I bothered to
unscrew the mains plug and check the fuse (UK 13A mains plugs have a
cartridge fuse inside). It was mising. Not blown, but removed. A new fuse
got the machine going again.

And I spent a very long time (as in several days) sorting out a PDP11/45
that would crash in strange ways after about an hour's running. I was
naive in those days and actually tried board-swapping. This made matters
worse as the replacement boards were a different revision and there was a
backplane modification (an extra clock signal IIRC) that you had to do.
Battled through the prints and did that, still no better.

After spending hours with boards on the extender card, checking signals,
etc, I finally thought to check the PSU. The +5V line to tbe memory (at
least) was sitting at 4.2V. Out came the screwdriver and I tweaked the
preset on the regulator. No more problems (even with the original CPU
boards).

Not a silly fault, but a silly thing to do was the time I had a
malfunctioning -15V regulator brick in the same machine. I put that on an
extender cable (this isn't an offficial DEC thing, but I have a cable
with an 8 pin mate-n-lock plug on one end and a socket on the other for
working on such bricks). I was trying to figure out why it was giving no
output into a minimal load and very foolishly I disabled the overcurrent
trip. Alas the reason it was giving no output was that the crowbar was
firing. With no overcurrent trip, the crowbar blew just about every
transistor on the PCB (and finally the fuse). The flash was spectacular....

Then there was the RK05 that would spin down during major disk activity.
When the head was stepping all over the disk, the thing would retract the
heads, and de-energise the spindle relay. Then it would start up again
and reload the heads. I spent quite a bit of time looking at the logic
(and this time I did check the PSU rails, all fine) before I thought to
check the interlock signal. It was being de-asserted. Now that comes from
2 microswitches in series, one detects that the door is closed, one
detects there's a pack in place. No, both swithces were fine, but some
idiot (not me) had cut one of the wires between them and just twisted the
ends together and bound them with insulting tape. Soldering that properly
cured the problem.

More stories when some others have owned up, I guess :-)

-tony
Received on Tue Jun 29 2004 - 18:32:48 BST