Reminds me of the bad old days, Bill.

Back in the 70's (while in the Navy), I had a "bizzare" problem with an
R390A HF receiver, a large, tube-type, 50 lb behemouth.  The trouble report
said that when the operator turned it on it would make horrible squealing
noises, which was surprising since it had no internal speaker.

I went over there, turned it on, and there were no problems.  It was late at
night, so I told the operator to get a large coffee and quit dreaming.
While I was talking, a trail of smoke was coming from the receiver, which
was sitting on a table.  The operator pointed out that maybe I should get
the coffee, I powered the receiver down and opened up the bottom of the
chassis.  After a few minutes of poking around, I found that a mouse had
crawled up on top of the tube sockets of the main regulator tubes (maybe
because it was warm?)  The operator had turned the receiver on and the poor
rodent suddenly found itself in the middle of a metal box with 400 or so
volts on one side and ground all around.  I had to take it back to the shop,
take all the electronics out of the chassis, and let it sit in an ultrasonic
"gunk tank" we used for the teletypes to get the mess off, while I picked
pieces of charred mouse out of the regutator circuits.  What fun :(

Must have been a Friday, too...

-----Original Message-----
From: DesignerCouncil
To: [log in to unmask]
Sent: 9/30/2005 2:31 PM
Subject: [DC] Friday oddities... :)

Why do Fridays always seem to be magnets for bizarre things to happen?

Yesterday in the late afternoon, I got a call from my wife at home that
the
Air Conditioning system had stopped working... she wanted to brainstorm
with
me over the phone so we could figure out what might be the cause... she
had
basically run out of ideas... and that's not easy, she's a very sharp
intelligent woman, not much gets passed her notice.

Hmmm... so we started eliminating possibilities...

She checked the circuit breaker to see if it had tripped, no, it was
fine.
She had checked the thermostat to see if it was faulty, nope... working
just
fine and indicating about 78 degrees in the house because the air
compressor
was not running. She tested the blower fan and it still functioned
admirably.. no fault there... just hot air coming out of it... Since it
was
hotter outside she left the blower going to circulate the air... but it
was
uncomfortable at best. She even checked the filter to see if it was
clogged
or anything... which it was pretty dusty so she took it out and washed
it
and re-installed it.. but no avail... still hot air.

After thinking about it some more we suspected the only other option was
something to do with the compressor itself. She went out to do a visual
inspection of the compressor unit and peered into the gratings on the
sides
to see if any thing could be seen from there... it was pretty dark in
there
and she couldn't see much... but wait... Oh my... it looked like there
was a
dead rodent in there... Hmmm... What would a rodent be doing in there to
begin with??? Maybe chewing on the cables? That would be a grim
thought...
So she reported that to me and I told her I would check it out when I
got
home in an hour or so...

Well, when I got home and retraced her steps double checking all the
things
we had discussed earlier and again I couldn't find any other reason for
the
unit to have failed and we couldn't really inspect inside of the
compressor
unit without disassembling the case and most likely voiding the warranty
on
the compressor, as this is a new unit we had just installed this spring
we
opted to call the company that installed it and have them come do a
service
call to fix whatever was the failure... They indicated they would be out
right away on Friday morning to take a look and get the unit back
on-line
for us. Of course that was a relief...

Well my wife just called today and relayed to me that they got it
working
again... (I bet you can't guess what made it fail.... ) We were
perplexed
because the service technician opened up the unit and disposed of the
rodent
carcass (using his pliers) and then looked for any damage inside and he
found NONE! No chewing, no bites... nothing! But when he tested the
compressor unit it still didn't work. This of course brought on some
'head
scratching' by the technician...

So, undaunted, he retrieved his trusty volt/ohm meter and started
checking
the power from the AC mains to the compressor. He traced the power from
the
circuit breaker to the main switch and into the utility box on the
housing
where a secondary remote switch was and it was all just fine, power
looked
good, no trouble. So why was it not working?

That was puzzling... So he checked the other side of the remote switch
that
runs to the motor and the power disappeared. No indication on the
meter...
dead. Hmmm... strange, so, being a persistent fellow and not willing to
accept defeat, he kept trying the power switch a few more times until he
heard an electrical ARC (zap!) and with a raised eyebrow, he took a
closer
look where the arc came from by uncovering the switch housing and
exposing
it's interior... And 'Lo and behold!'... there inside the contacts of
the
switch was a carbonized mass of hundreds of ANTS!!!... Who would have
believed it! They clogged the entire box where the switch contacts
were...

Apparently the ants were going for the 'food' in the bottom of the
compressor housing and made the unfortunate choice to use a path across
the
switch contacts as the way to get there... So, as the air conditioner
was
turning on and off over the last week or so... the build up of crispy
ants
that just happened to be traveling between the contacts of the switch as
it
was energized created a carbonized insulation between the contacts and
eventually the unit failed to start again due to an open circuit...

He cleaned the switch out and sprayed the perimeter of the compressor
unit
with ant poison and charged us 130 dollars for the service call which
took
him about 2 hours to solve. The air conditioner is working again, and
all
for a very strange set of environmental circumstances that brought about
the
failure of an electrical device that I'll bet the test engineers never
dreamed of... Isn't engineering a fascinating career?

Have a great weekend... :)


Bill Brooks - KG6VVP
PCB Design Engineer, C.I.D.+, C.I.I.
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510
Datron World Communications, Inc.
_______________________________________
San Diego Chapter of the IPC Designers Council
Communications Officer, Web Manager
http://dcchapters.ipc.org/SanDiego/
http://pcbwizards.com

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