[mythtv-users] shocking hardware issue

Brian Wood beww at beww.org
Tue Mar 10 21:27:19 UTC 2009


On Tuesday 10 March 2009 14:13:15 James Crow wrote:
> On Tue, 2009-03-10 at 15:25 -0400, Josh White wrote:
> > One of my frontends has been acting up lately, and today I found a
> > disturbing symptom:  I went to restart the machine, by pressing and
> > holding the power button (since it seemed to be unresponsive) and as I
> > approached the button with my finger, I received what felt like a
> > static shock, and the machine reset, without my actually touching the
> > machine at all.
> >
> > When the machine booted, it said had lost he BIOS settings, and I had
> > to press F1 to adjust my settings, or F2 to run the defaults.  The
> > case is an Antec Fusion (silver), and it uses the stock 400w power
> > supply that came with it.  Until now, the machine has behaved
> > normally.  I did recently add a tuner card to the machine, which has a
> > coax analog cable cord connected to it.  In addition to the tuner
> > card, I have a PS/2 keyboard, USB mouse, Cat 6 network wire, USB
> > Windows MCE remote receiver, and a normal power cord connected to it.
> > It connects to my TV with a RGB TV-out from an 8400GS card, and
> > stereo, analog audio.
> >
> > I have little reason to believe there is a probelm with the ground
> > wire (in the wall outlet), but could this be a first symptom of that,
> > or perhaps an issue with the powerstrip it's plugged into?
> >
> > FYI, the machine did boot fine after this, and seemed to work.  The
> > only symptom I had noticed before this is that my remote control would
> > seem unresponsive, and then minutes later, would suddenly act on all
> > the input I had tried to give (like to skip a commercial; nothing
> > would happen during the commercial, but a few minutes into the next
> > segment, it would skip ahead as though I had pressed the forward
> > button several times).
> >
> > Any thoughts?
>
> It is possible to get a voltage potential from the coax shield of a
> cable line. I have mine grounded at each wall jack because I would
> occasionally get a small shock from the coax. It also helped with an
> audio hum by an old non grounded TV.

The fact that this happened jus after he added the tuner card makes me suspect 
it is indeed a problem with the cable TV ground vs. the AC power ground.

I'd try measuring the voltage between the CATV shield and the AC ground, if it 
is more than a fraction of a volt I'd investigate further. 

It may have been, as the OP alluded to, a static shock. These can build to 
very high potentials and are a common cause of problems, but the power switch 
really should be insulated from anything in the case, as was pointed out they 
are usually just plastic plunger that presses on a microswitch or similar 
device.

I'd also make sure the power cord is properly grounded and not fed by any sort 
of a "cheater" device or improper adapter, and that the polarity of the 
outlet feeding the machine is correct (The larger blade is the neutral, the 
smaller one is "hot", and there should be no significant potential between 
neutral and ground. This assumes the OP is in the USA or a country with 
similar standards.

Most of Europe uses 220 volt, not 110 (117 nominal actually). This is because, 
when most of the electrical grid was re-built after WWII, 220 was chosen in 
order to save on the copper cost by not running neutral conductors. 50 cycles 
was chosen instead of 60Hz. to mnimize the inductive reactance of the 
transmission lines, while still not causing significant flicker in lighting.

Interestingly, overhead railroad "catenary" lines once used 17Hz., as it 
minimized inductive reactance even more, and the RRs didn't care about 
flicker. Once the RRs started buying power from commercial sources, instead 
of generating their own, they were forced to go to 50/60 Hz., which is what 
they use now. Most "third rail" systems, like the NYC subway, use DC on the 
third rail so the rectifiers will be at the (presumably stationary) power 
plants instead of having to be dragged around by the rolling stock. Back when 
the recitifiers were mercury pool or Ignitrons, this was a significant 
factor. AC motors were not used because it was too difficult to control the 
motor speeds.

Modern deisel locomotives actually use AC traction motors these days, and they 
control the speed by varying the frequency of the AC going to the motor. 
There are still a lot of older DC motor locomotives around though.

AC traction motors do not have the tendency to overspeed when the load is 
reduced that DC motors have. "Wheel slip" is a major problem for DC motor 
locomotives, and is one reason they have sanders to sand the rails and 
minimize slip. 

Steam Locomotives had sanders as well, for similar reasons.

The newer subway cars also use AC traction. They actually chop the DC input to 
a variable-frequency AC waveform using large SCRs or Triacs about the size 
and shape of hockey pucks.

(My brother is a locomotive engineer, I hear more than I ever want to know 
about them)

-- 
beww
beww at beww.org


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