[mythtv-users] shocking hardware issue
Adam Stylinski
kungfujesus06 at gmail.com
Mon Mar 16 20:08:48 UTC 2009
On Tue, Mar 10, 2009 at 02:27:19PM -0700, Brian Wood wrote:
> 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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Your post made my day.
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