[mythtv-users] shocking hardware issue

[Adam Stylinski]

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
> _______________________________________________
> mythtv-users mailing list
> mythtv-users at mythtv.org
> http://mythtv.org/cgi-bin/mailman/listinfo/mythtv-users

Your post made my day.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 198 bytes
Desc: not available
URL: <http://mythtv.org/pipermail/mythtv-users/attachments/20090316/c2a8cc9e/attachment.pgp>