A DVD player, a TV, composite cables, and a spark

April 22, 2006 7:57 PM

Connecting my DVD player to my TV apparently damaged the latter's composite cable ports. Now I'm wondering if this could happen to other devices.

When reconnecting my DVD player (a Cyberhome CH-DVD 402) to my TV's jacks using a composite cable (those red, white, and yellow RCA jacks) I noticed a spark (possibly at the DVD-player end, actually, but I actually can't remember now); since then images from any source connected via the composite jacks are completely scrambled. The TV still works normally with standard cable-TV input and had worked normally with this and other DVD players previously (when connected with the same cables, I believe).

I'd like to connect the same DVD player to another device. I know nothing about the electrical internals of anything so: could the DVD player or the composite cables have at some point become (permanently) faulty in such a way that they are more likely to permit damage to other electronic equipment (via static electricity, say, since that's likely the cause)?

posted by yz to technology (5 comments total)

Sounds like static damage, all right.

I know of no failure mode on the outputs of a normal video device that would make it more prone to damaging other things you hook it to. Of course, there are abundant failure modes for all kinds of electronic assemblies, so you might have something unusual.

One way to check... use a voltmeter to check for any large AC or DC voltages present on the outputs (i.e., at the end of your RCA cable (the yellow one= video). A normal video signal is nominally a volt or so, and is a complex waveform, so it may give you a reading, but not more than volt, under normal circumstances. If you measure something large, it's abnormal.

If you have an technoid friend with an oscilloscope, you can verify the presence of normal video waveforms on the output. Have him/her look for really big DC offsets.

Generally there is no high voltage in a DVD player except for the standard 120V, but that's AC and a long way from the outputs, which are all low voltage signals.

There IS high voltage in a TV.... if it's a big, standard TV, the anode voltage on the picture tube is 30,000 or more. Static from your carpet could get you that high, too.

I'd be willing to bet that you were nicely charged up from wiggling around on the carpet in back of the TV and static killed the video input to your TV. This would normally be to an input buffer amplifier USUALLY protected by electrostatic discharge suppression (ESD) components. If you saw a spark, it was a big charge, but it is still surprising it killed it. ESD can kill some senstive components with just a few volts, though, depending on where it hits.

If you have s-video inputs to your TV, they may be undamaged. Check to see if your TV and DVD have s-video also, instead of just composite, and you may be able to use them instead. Cables available at Radio Shack or the local hardware store.

Make sure you get rid of static first next time. I usually touch the little screw on the plastic cover of the wall outlet, which is usually grounded in modern homes. Perfectly safe to do so.

More info than you wanted, I bet. Good luck!
posted by FauxScot at 8:35 PM on April 22, 2006

The cable can not become more likely to cause damage. It can fail, but that is all.

The DVD player probably isn't going to cause further damage either, but I would be more careful with it anyway. Make sure it still turns on and picks up the disc successfully. If so, test the output as suggested by FauxScot, or hook it up to something you can afford to damage.. You could also hook a resistor in series with the signal pin on the cable, but that will degrade the signal quality, and you are about as likely to have that around as a multimeter, so..

I agree that the cause of your problem probably was static electricity, but it may have been leakage current - basically, the problem may be very similar to the one described in this recent question: Eject the core!.

Leakage current and static electricity can occur at any time, and the correct answer to those problems is to properly ground yourself, and only connect equipment that is turned off. I don't do those either :P And I blew up the output on my VCR because of it..

If anyone can elaborate on practical aspects and implications of the leakage current issue (read the linked question for details), that would be very interesting.
posted by Chuckles at 10:11 PM on April 22, 2006

The leakage current thing can get a little complicated.

What the folks in the other post are talking about is systems that are powered by the same AC mains supply having different potentials at the point of interconnect. This can come from faulty components, bad designs, certain types of component failures, etc. but most folks are probably talking about DC potentials that cross the isolation barrier that the power transformer is supposed to provide. This is a measured and verfied specification on consumer equipment, and it's important enough to more or less, keep Underwriter's Laboratory (UL) and CSA in business. One difference between say a medical device in a hospital and your stereo is that the medical leakage current specs are super stringent.

What is supposed to happen is that there is NO DC path back to the mains side of things, nor any connection of the hot AC lead to the outside/accessible points of the device. A two wire plug usually connects AC directly (via a switch and fuse) to two leads of a transformer, and that is why it doesn't fuctionally matter which way they are connected. The third, green (in USA) wire DOES go to ground, and it is usually connected to the same surfaces that may be accessible to the outside of the system (VCR). It safely shorts any leakage voltages to earth ground.

The thing is, in video and audio equipment, the outputs are 'floating' with respect to earth ground. A voltmeter connected between earth ground and a video output should have no reading. There SHOULD be a reading measurable between the two leads of the outputs. One of those, the outside one, is usually called Signal Ground, and all of your video/audio gear interconnects them via the outside connector on an RCA jack. If there are slight AC or DC potentials between them, this is supposed to eliminate them.

If you get a perceptible tingle between your equipment, I'd suspect something serious, either on the primary or secondary side of the power transformer, a defective switch, failed wiring insulation, etc.

BTW, in the old days of vacuum tubes, it was common NOT to have a power transformer in some designs. In those, the actual AC power line might have been connected to the chassis of the device, and the difference between which way the plug went in the wall could have been significant and potentially lethal. I have been nailed a bunch by this problem during troubleshooting old hardware, but in Darwinian fashion, survived and got a lot more cautious.

That said, I'll probably get electrocuted by my electric shaver tomorrow.

I would generally recommend doing interconnect wiring with the AC power disconnected. A major failure will smoke the interconnecting cables when you eventually turn things on and be an unambiguous testimony to the presence of a problem without getting you killed. Voltage is invisible and it is not your friend in the wrong places.

I could go on, but I am sure by now you're hoping I won't!

Feel free to email me if you have specific questions about your setup.
posted by FauxScot at 1:15 PM on April 23, 2006

Thank you both for your patent expertise; this was all very helpful. I don't have any other devices with which I can test this; and that combined with the expense and trouble of properly testing the jacks and the (distant) possibility that the player's ESD protection (however it's implemented) has somehow failed is tempting me to consider a new player. Perhaps I'm being excessively cautious here but the cost of an inexpensive new player could, I guess, be reasonably small relative to the consequences of permanently damaging the display's components. Maybe I'll change my mind again, though, before the stores open. . . .
posted by yz at 5:52 PM on April 23, 2006

Probably no one's reading but I did just realize while looking at my profile that I never concluded this with a further update. I did change my mind and did try connecting my current DVD player to my new display (with a little more attention to possible sources of static electricity); there have been, fortunately, no problems. Thanks again to the posters above.
posted by yz at 7:19 PM on July 11, 2006