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TOSHIBA 57HC85 voltage coming through HDMI - grounding prob?    
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jcfuller



Joined: 19 Dec 2010
Posts: 4

PostPosted: Sun Dec 19, 2010 5:04 pm    Post subject: TOSHIBA 57HC85 voltage coming through HDMI - grounding prob? Reply with quote

The situation:

I have a Toshiba 57HC85 projection HDTV.

My old cable box was constantly resetting and finally would not turn on. The cable technician replaced it with a new Motorola DC3200X-M cable box. He informed me that he was reading 50 volts coming through the HDMI cable from the T.V., and that was likely to be what fried the cable box. He then hooked it up with component cables so that hopefully it wouldn't fry it again.

He told me that there may be a problem with the house wiring or the TV's ground, so I bought a simple outlet tester to find out if there is a problem with the house wiring. The outlet tester shows that there is indeed a problem with these outlets, so I will call an electrician to take a look. However, I found an outlet in another room that the tester showed to be working properly and ran an extension cord to the TV in order to check whether switching the TV over to a known good outlet would solve the voltage problem. Unfortunately, it didn't.

Using a multimeter, I've found that there is around 43V coming through the HDMI cable from the TV. I then tested the component cables and found the same 43V coming through. It seems that the issue is with my TV and occurs for both the HDMI and component. I would gladly buy a new one if I had the money, but I'm hoping for a cheaper fix.


Questions:

1. Is it safer to hook up the cable box to my TV with the component cables, or will it eventually fry the box again either way?

2. Is there any kind of go-between device that would be able to 'eliminate' the voltage in my HDMI cable? Or, is there any way to divert it or properly ground the TV (if there is a grounding problem)?

3. Does the Toshiba 57HC85 typically have grounding issues or electrical problems? Especially considering that it only has a two-prong plug, that might make sense. If so, what can be done to fix it?

4. Could the problem with the house wiring have damaged my TV, or is 43V coming out of the cables normal for a Toshiba 57HC85?


Please respond as soon as possible, and thank you for your time.
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kuhurdler



Joined: 28 Dec 2006
Posts: 2862
Location: Overland Park, KS

PostPosted: Sun Dec 19, 2010 6:05 pm    Post subject: Reply with quote

just curious what reference you are using to find the 4x volts? Is this AC or DC voltage.
Your TV should be creating its own voltages on the secondary side of the power supply. The circuit would then be floating and completely isolated from the wiring of your house. You may be seeing phantom voltages, and there may not really be a problem at all. IT all depends on how your measuring it.


If it is truly putting a voltage (with current) out of the hdmi input, I would suspect a solder splash on the circuit board that is causing it, or else a loose piece of metal inside the TV has shorted something.

My best Answers:
1. Safe as long as the TV isn't sending voltage through the component cables too.
2. its possible, but I think you're headed down the wrong path. Fix the problem, not the symptom.
3. no. Most TVs and electronics have a two prong cable.
4. depends on what the problem was with the wiring. I think it is unlikely.

someone else may weigh in on this too.
jcfuller



Joined: 19 Dec 2010
Posts: 4

PostPosted: Sun Dec 19, 2010 6:37 pm    Post subject: Reply with quote

Thanks for your quick reply, kuhurdler.

I used a Commercial Electric M1015B multitester on 50 V~ AC mode. I touched one lead to the HDMI cable end and the other to a metal screw on the cable box (which is what the cable technician did, so I did the same). The readout was about 43V. Unfortunately, there appears to be the same amount of voltage coming through the component cables when I test them in the same fashion.

If it is a short in the TV, would you say that it could be fixable? And if so, is it something that I can do myself with relatively little experience?
kuhurdler



Joined: 28 Dec 2006
Posts: 2862
Location: Overland Park, KS

PostPosted: Sun Dec 19, 2010 7:17 pm    Post subject: Reply with quote

I'm still not convinced you have a problem. I'd first suspect it is just a phantom voltage from the floating power supply. Especially since it's AC voltage.

otherwise, you'd feel a shock when you touched both those locations simultaneously .

I couldnt find a great explanation of what a floating power supply is, but here is the best I could find doing a search:
http://www.ese.upenn.edu/rca/instruments/HPpower/PS3631A.html
jcfuller



Joined: 19 Dec 2010
Posts: 4

PostPosted: Sun Dec 19, 2010 7:38 pm    Post subject: Reply with quote

Oh, I do feel a shock... haha

So a floating power supply is not connected to the ground and should only be outputting the correct voltage for the device it's designed for? I'm not very experienced in electronics engineering.

A response from another forum where I posted my question was that it might be reversed polarity. The outlet tester (GRT-3500) showed that there is reversed polarity (middle light is on, left(red) and right lights are flickering) in the outlets in this room, but I would think that since I ran an extension cord to an outlet in another room that tested to be ok by the same tester (middle and right lights on), it shouldn't affect it? Maybe that's the wrong track too?
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kuhurdler



Joined: 28 Dec 2006
Posts: 2862
Location: Overland Park, KS

PostPosted: Sun Dec 19, 2010 9:32 pm    Post subject: Reply with quote

There is a shock, huh? I change my answer then. There is something wrong.

I'm not sure what it could be though without looking over the schematic. This is the kind of problem I like to troubleshoot.

I'd probably start with my Oscope and see what kind of voltage your seeing on that hdmi output. If it looks a clean sine wave or not, you'll know which portion to chase down. Without an Oscilloscope, I'm not sure where to point you toward except checking for any weird solder splashes near the signal board.
jcfuller



Joined: 19 Dec 2010
Posts: 4

PostPosted: Mon Dec 20, 2010 1:14 pm    Post subject: Reply with quote

Here are some very strange facts from today.

First of all, I'm starting to think that my tester is worthless or I am still not using it properly, although I can't imagine that I can screw up using such a straightforward device... However, that doesn't negate the fact that I was feeling the shock when I held both ends in my hands.

I tested all of the outputs on the back of the TV (component, hdmi, composite, s-video, audio, etc.) by placing one lead on the outside of the port and the other on ground, and they all tested to have 43V coming out of them, so I pulled all of the cables out of the ports immediately. I then re-tested the outputs with no cables coming out of the TV, and they read 20V for each of them. Then, I used the tester on the cable ends coming out of my other devices (a dvd player and a couple game consoles), and the readings varied all over the place - at first, between 20V and 30V.

Just taking shots in the dark, maybe I've ruined the tester (it was cheap, so not a big deal), or there is voltage coming out of basically everything (which would be a huge deal), or the TV has been damaged by the house wiring, etc.

The electrician will be coming in a few days to look at the outlets, and hopefully he'll know much more about what he's doing than I do. I'll let you know what he says. However, when the cable technician was here a week ago, he mentioned that this "voltage coming through the HDMI cable and frying the cable box problem" is common for them, so it must be a real problem somewhere. In the meantime, if you can think of anything else or a more general situation that might be causing all of this, please let me know. I very much appreciate your help and input.
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