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Vizio LCD TV - has sound but intermittent picture

Last response: in Home Theatre
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April 22, 2012 12:50:36 PM

TV is vizio GV42L FHDTV10A

Source of signal is directly connected Comcast cable and tuned in via the TVs QAM tuner

The symptoms were that for a week or so, the picture would be good but then I would hear some strange, almost interference-like humming sound on random channels and the picture would be slightly lower quality (almost like it wasn't quite tuned.) Not enough that I couldn't watch but it was noticeable compared to regular HDTV. This would happen for around 30 minutes an evening so I didn't really worry about it. I put it down to Comcast

One day, I turned the TV on and there was no picture but the sound was OK. This would happen across TV/HDMI/Component inputs. The backlight was on and the little "Vizio" logo on the front would glow white (as it should) and back to orange when the TV was switched off.

I did some research online and the consensus was to change the main board which I did. After turning the TV on (without the covers etc) I got my blue "no input" screen back (which is expected with no cable signal) and I thought it was fixed but I still heard that strange interference sound.

I added the safety covers and was about to put the plastic back cover on when I thought I'd better test again - this time there was no blue background and I was back to the original problem (but with the new board) - and the "interference" sound was still present.

I thought I'd try again with the original - presumed bad - board and popped that one back in. When switching on this time, there was no blue background (which I expected, since I thought it was the cause of the original issue) and the "interference hum" was present. I left it on for about a minute so I could try and locate the source of the noise and then it stopped and the blue background popped up.

So now I feel that the humming interference is the root cause but I don't know what it could be. The speakers are disconnected so I know it's coming from an internal component, but what? I assume that the power-supply would be the most likely culprit but I would love to hear some other ideas/advice.

I do have a multimeter, but I would need *explicit* "DMM for Dummies" advice on how to use it in this situation.

Thanks for reading
a b x TV
April 23, 2012 4:48:35 PM

I would agree, power supplies are often the cause of 60 cycle hum and interferrence. I'm not sure how deep you want to get into it, but you should consider getting the service manual as well. Those often list correct voltages, and resistances.

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April 23, 2012 7:25:58 PM

thanks for responding... I do have the service manual - I'm just not sure exactly what to do with it :-) I'm a little leery of jabbing my DMM probes into a live PSU

I see where the power comes into the board (a connector with a blue and brown wire) so I assume I set the DMM to AC volts, stick the black probe on the ground screw and the red probe to each wire.... but after that, I'm kind of lost. Also, since the problem now appears to be intermittent I'm guessing I really need an oscilloscope

I think I'll just get a new power board and eat the cost of a return if it makes no difference
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June 11, 2012 7:52:18 PM

Apologies for the delay... just wanted to update this for anyone with the same issue.

The squealing was ultimately coming from the t-con board that is just behind the power-supply.

Replacing the t-con resolved the issue.
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