Wednesday, October 24, 2012

TV changing channels or volume by itself, where is the problem?

This article is about a not very common yet definitely annoying problem that can develop in any TV - LCD or plasma - and a basic troubleshooting guide to help narrow it down to a particular module.

It was prompted by a recent experience followed by a user question that has just arrived.

Dear Coppell TV Repair,
I had you service the Y-SUS board in my HP PL4260N and it works fine.

I am having another problem with the TV now and was wandering if you have heard of it and if so how would I solve the problem.

The tv will start changing channels on its own along with volume and flash the menu.I ruled out the remotes and covered the ir, disconnected all cables and it still does it.

Do you have any ideas what it could be?

The described problem of random channel change or volume increase or volume decrease can happen on any TV and I have seen it happen on different kinds of TVs.

There may be some that may be more prone to it - a common production defect is always capable of causing something like it - but I am not aware of any particular TV model where this is a known mass problem, including the HP PL4260N.

The good news is that on all TVs the troubleshooting process is relatively similar and also relatively simple.

The TV - be it LCD, plasma, LED, most likely projection TVs as well (although we don't do those) - can receive a command to change volume or channel in two major ways - the IR sensor or the on-unit navigation control, which in most TVs is in a form of several buttons.

Both these sources travel , usually independently from each other, to the main board where they are processed by the TV's "brain", the main board's micro controller.

Which means that the distortion can come from:
1) the TV's IR control unit including all the way to the main board
2) the TV's manual navigation unit, including wires all the way to the main board
3) something on the main board itself; because of the way the IR and the buttons modules operate this is more likely to be something in the navigation button's circuit than the other one, but either way it would be something on the main board.

To rule out the IR board you would want to just disconnect it from the main board. You can often do that at the plug on the IR board itself, but it is usually easier to just trace the cable from it to the main board and disconnect it there.
You can power on the TV and disconnect it then or you can disconnect it and power on automatically.
Either way you are trying to see if disconnecting the IR board will cause the volume or channel change to stop.

You do the exact same to rule out the buttons navigation board. Disconnect and power on through remote (of course IR board will have to be connected) or power on and disconnect from the main board.
If volume increase (or decrease) stops or channel flipping (or menu activation) stops, then it's something on the board or cable.
Literally two weeks ago I saw for the first time in my life a volume up button which has failed and had a constant 50 ohm resistance, resulting in a constant "vol+" response from the TV. TV owners said they don't ever use the button which is believable since almost everyone these days uses a remote. And yet it has failed.

At least once before I have seen the cable from the buttons panel to the main board get squeezed by and subsequently cut by the TV's bezel. I have also seen that same problem in other Chinese devices. Good companies - I think of Philips - design wire paths inside the body to prevent things like those from happening.

Anyway, if you rule out both of those then the problem is in the main board.

I have seen at least one case where a resistor has failed, near the buttons connector, and was causing the problem. Most if not all modern TVs do not have dedicated wire for each button on the panel; instead, they use an analog to digital converter on the main board to convert resistance to digital level.
Buttons on the power board are simply resistors that get connected to a chassis; different buttons connect different resistance values, the main board converts that to a digital number and the main board maps a number to a function.
Stands to reason that any significant impedance (resistance) change in the circuit will lead to misfire.

Sometimes the problem may not necessarily be a resistor; it may be an active component, i.e. the ADC (analog to digital converter) itself.

Troubleshooting the power board is beyond the scope of this article.

Hope this helps!


Unknown said...
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Unknown said...

One of my friend have the same kinds of problem which you mentioned over here. I'll recommend this post to him. It will surely help to him.
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