Do LCD tv's have built in test patterns ?

[mmartell]

There is a pad labelled TST_PGM on my T-Con that may stand for Test Program and was wondering if that was for displaying test patterns and how you would enable it.

I have yet to see anything on the screen of this lg 42lb5df-uc. Was hoping to rule out a bad panel and/or implicate a failed mainboard.

[tom66]

Re: Do LCD tv's have built in test patterns ?

Very few do.
Some Samsung LED-LCD with FRC do have, but not many.

[mmartell]

Re: Do LCD tv's have built in test patterns ?

So my TST_PGM pad is not for that then...

I can only get the backlight to fire up on this set by disconnecting the 8-pin flat cable going from the T-Con to the Master Inverter. Does that mean anything to you ?

[tom66]

Re: Do LCD tv's have built in test patterns ?

That is the inverter sync cable.

All LCD panels do blur a little when they change states, which is undesired as it makes the picture look smudgy. So a few panel manufacturers thought it would be a neat idea to design an inverter which blanks the screen while it is changing, which makes it look like it changes faster. It lights up for 50% of the time but 2x brighter. So, if the inverter does not light up when the t-con is connected, it stands to reason the t-con is not sending the sync signals the inverter needs. (The inverter will self-synchronise when the cable is removed but not with a dud t-con.)

[mmartell]

Re: Do LCD tv's have built in test patterns ?

Wow great information! I've already replaced the T-Con with a used board and had the same result. I was wondering if i got a bad replacement board...

On the inverter there are three active pins: Dim_in, Dim_out and Sen_CB. With the sync cable unplugged there is 5v on the Sen_CB pin but the matching tcon pin is grounded - that's why I thought I might have got a bad board.

Any of that make sense ?

[mmartell]

Re: Do LCD tv's have built in test patterns ?

Actually how come I still don't see a picture with the sync cable unplugged ? I mean there's nothing there whether the sync cable is plugged in or not.

[tom66]

Re: Do LCD tv's have built in test patterns ?

I think the t-con is defective in both cases. I guess the inverter uses SEN_CB to SENse the CaBle is connected and use that for sync info.
You get backlight with sync cable unplugged right?

[mmartell]

Re: Do LCD tv's have built in test patterns ?

Yes I do.

[tom66]

Re: Do LCD tv's have built in test patterns ?

You should see an image then if the t-con was OK, even if the sync cable caused issues.

[mmartell]

Re: Do LCD tv's have built in test patterns ?

Ok since there are three voltages on that sync cable (at least at the inverter anyway) Dim_in and Dim_Out (each around 2v and Sen_CB at 5v would it be fair to say none of those pins should be grounded on the T-Con ?

Because that's what I've got on the two boards I have now - Sen_CB tied directly to ground.

[tom66]

Re: Do LCD tv's have built in test patterns ?

I think Sen_CB is for sensing the cable is connected so it can work with or without it. The DimIN is likely the same signal the main board outputs and the T-con will modulate the DimOUT with its blanking signal.

[mmartell]

Re: Do LCD tv's have built in test patterns ?

So it's not harmful to sink that 5v sense voltage directly to ground ?

[tom66]

Re: Do LCD tv's have built in test patterns ?

It would probably have a few kilohms impedance behind it so current will be low.

[mmartell]

Re: Do LCD tv's have built in test patterns ?

Was thinking that was how I could tell that it was faulty - but it was designed that way.

Will order up another T-Con.

[mmartell]

Re: Do LCD tv's have built in test patterns ?

Ordered.

[mmartell]

Re: Do LCD tv's have built in test patterns ?

Third T-Con, same result. What's the definition of insanity again ? Lol. I think I can safely say the T-Con is not the issue here.

It seems like the video signal is not reaching the T-Con, so maybe mainboard or even LVDS cable.

What's confuses me is that the backlight won't come on if the sense cable is plugged into the T-CON - REGARDLESS OF WHETHER THE T-CON IS PLUGGED INTO THE MAINBOARD OR NOT. Which makes me think the Inverter is supplying voltages in the DIM circuit that it shouldn't be.

Frustrated. Any further thoughts out there ?

[mmartell]

Re: Do LCD tv's have built in test patterns ?

The problem was solved by replacing the mainboard. The dimming cable was left unplugged. Cheers !