RCA RLED4778A No Backlight no Pic No Response

[mmartell]

Standby light lit and set responds to power up by changing red standby light to blue. Other than that no indication of any life. Will not go back into standby and power must be cycled at the mains to go back into standby.

All voltages present at PS after power up.

I'm thinking main/driver board/tcon in that order.

This is an LG panel and probably other internals. One and a half years old.

Need advice on how to narrow this down as I'm not familiar with failure modes on this set and there are no schematics/manuals that I can find.

Pics are of PS, main, driver board. Tcon buried atm.

[mmartell]

Re: RCA RLED4778A No Backlight no Pic No Response

Is there any common or generic fault with LG led tvs that would cause this symptom ?

And any idea on how to test the backlights on this ?

[mmartell]

Re: RCA RLED4778A No Backlight no Pic No Response

By the way the fault was degenerative - it got worse over time to the point it will no longer power on and work normally.

As an aside the main board in about the size of a pc video card !

Could use some troubleshooting tips here...

[paulstef]

Re: RCA RLED4778A No Backlight no Pic No Response

By the way the fault was degenerative - it got worse over time

Sounds more like resistors or capacitors. Any way you could get it working by heating or cooling the boards?

[mmartell]

Re: RCA RLED4778A No Backlight no Pic No Response

I've tried heating all the boards (except the tcon) to no avail. Cooling seems doubtful to me as a dead set is pretty cool to begin with lol.

Perhaps I should have a look at the tcon. I've been avoiding tearing off the shield because that means taking off the stand as well. Can a faulty tcon cause these symptoms ??

[tom66]

Re: RCA RLED4778A No Backlight no Pic No Response

Not from my experience.

[mmartell]

Re: RCA RLED4778A No Backlight no Pic No Response

There is a three pin jumper located on the main board just beneath the horizontal power connector at the top right of pic four above. The jumper can be moved over to short the top two pins instead of the bottom two pins. Any idea what that's for and how to use it ? Wondering if it's some sort of "reset" - not that it makes for any real solution.

[paulstef]

Re: RCA RLED4778A No Backlight no Pic No Response

Cooling seems doubtful to me as a dead set is pretty cool to begin with

Depending on where in the circuit a temperature sensitive component is located cooling it can have an effect, yes. 20 deg C is cool. -40 C is cooler. Just a thought.

[mmartell]

Re: RCA RLED4778A No Backlight no Pic No Response

I got ya. Just more of a believer of the "heat expansion for intermittent contact" in this situation as the set doesn't work for a period and then shut down.

Well, that and I'm out of canned air lol.

[paulstef]

Re: RCA RLED4778A No Backlight no Pic No Response

Not only this. There are many failure modes for all kinds of components. I would always go in both temperature directions. From experience. Easiest thing to do before serious troubleshooting.

[mmartell]

Re: RCA RLED4778A No Backlight no Pic No Response

Will get some dust spray tomorrow and giver er a try.

[paulstef]

Re: RCA RLED4778A No Backlight no Pic No Response

I'm sure you know that inverting the dust spray can it gets really cold. Just saying.

Otherwise, cooling spray.

[mmartell]

Re: RCA RLED4778A No Backlight no Pic No Response

Yes that's why I'm getting Dust Spray. Will report back tomorrow.

[mmartell]

Re: RCA RLED4778A No Backlight no Pic No Response

Hosed down the main board with cold air - no change.

I suspect component failure on the main board. Since it's quite small with few components I'm going to go over it with my meter and see if anything presents.

[mmartell]

Re: RCA RLED4778A No Backlight no Pic No Response

Verified I have good standby voltage and good PSON when power button pressed. After that the red standby led turns to blue all voltages are present except there is NO BLON from Main and set becomes frozen. It's possible the main IC is toast.

Generally does the main board interact with the tcon before issuing BLON ? I ask because that's the only board I haven't looked at yet.

I can find no shorts on the Main board. All five regulators measure good.

Am going to recap it as I can't find the board anywhere. Maybe there is ripple even though the esr looks good ?

[budm]

Re: RCA RLED4778A No Backlight no Pic No Response

"Generally does the main board interact with the tcon before issuing BLON ?" Most are not, some may use T-CON error to shut down the backlights and the other circuit at the same time. See if you get voltage at the T-CON since the processor also send the panel on signal to turn on the MOSFET on the main board that supplies the voltage to the T-CON fuse. It sure sounds like you have main board problem. You can also cut the BL_ON wire to allow you to force on the backlights circuit, once you have backlights and nothing else on the main board is bad you should see picture on the screen.

[mmartell]

Re: RCA RLED4778A No Backlight no Pic No Response

Well found NO VOLTAGE at the tcon fuse. In conjuntion with what you said above and the fact the set appears frozen after PSON command I would say the processor is not running or related components have failed or are without voltage.

I will check for voltages at the flash, ram, eeprom and processor. Beyond that I'm not sure how to verify their operation.

[mmartell]

Re: RCA RLED4778A No Backlight no Pic No Response

Capacitor E6 (100uf 16v) would not read in circuit with my esr meter. Read fine after taking it out. The cap is fed by a 1.8v regulator. It looks like it *may* feed the processor.

The circuit that it's in measures about 65ohm to ground. Is that possible or does it indicate a problem (a leak to ground) and where would I look for it ?

[mmartell]

Re: RCA RLED4778A No Backlight no Pic No Response

Some pics.. I would replace it but can't find it anywhere.

[mmartell]

Re: RCA RLED4778A No Backlight no Pic No Response

Need some help troubleshooting this...