LG 42LC7D-UB power problem.

[dietzcj]

Hello !

I am currently troubleshooting an LG 42LC7D-UB tv.
The TV shuts down after about 10 min of operation. The power light goes to red standby.
When I try to turn the TV back on, it immediately shuts off.
If I leave teh TV unplugged for about 1 hour I can get it to come back on for another 10 min.
I have taken it appart to view teh power supply and there are not bulgin capacitors..
I have done further testing by just leaving the Power supply and the logic board and the spearkers (No back lights or T-con board) and get the same result.
This is a TV that was given to me and the person went an bought a new TV.

I do not want to purchase both the power supply and a logic board if only 1 of them is needed.

My suspician is that the logic board is telling the power supply to shut down, but why I do not know.
THe Sleep timers are off and there no software updates for this TV.
I have tested the 24v 6.3 v and other points with my multi meter and they did test okay until the unit shuts off.

Any ideas ?

Chris

[PlainBill]

Re: LG 42LC7D-UB power problem.

Only one idea - something is overheating, which causes the shutdown.

Usually there is a control line from the main board back to the power supply that turns on the main supply. Common names for this include PS_ON, ON, ON/OFF, etc.

My suggestion would be to identify this line and interrupt the control signal from the main board. Then pull the control line 'on' while hitting the On button for the TV. When the TV shuts down an hour later quickly check the output voltages of the power supply. If all are correct, it's most likely a main board issue. Whatever the results, don't leave the power supply active any longer than necessary.

PlainBill

[dietzcj]

Re: LG 42LC7D-UB power problem.

Good advice, but if the ps line is in a socket with other line feeds how do I disconnect it with out the other lines as well ?
I see what you are trying to do I just don't know how to do what you propose. That being said, if it is over heating then it is either the PS or the logic board. Of course I do not know if it is a transformer or any other component on the PS. I cannot imaging the logic board over heating.

Chris

[PlainBill]

Re: LG 42LC7D-UB power problem.

Good advice, but if the ps line is in a socket with other line feeds how do I disconnect it with out the other lines as well ?
I see what you are trying to do I just don't know how to do what you propose. That being said, if it is over heating then it is either the PS or the logic board. Of course I do not know if it is a transformer or any other component on the PS. I cannot imaging the logic board over heating.

Chris

It won't necessarily be pretty but there are many ways to do it. Is this a ribbon cable, or is it a bundle of wires? If it's a bundle of wires, just cut the one wire and strip enough insulation to make contact with the conductor. When you are done, splice it and cover it with heat shrink tubing. If it's a series of wires, you can often release an individual contact from the shell holding them.

If it's a ribbon cable, you will have to work on the PC boards. Cut a trace or lift a jumper.

If you are unable to conceive of an IC getting warm enough to shut down, this effort is doomed to failure.

PlainBill

[dietzcj]

Re: LG 42LC7D-UB power problem.

so if the voltages are not dropping then the PS is good and the logic are is bad. But if the voltages are dropping then it is the PS ?

Sounds logical.

[dietzcj]

Re: LG 42LC7D-UB power problem.

So here is what I did:

Spliced the line that was labeled on/off from the socket (it was bunched up wires and not a ribbon) , stripped the ends so I could reconnect them. I twisted them back together and then power on the Unit.
I then pulled them part to test like PlainBill said.
The unit immediately shut of once that line was disconnected......

[PlainBill]

Re: LG 42LC7D-UB power problem.

So here is what I did:

Spliced the line that was labeled on/off from the socket (it was bunched up wires and not a ribbon) , stripped the ends so I could reconnect them. I twisted them back together and then power on the Unit.
I then pulled them part to test like PlainBill said.
The unit immediately shut of once that line was disconnected......

Good first step. That's exactly what I would expect to happen. Now for the real test.

With the cut wires twisted together, plug in the TV and press the power button. While the TV is on, measure the control voltage, either at one of the connectors, or at the twist. I usually use the head of a screw holding a circuit board in place as ground. The voltage will probably be about 3 to 3.3 Volts. Now turn off the TV.

Next, prepare a voltage source. Most people don't have a variable power supply sitting around. Panasonic suggests connecting two AA cells in series to produce 3 volts. One fancy way to do this is Radio Shack 270-408, a battery holder with leads for 2 AA cells.

Now connect the - lead from the battery to a ground point (the screw mentioned above). Plug in the TV, and connect the battery to the side of the control lead going to the power supply as you are pressing the power button on the TV. The TV should turn on and stay on.

Nest, wait until the TV shuts down. Immediately check the voltages out of the power supply. If any are missing, it's a power supply problem. If all are correct, it's a main board problem.

PlainBill

[dietzcj]

Re: LG 42LC7D-UB power problem.

I will also add some alligator clips and shrink tubing to my shopping list as well.
I will post my findings later today !

[dietzcj]

Re: LG 42LC7D-UB power problem.

So, by placing the battery circuit to the power supply (the logic board says "stay on" to the power supply) wouldn't the TV just stay on since the logic board cannot cut the voltage to the power supply?
If it does just stay on, would that indicate a bad logic board ?

[dietzcj]

Re: LG 42LC7D-UB power problem.

Okay, I did the battery trick and the TV did turn off as it normally did. So the logic board could not tell the power supply to shut down because I had that battery interrupting the On/off circuit.
Checking some of the voltages, the 3.4 is not even registering.
Bad power supply ?

Now what is over heating ?
I actually have a power supply board (working) from an LG4250 that I can use for parts (Since I cannot sell it on ebay , tried to 7 times).

Chris

[PlainBill]

Re: LG 42LC7D-UB power problem.

Okay, I did the battery trick and the TV did turn off as it normally did. So the logic board could not tell the power supply to shut down because I had that battery interrupting the On/off circuit.
Checking some of the voltages, the 3.4 is not even registering.
Bad power supply ?

Now what is over heating ?
I actually have a power supply board (working) from an LG4250 that I can use for parts (Since I cannot sell it on ebay , tried to 7 times).

Chris

Good diagnosis. SMPS power supplies are often repairable. If the 3.4 volt output is shutting down (and some other outputs are good), troubleshooting should be fairly easy.

Standard suggestion at this point: Clear, high resolution (as close to 2000 x 2000 pixels as possible preferred) of top and bottom of the power supply. Use 'Manage Attachments' (below the text entry area) to attach them.

PlainBill

[dietzcj]

Re: LG 42LC7D-UB power problem.

Here are the pics

[PlainBill]

Re: LG 42LC7D-UB power problem.

I'm going to need a couple more pictures. The first should cover the same area as the third picture you attached. On that one I am mainly interested in the labels on the output connectors - I want to be able to identify which pins carry the 3.4 volt output. The second should cover the under side of the board.

PlainBill

[tom66]

Re: LG 42LC7D-UB power problem.

I fixed an IDENTICAL power supply in my LG 42LC46.

Replace the four 2200u 10V capacitors, two around the standby section, and two in the secondary side. I've highlighted them in red.

It brought this TV (dumpster find) back from the dead... (it wouldn't power on at all - but I wouldn't be surprised if it exhibited this problem before it was thrown in the trash.)

One of the caps filters the 3.4Vstby line which runs off the 5Vstby line (there's a buck converter on the other side of the board - note two SMD FETs and a buck controller IC.) If this is bad, the TV will reset as the CPU requires it. But it's probably best to replace all four of these because mine had them all bulging - they are going to fail at one time or another.

I bet they are the cheap Samwha caps that LG are so keen on using.

I replaced them with Rubycon YXF.

[dietzcj]

Re: LG 42LC7D-UB power problem.

This is about as good as I can get it.
Only the 2 lower sockets are used.

[PlainBill]

Re: LG 42LC7D-UB power problem.

This is about as good as I can get it.
Only the 2 lower sockets are used.

OK, that gives me the pinout. Now I need one of the reverse (back, bottom, circuit, green) side of the board.

Also note tom66's suggestions.

PlainBill

[tom66]

Re: LG 42LC7D-UB power problem.

This is about as good as I can get it.
Only the 2 lower sockets are used.

Can you give a P/N of that PSU, may be possible to find a schematic. I can't remember it since I fixed my TV, a few months ago.

[dietzcj]

Re: LG 42LC7D-UB power problem.

Here is a more detailed photo< I used MS paint to make it more clear.

[dietzcj]

Re: LG 42LC7D-UB power problem.

EAX32268301/8
That is the part number.,
And I have replaced all of the 2200, 10 V capacitors as was suggested.

[tom66]

Re: LG 42LC7D-UB power problem.

EAX32268301/8
That is the part number.,
And I have replaced all of the 2200, 10 V capacitors as was suggested.

Still not working?

What caps did you use?