Need help with faulty PSU board for LG Plasma TV

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klrman
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Need help with faulty PSU board for LG Plasma TV

Any electronic wizards here that would know how to test and repair my original board?  Something internally went wrong with one of the components of the board, but no visual signs that I can see to identify the problem.  All capacitors etc look fine.  I wouldn't know how to test each part of the board to isolate the fault myself unless there is a trick to do it simply?  I have a dmm.  

 

Here is what the board looks like:

 

Edited by: klrman on 05/30/2018 - 16:37
Dusty
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It might help if you describe the symptoms, but if it’s no power, there appears to be a fuse in the lower left corner.

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klrman
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When I turn the tv on, the tv automatically then turns itself off again.  There are two fuses on the board, but can't tell if they are bad just by looking at them as they seem to be solid fuses?  After doing some research online, it all goes back to the psu board, some with blown fuses or caps, or other parts of the board.

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Look beneath the caps too. Sometimes they’ll bulge that way instead. I’d start with the caps in the upper right corner that are suspiciously tilted.

Punch the part number for the PSU into ebay. Sometimes you can get a new one for less than the cost of the replacement caps.

The low mode should be lower.

Reviews: Efan IMR18350 700mAh 10.5A, <a href="http://

klrman
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I did look and they look fine.  I got a replacement board from ebay that was "new" but when it arrived today, it was badly used, smelled terribly as like it was in a real dirty house and had wear marks on it as if it had been around the block.  Now I have to file a PP against the seller, but that doesn't help me much to fix the problem.  Do those fuses show any visible signs if they blow since they are not glass, can't see whats going on with them.

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Without a schematic,  you have a tough row to hoe.

 

Checking the fuses can be easily done with the DMM.  However, even if you found an open one, it could have been blown by some other component - thus replacing the fuse won't get you anywhere.  Some of the videos I watched have found bad IC regulators, like the ones screwed onto the big heat sinks I see in your photo.  Without schematics and replacements parts, I can't think of any easy way to check them.  When I was a tech, I never liked working on analog power supplies. One bad part tends to cascade the incorrect voltages all over the circuit, making it hard to isolate where the true problem originated.

 

There seems to be replacement boards available for around $50.  Edit: Oops, I see  you got a new board, which didn't go so well.

 

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Using your DMM, touch each side of the fuse with your leads and check for resistance it should be very low if the fuse is good.

klrman
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Thanks for the info.  I know used boards for about $50 or so, but then I am in the same boat as the one I just got today that was supposed to be new and was faulty.  

klrman
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moderator007 wrote:
Using your DMM, touch each side of the fuse with your leads and check for resistance it should be very low if the fuse is good.

 

When I get back in a couple of hours, it will be the first thing I try just to see if anything shows up thanks.

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Check here: https://www.youtube.com/watch?v=BYQW9TwNgkk

Changed caps on a number of PC motherboards. One of them the some caps were obviously bad. Got a kit with the common ‘problem’ caps for that MB. Replaced the obviously blown ones. It acted exactly the same. Went back and replaced the rest. > Fixed. Could see absolutely no problem with those caps but they were evidently bad.

Had a TV with similar problems and assumed it was the same. No bad caps found. Turns out it was probably just one of the ribbon plugs had gotten high resistance over time and simply taking them out and plugging back in fixed it.

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If you can’t see any visible signs of something blown or over heated on the back or front its gonna be pretty hard to nail it down without some knowledge of where and what to probe. I had a TV that had quit working because of some glue the factory puts on the larger capacitors to help hold them in place. The glue gets old and hard and becomes conductive. One night with the TV off we here a loud bang like a 22 went off in our bedroom. Couldn’t figure out what the crap it was until we went to turn the TV on, Nothing but the power light came on. I took it apart and looked and looked and read up on my model TV and sure enough when I found the glue on a large capacitor there was a fusible resistor right beside it that looked charred. The capacitor had discharged and jumped thru the fusible resistor and poof no TV. The dust that had accumulated in there probably didn’t help matters either. Cleaned out the dust, scrapped off the capacitor glue and ordered the fusible resistor from digikey. When it arrived soldered the new one in and reassembled. It has been working for 2 years without problems.
Mine was only a glue at fault. So there’s really no telling with yours but the best advice I can give you is research your particular board and all the problems that are associated with that particular board. That will narrow it down to more common faults to look for. It took me days to find the problem.

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Inspect all the solder joints of the capacitors on the back side of the board. i have repaired dozens of TVs and the predominant failure mode is bad caps and bad solder joints.

Here is a good joint:

And this one is bad, notice the tiny black ring which is a crack in the solder. This makes for an intermittent junction that sometimes works when you gobsmack the side of the TV—but eventually the gap gets too big and smacking doesn’t work anymore. Get a hot soldering iron and touch up all the solder joints, adding solder where the joints look cracked or need more to cover the pad and “wet” the leads.

klrman
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flydiver wrote:
Check here: https://www.youtube.com/watch?v=BYQW9TwNgkk Changed caps on a number of PC motherboards. One of them the some caps were obviously bad. Got a kit with the common 'problem' caps for that MB. Replaced the obviously blown ones. It acted exactly the same. Went back and replaced the rest. > Fixed. Could see absolutely no problem with those caps but they were evidently bad. Had a TV with similar problems and assumed it was the same. No bad caps found. Turns out it was probably just one of the ribbon plugs had gotten high resistance over time and simply taking them out and plugging back in fixed it.

 

Thanks for the vid!  Had another look at all the caps, tried the ribbon connections, but still same problem.  Clicking sound comes from psu board and I have read that usually means the psu board is the problem, but you gave me an idea with the other ribbon plugs.  Going to unplug all of them and plug them in again and see how that works out.

klrman
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moderator007 wrote:
If you can't see any visible signs of something blown or over heated on the back or front its gonna be pretty hard to nail it down without some knowledge of where and what to probe. I had a TV that had quit working because of some glue the factory puts on the larger capacitors to help hold them in place. The glue gets old and hard and becomes conductive. One night with the TV off we here a loud bang like a 22 went off in our bedroom. Couldn't figure out what the crap it was until we went to turn the TV on, Nothing but the power light came on. I took it apart and looked and looked and read up on my model TV and sure enough when I found the glue on a large capacitor there was a fusible resistor right beside it that looked charred. The capacitor had discharged and jumped thru the fusible resistor and poof no TV. The dust that had accumulated in there probably didn't help matters either. Cleaned out the dust, scrapped off the capacitor glue and ordered the fusible resistor from digikey. When it arrived soldered the new one in and reassembled. It has been working for 2 years without problems. Mine was only a glue at fault. So there's really no telling with yours but the best advice I can give you is research your particular board and all the problems that are associated with that particular board. That will narrow it down to more common faults to look for. It took me days to find the problem.

 

I heard a loud bang like that and it  was one of the bigger caps that blew on one of my pc psu.  Scared the daylights out of with when it happened as I thought it was a driveby and my window was the target.  Anyway, good advice thanks.  I've read a few different problems with this board but so far no visible signs  yet.  Will keep looking to see if I can locate something.

 

If I did it correctly, I just tested resistance on both fuses and dmm reads 000.1 for both of them.  Does that sound right?

klrman
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kennybobby wrote:
Inspect all the solder joints of the capacitors on the back side of the board. i have repaired dozens of TVs and the predominant failure mode is bad caps and bad solder joints. Here is a good joint: ![img]https://i.imgur.com/gAz0HYA.jpg[/img]! And this one is bad, notice the tiny black ring which is a crack in the solder. This makes for an intermittent junction that sometimes works when you gobsmack the side of the TV--but eventually the gap gets too big and smacking doesn't work anymore. Get a hot soldering iron and touch up all the solder joints, adding solder where the joints look cracked or need more to cover the pad and "wet" the leads. ![img]https://i.imgur.com/aB61O11.jpg[/img]![/quote]

 

Great idea thanks!  Last Winter I had problems with my brothers DRL on his honda and after some research, I ended up resoldering some cracked solder joins in the control module and everything worked again.    Thanks to all the tips in this thread I'll keep at it and see where it goes.   

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If it turns on then turns itself off, it sounds almost like too much of a load that the supply “folds back” shortly after.

Ever power up the supply with a dummy load (resistive), see if it holds steady?

If it does, increase the load to what the teevee should be drawing, see if it keeps holding steady. If yes, then a different board might be trying to draw too much.

Coupla new(ish) laptops they gave me from work have these weird-ass barrel connectors, not the usual 12V kind, but 18V or more, and huge (but flimsy) plugs. Squishable-between-your-fingers flimsy.

Anyhoo, tried powering up some strip-lights, evidently tried drawing more amps than the beastie was rated for, and just stopped. No voltage. Oops. Power it off (ie, unplug it), wait a few secs, then plug it back in, works again. Repeat the experiment, same thing. Shuts down, doesn’t come back up ‘til unplugged and “rested” a few secs.

That’s when I look up the ratings of the strip-light. Whups… Big Smile

09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0

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Both fuses are good. Does your DMM measure capacitance?
I also noticed in the bottom middle of your pic there is a black box. Looks like a relay to me. I had one of those go bad on a older TV before.

klrman
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My DMM is a AN8002 but since I have never measured capacitance before, I wouldn't know if it does or not?

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klrman wrote:

My DMM is a AN8002 but since I have never measured capacitance before, I wouldn't know if it does or not?

 

Looked it up, it does measure capacitance. Ideally, caps need to be measured out-of-circuit. Without knowing what the capacitors are in parallel with, the readings may or may not mean much.  

klrman
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Terry Oregon wrote:

klrman wrote:

My DMM is a AN8002 but since I have never measured capacitance before, I wouldn't know if it does or not?

 

Looked it up, it does measure capacitance. Ideally, caps need to be measured out-of-circuit. Without knowing what the capacitors are in parallel with, the readings may or may not mean much.  

 

Unless I desolder the caps, there is no other way of isolating them then to measure correctly is there?

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klrman wrote:

Terry Oregon wrote:

klrman wrote:

My DMM is a AN8002 but since I have never measured capacitance before, I wouldn't know if it does or not?

 

Looked it up, it does measure capacitance. Ideally, caps need to be measured out-of-circuit. Without knowing what the capacitors are in parallel with, the readings may or may not mean much.  

 

Unless I desolder the caps, there is no other way of isolating them then to measure correctly is there?

 

Not easily.  If you look up "how to measure caps in circuit", there are some rather sophisticated ways that might give you some ballpark values in some circumstances. This is not going to be practical for you.  https://www.youtube.com/watch?v=qDABYKoVO4Q

.

Even though your research "goes back to the PSU", it's certainly possible the issue is with the TV.  If a component in the TV shorted out, pulling too much current, the PSU may shut down as a safety feature.

,

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Lightbringer wrote:
If it turns on then turns itself off, it sounds almost like too much of a load that the supply "folds back" shortly after. Ever power up the supply with a dummy load (resistive), see if it holds steady? If it does, increase the load to what the teevee should be drawing, see if it keeps holding steady. If yes, then a *different* board might be trying to draw too much. Coupla new(ish) laptops they gave me from work have these weird-ass barrel connectors, not the usual 12V kind, but 18V or more, and huge (but flimsy) plugs. Squishable-between-your-fingers flimsy. Anyhoo, tried powering up some strip-lights, evidently tried drawing more amps than the beastie was rated for, and just *stopped*. No voltage. Oops. Power it off (ie, unplug it), wait a few secs, then plug it back in, works again. Repeat the experiment, same thing. Shuts down, doesn't come back up 'til unplugged and "rested" a few secs. That's when I look up the ratings of the strip-light. Whups... :D

 

I don't think it would be that easy to test it with a dummy load as it has three ribbon connectors so could be complicated trying to give power to it.  Would be disappointing if you were right and another board is faulty of the plasma screen itself as are drawing the extra current that is switching off the psu.

klrman
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Terry Oregon wrote:

klrman wrote:

Terry Oregon wrote:

klrman wrote:

My DMM is a AN8002 but since I have never measured capacitance before, I wouldn't know if it does or not?

 

Looked it up, it does measure capacitance. Ideally, caps need to be measured out-of-circuit. Without knowing what the capacitors are in parallel with, the readings may or may not mean much.  

 

Unless I desolder the caps, there is no other way of isolating them then to measure correctly is there?

 

Not easily.  If you look up "how to measure caps in circuit", there are some rather sophisticated ways that might give you some ballpark values in some circumstances. This is not going to be practical for you.  https://www.youtube.com/watch?v=qDABYKoVO4Q

.

Even though your research "goes back to the PSU", it's certainly possible the issue is with the TV.  If a component in the TV shorted out, pulling too much current, the PSU may shut down as a safety feature.

,

 

On occasion I have heard of the plasma screens themselves giving out so that is a real possibility.  If there is a short elsewhere, well that could be too, that's  why I ordered the psu board just to give it a go, but they cheated me and sent me a beat up used one instead of a new one.

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Apples and oranges, buuut, my LED backlit TV stopped working and I wasn't getting voltage at the power supply output, bought a used PSU and same thing happened.
Turns out it senses when one of the series LED lights goes out and shuts down. Bought some new LED strips from Shopjimmy.com and it's better than new.
Could be something downstream.
Shame you can't take all the tubes down to the drugstore tube tester or clean/replace the tuner knob. (sorry, joke for old people)

Mayor Glenn Welsch: There's a saying in Missouri, if you don't like the weather just wait five minutes. In Blaine, with hard work, I think we can get that down to three or four minutes.

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look for a short to ground on the va and vs lines.
if you find one unplug ysus from the power supply.
if still there unplug zsus.
y=big board on left as you are behind set.
z=the one on the right.
if no short leave va,vs unplugged from power supply and power it up.
measure from ground to va,vs.
if nothing possibly that section is not being commanded on or blown up.
or apfc not running and that section is in uvlo.
apfc output is at the 2 big caps on their side and is around 400vdc

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oh and big warning!
these power supplies are dangerous due to both high voltages and high surge current due to big filter caps on va and vs.
apfc is around 400vdc and backed up by big caps too.

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Thanks snakebite will give it a try.  Do the big caps still retain high voltage when unplugged?

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assume that they do.
measure before touching.

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Since you tried another PSU board that was likely okay even if dirty, and you got the exact same symptoms—then i’m gonna say your problem is NOT on the PSU but on one of the other boards.

You didn’t really troubleshoot this problem and determine that the PSU was bad, you just assumed it was bad, based on stuff you read on the websticle. You obviously don’t have the equipment or skills to troubleshoot it, especially without a schematic. This is too dangerous for a novice.

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Yeah, I took a chance that it was the psu after what I read and since the clicking sound came from that board, but as the short could be elsewhere, it could be a problem.  TV is at my brothers house, so Monday I will put the original psu back and unplug one at a time the other three boards that are plugged into the psu board to see if that isolates the problem possibly to a board and I can go from there.  I'll do my best to avoid a big voltage shock as I like my life at the moment!

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snakebite wrote:
look for a short to ground on the va and vs lines. if you find one unplug ysus from the power supply. if still there unplug zsus. y=big board on left as you are behind set. z=the one on the right. if no short leave va,vs unplugged from power supply and power it up. measure from ground to va,vs. if nothing possibly that section is not being commanded on or blown up. or apfc not running and that section is in uvlo. apfc output is at the 2 big caps on their side and is around 400vdc

 

I plugged in the original psu board and started unplugging each board to test like you said and the psu works again as long as the large board on the left in the pic is unplugged.  I could not see any faulty caps or anything though.  Is it worth the risk to try and get a replacement board or can I troubleshoot further now that we know something is going on with the left board?  Looks like all the plasma screen lines are plugged into that big board, but how would I test to see if  the problem is the board or what is connected to it?

 

Pic below psu is not in, but you can see the big board on the left.  PSU will work if I unplug the top wire to it, or

if I unplug the top wire of the right board where the left board plugs into.

 

Close up of left board

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