EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: videobruce on October 07, 2014, 03:16:33 pm
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I have a bad PS from a LG 60PK250 Plasma TV. There was no standby voltage. I replaced that with a used supply and the set works fine.
Under a magnifying lamp, I found no exceptions to the original board (mfg, date 3/10) including bulged caps, nor did I find any cold solder joints.
Are the only components that can be tested in circuit are diodes? I have no schematics, only block diagrams from their training manual which is far more than what their so called service manual shows. >:(
I don't want to guess and start pulling components randomly to test or just replace.
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The standby supply SHOULD be pretty simple to troubleshoot. Often a small SMPS or linear transformer/rectifier/regulator fed directly off the incoming line ahead of the main power relay. Sometimes part of the main PSU board, sometimes not.
Once you find it, look for an open fuse feeding it. The fuses used here are usually soldered in "picofuses" that look like resistors.
You really want to get a schematic/service manual, which seems to be available from a few places online.
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Are the only components that can be tested in circuit are diodes?
No.
Because most components fail either open or dead short you can always do resistance measurement on unpowered boards.
You seem to have a defective power supply and an identical working one.
This makes it possible to compare component measurements without schematics and without desoldering.
Whatever component you are measuring on the known good board, you should get similar ohms readings on the defective board if the component you are measuring is not broken.
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You really want to get a schematic/service manual, which seems to be available from a few places online.
Yes, I really wanted to get one except for the minor point; there aren't any! :--
I have searched three times and the best I managed to find is a training guide for their top end set (in that series) that has the same panel, but a different chassis. It's a help, but there are no schematics for either the PS, or the Y or Z boards which comprise 90% of the circuity of the set. I might add I have three similar service manuals and all they have are schematics of the "Control" board and the "Main" board.
I wanted to avoid removing that board since that is the main TV and I don't know the discharge time for all of those caps. ;)
It could be worse, it could be a Vizio and I would have neither. There are no service manuals (let along schematics) available for any of their models in the past 3 or so years, just the older sets.
Here is the board;
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The standby supply SHOULD be pretty simple to troubleshoot. Often a small SMPS or linear transformer/rectifier/regulator fed directly off the incoming line ahead of the main power relay. Sometimes part of the main PSU board, sometimes not.
Once you find it, look for an open fuse feeding it. The fuses used here are usually soldered in "picofuses" that look like resistors.
You really want to get a schematic/service manual, which seems to be available from a few places online.
As N2IXK says they can be quite simple to troubleshoot.
Find the datasheet for the IC and often there is a "typical application" that is often copied on PCB's.
Often the VCC cap for the IC ages and needs to be replaced.
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IC?? There are none on that board. :-//
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There is more than enough stuff on this series on the web. You just need to use various google keywords e.g. the model, the smps board, the panel, part numbers, other models in series, other models of a similar series.
http://elektrotanya.com/lg_60pk750_chassis_pu02a_panel_60r1_exploded_sch.pdf/download.html (http://elektrotanya.com/lg_60pk750_chassis_pu02a_panel_60r1_exploded_sch.pdf/download.html)
http://forum.iwenzo.de/attachment_processor.php?mode=download_file&id=7929 (http://forum.iwenzo.de/attachment_processor.php?mode=download_file&id=7929)
https://www.scribd.com/doc/111967907/Lg-60pk250-60pk540-60pk550-60pk560-60pk750-60pk950-Chassis-Pu02a-Panel-60r1-Alignment (https://www.scribd.com/doc/111967907/Lg-60pk250-60pk540-60pk550-60pk560-60pk750-60pk950-Chassis-Pu02a-Panel-60r1-Alignment)
Different model but similar board.
LG 50PK540 plasma TV Repair power supply EAY60968801 Part 1-2 (https://www.youtube.com/watch?v=I9wF0OLyV0k#)
LG 50PK540 plasma TV Repair power supply EAY60968801 Part 2-2 (https://www.youtube.com/watch?v=vHrv5XaDULI#)
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I'm sure you already checked, but are those four fuses in the middle of the board good?
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Yes on the fuses, the 2nd thing I checked after a visual inspection of the caps. The standby light would light for 2 seconds then go dark after it was plugged in. No relay clicks.
Shock;
Thanks for the links but I already had all of those. That link for the SM is for a different chassis. As I stated, I already did more than one extensive search. None of those have any PS schematics which is really ridiculous.
I did search u-boob, but I only found what you posted. Some guy with a crap shaky camera phone that was pedaling his $70 service to replace four components for a different problem (the clicking relays problem which is more common).
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I've seen bad thermistors in LG psu's more than once or twice. Just figured I'd chime in. GL
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I'll remove the power cord, let it sit overnight to be sure any residual charges are gone and start taking readings to compare with the good board.
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I'll remove the power cord, let it sit overnight to be sure any residual charges are gone and start taking readings to compare with the good board.
BANG! You just got electrocuted or damaged your multimeter because you expected the circuit to discharge overnight.
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See if you can find the bootstrap resistor. Look for a large value 1/2W or 1W resistor, eg, 100K-500K
It's used to start the main IC when power is first connected. They are often under-size and fail open
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But not if I test for residual voltages first with a $20 DVM. ;)
I'll look for that resister. There is a IC on the bottom of the board that I forgot about. That will be a starting place though with no power, I don't know if I would see a difference between boards.