LG tv powesupply dead - Page 1
Forum section: electronics / repair

author: Sjokolade
author_rank: Regular Contributor
timestamp: June 27, 2021, 12:38:03 am
content: Being a novice in repair I'm posting here asking for help - this LG tv have no standby led and is completely dead.Measuring ac on the bridge rectifier when plugging it in to the wall the ac voltage starts off at 230v and drops to 0 within a few seconds after relay clicks - I can't see or find anything that is blown but being a novice this is a too complicated board.Fuses are fine, rectifier measures fine, resoldered some parts incase there were cracked solder, measured a few mosfets , measuring across mains input I get 3.8M ohm.Any tips on how to proceed ?Link to dropbox for bigger pictures -https://www.dropbox.com/sh/3uieewey66whrbw/AACvCoYEbYL7a8AMV7JBmFE2a?dl=0

author: bob91343
author_rank: Super Contributor
timestamp: June 27, 2021, 03:21:24 am
content: Those kinds of equipment generally have two power supplies. One is always on, ready to accept a command to turn on the other one. So you need to see where these sections are and why the small one isn't getting the main one to energize.

author: KevinA
author_rank: Regular Contributor
timestamp: June 28, 2021, 06:22:49 pm
content: Never even bothered trying to repair one of these. Around £35 for a working board pulled from a working TV, that equates to 30 minutes labour.dJust checked on my parts database from my old employer, he had four in stock four years ago.

author: RayRay
author_rank: Frequent Contributor
timestamp: June 28, 2021, 08:29:01 pm
content: Tested the diodes with your multimeter? Do you have an LCR/ESR meter? Capacitance testing ability in your meter (with a good range)? If you don't have sufficient testing gear or time to work on this, you might wanna consider buying a replacement board instead:https://www.ebay.com/itm/324321842826It'd cost you 74 euros (incl delivery) which is a bit pricey from one hand, but would save you quite a lot of time from the other!

author: Sjokolade
author_rank: Regular Contributor
timestamp: June 28, 2021, 09:24:58 pm
content: Quote from: KevinA on June 28, 2021, 06:22:49 pmI did consider that but with the new tax system on imports over here the board RayRay links to would cost close to 93 euro sadly - I might go for a used board if I can't fix it.Quote from: RayRay on June 28, 2021, 08:29:01 pmYes I did test the diodes and they seem fine, I do not have a LCR meter but I do have a ESR micro v4s and I did measure the caps in circuit and they also seems fine.I have some free time, weather forecast looks bad but I might consider buying a used board if this one can't be fixed.

author: RayRay
author_rank: Frequent Contributor
timestamp: June 28, 2021, 10:31:58 pm
content: Quote from: Sjokolade on June 28, 2021, 09:24:58 pmIncluding the two I've marked on the attached pic? In diode mode on the meter and in both forward and reverse biases? For regular (black) diodes, you're suppose to get a reading only in the correct polarity, if you get it in both, then it's either something in the circuit affecting it, or the particular diode is faulty (only way to tell in such a case would be to desolder it and test it out of the circuit). Also, does your ESR micro only measures ESR, or capacitance too? Good ESR alone doesn't neccessiarily mean the cap is okay. And also, Did you measure all the electrolytic caps (incl the 4 main, largest ones)?

author: Sjokolade
author_rank: Regular Contributor
timestamp: June 29, 2021, 07:22:59 pm
content: Yes both measures fine in both directions, the ESR micro measures both esr and capacitance. I did measure all electrolytic caps, the 5 main caps seems fine and measures 0.3ohms and 203µF.Two caps might be on the edge but I'm not completely sure, they measures 7.3ohms and 10.9µF in circuit - the datasheet does not list esr for the NFC series.Link to dropbox for datasheet -https://www.dropbox.com/s/3le1svud8s7p7nw/NFC.pdf?dl=0

author: kingsolmn
author_rank: Contributor
timestamp: June 29, 2021, 07:43:00 pm
content: Quote from: Sjokolade on June 27, 2021, 12:38:03 amHave you tried following the money on that relay? Sounds like the relay is part of the init post, and then shuts down waiting for the "on" command. If that makes sense?Also, I'd also take a close look at the standby/power switch and follow that to the trigger point. And maybe the switch itself is bad? Or an associated component on its daughter board?Just spitballing here, I usually have to poke around quite a bit before I find that something with these symptoms is usually a faulty switch or cracked solder joint or something where the "200 lb gorilla" meets the button - to borrow from AvE's vernacular. Lol Good luck!

author: RayRay
author_rank: Frequent Contributor
timestamp: June 29, 2021, 09:53:51 pm
content: Quote from: Sjokolade on June 29, 2021, 07:22:59 pmThat part may or may not be normal, I'll explain. ZD = Zener Diode. When testing Zeners with a multimeter in diode mode, if for example the testing voltage of your meter is 3V, and the diode is 2V (rated lower than your meter's testing voltage) then you'll get a reading in both directions, while the reading in the reverse direction would be higher than the other! However, if the diode is rated higher than your meter's voltage, then youre suppose to get a reading only in the correct polarity and not in the other. So to sum it up, you have to know the specs of the diode in order to determine if that's normal or not! Try looking at it with a magnifier and some light, if you can't get the model from that, then I'd advise you to desolder it so you could have a better look.As for the caps though, it's possible that they're connected in series (and that youre getting a reading of both, and not just each standalone) not to mention there could be other things in the circuit negatively affecting the measurement. I'd say it'd be best you desolder em and test em again. As for their datasheet, if the esr/impedance isn't mentioned, then they're prob not low ESR (which is not surprising, as that's fairly rare for such low capacitance)

author: Sjokolade
author_rank: Regular Contributor
timestamp: July 02, 2021, 10:45:37 pm
content: Quote from: kingsolmn on June 29, 2021, 07:43:00 pmNo I have not hade a look at the relay - I agree that it might very well lead to the source problem. I did resolder some joints but have not had any luck yet - Thanks.Quote from: RayRay on June 29, 2021, 09:53:51 pmOne of these two zeners are 1N5246 and measures 0.64v and the other one is a 1N5252 and measures 0.62v - I get that reading only in one direction with a Brymen 257.Desoldered the two caps too and they measures 13.4ohm/5.4µF and 11.6ohm/5.59µF.

author: RayRay
author_rank: Frequent Contributor
timestamp: July 03, 2021, 12:19:31 am
content: I'm slightly confused, I thought you previously stated that those Zener diodes measured in both directions? Given the fact your meter has a 1V range for diode mode, and that those particualr ones go way beyond that (16v for 1N5246 and 24v for 1N5252) you should only be getting a reading in one direction, with the correct polarity applied. As for the caps, they're probably fine! Just for reference, here's a Kemet axial cap (low ESR) of the same specs:https://www.mouser.com/ProductDetail/KEMET/PEG124YD1470QL1?qs=sGAEpiMZZMukHu%252BjC5l7YR00Qowwle8IcWZDkI03Qng%3DIt's said to have 11 ohms ESR (and your measurements aren't too far off from it) plus, there's a 20% capacitance tolerance, which neither goes over, also, you might wanna check your ESR meter's specifications for it's accuracy ratings on that range. Anyhow, I'd suggest you check all the transistors/mosfets under the large heatsinks, as well as the NTCs, also measure the capacitance of the red film caps & blue ceramics, as well check the resistors (they're color coded for a reason, look up "resistor color code calc" to help you determine their values). Hope you'll be able to narrow it down eventually. Good luck.

author: Sjokolade
author_rank: Regular Contributor
timestamp: July 03, 2021, 12:29:44 am
content: What I meant was that they measure fine in one direction with correct polarity and no connection with reverse connection.Yes I will have a look at the mosfets and other caps - thanks for your help.