LG LCD power supply

[Wrog]

Finally had some time to investigate farther into this one. I had posted previously on this one, trying to identify some of the "H" caps to head off any future problems. I plugged it in to take some measurements, and... nothing happens! I see 120VAC at the mains plug, the line fuse measures good, but I don't see 120VAC on the inner legs of the bridge rectifier (I should say, I see it on one leg but not the other). Following the traces, the voltage seems to stop at the relay. Can someone suggest test points and test methods to verify? Thanks!

[momaka]

Re: LG LCD power supply

It seems that this PSU has two separate circuits - one for standby power and another for the main power that runs when the relay is turned on. The relay is probably kicked on from the standby circuit. Do you hear the relay click when you plug in the power supply? If not, start by checking the standby circuit.

Find Q123 and *carefully* measure the voltage by placing your multimeter's negative probe on heatsink HS2 (or the anode side of ZD110 if you prefer), and the positive probe on the top pin of Q123 (the one closest to the big electrolytic capacitor). What voltage do you get?
(remember, this is the hot/high voltage area of the PSU - the last thing you want to do is touch something with your hands and get shocked!)

If you get no voltage at all, check D500 and D504 with the diode test on your multimeter (with PSU unplugged from the wall, of course). Also, check resistance on TH500 (looks like a NTC thermistor, should show very low resistance). If these are all good, measure the (DC) voltage on capacitor C500. I *think* you should see either fully rectified line voltage on it (i.e. ~165v DC since your line voltage is 120VAC) or half of that. Post here what voltage you get - again be careful, this is a high voltage area.

[Wrog]

Re: LG LCD power supply

It seems that this PSU has two separate circuits - one for standby power and another for the main power that runs when the relay is turned on. The relay is probably kicked on from the standby circuit. Do you hear the relay click when you plug in the power supply? If not, start by checking the standby circuit.

Find Q123 and *carefully* measure the voltage by placing your multimeter's negative probe on heatsink HS2 (or the anode side of ZD110 if you prefer), and the positive probe on the top pin of Q123 (the one closest to the big electrolytic capacitor). What voltage do you get?
(remember, this is the hot/high voltage area of the PSU - the last thing you want to do is touch something with your hands and get shocked!)

If you get no voltage at all, check D500 and D504 with the diode test on your multimeter (with PSU unplugged from the wall, of course). Also, check resistance on TH500 (looks like a NTC thermistor, should show very low resistance). If these are all good, measure the (DC) voltage on capacitor C500. I *think* you should see either fully rectified line voltage on it (i.e. ~165v DC since your line voltage is 120VAC) or half of that. Post here what voltage you get - again be careful, this is a high voltage area.

Thanks for the suggestions Momaka. I do not hear a click when the power is applied (I should have stated that explicitly in my original post). Measurements:

Q123 / HS2 = 48.1VAC (22.1VDC)

I continued to measure the other areas even though I was able to get a reading:

D500 = 0.595
D504 = 0.584
TH500 = 6.2 ohms
C500 = 154.2VDC

Please let me know if anything catches your eye that can be tested next. I appreciate the help!

[momaka]

Re: LG LCD power supply

The measurements look good. Not sure why you're getting 48.1VAC from Q123 / HS2, but the DC value of 22.1v seems okay. Means the standby circuit supply is likely working fine. Perhaps check diode D126 just in case, although I'm sure it should be fine (it's located in front of the smaller transformer, T500).

If it's good, check the SMT diodes, D160 (across the relay), D161, and also verify that resistors R170 and R171 are not open. Check ZD110 as well.

If all these components test normal, see if there is standby voltage present on one of the connectors P201, P202, and P203. Just follow the output pins of the smaller transformer, T500, and see which rail and connector it goes to - this should be the standby power and should always be present.

Lastly, can you read the labels for the rails on connectors P201, P202, and P203? Also, would it be possible to post two less zoomed-in pictures that cover the entire PSU (front and back). Thanks.

[Wrog]

Re: LG LCD power supply

I will check your suggestions tomorrow evening, work comes early! In the meantime, here are the full top/bottom sides with the connector voltage silkscreens (they should be readable in these photos). Thanks for your help thus far!

[Wrog]

Re: LG LCD power supply

The measurements look good. Not sure why you're getting 48.1VAC from Q123 / HS2, but the DC value of 22.1v seems okay. Means the standby circuit supply is likely working fine. Perhaps check diode D126 just in case, although I'm sure it should be fine (it's located in front of the smaller transformer, T500).

If it's good, check the SMT diodes, D160 (across the relay), D161, and also verify that resistors R170 and R171 are not open. Check ZD110 as well.

If all these components test normal, see if there is standby voltage present on one of the connectors P201, P202, and P203. Just follow the output pins of the smaller transformer, T500, and see which rail and connector it goes to - this should be the standby power and should always be present.

Lastly, can you read the labels for the rails on connectors P201, P202, and P203? Also, would it be possible to post two less zoomed-in pictures that cover the entire PSU (front and back). Thanks.

Measurements:

D126 = 0.547 / 0L
D160 = 0.630 / 0.560
D161 = 1.566 / 0.695
R170 = 50.1
R171 = 50.1
ZD110 = 1.594 / 0.818

I suppose that in order to test the diodes properly, they need to be removed from the circuit? D160/D161/ZD110 should only show a voltage drop in one direction, right?

Unfortunately didn't have time to take measurements with power applied, will do that tomorrow. Is it better to have all boards hooked up to the power supply while measuring or can I do it separately? (If the latter, then I will probably bring it in to work and do it over lunch).

Examining the PCB a little closer while doing the measurements, I noticed there are two resistors R154 and R510 that are seriously black. One of them has a single orange band. However, there doesn't seem to be any indication of excessive heat. Are these supposed to be that way?

[Wrog]

Re: LG LCD power supply

Update: P201 has two pins marked "ST 5V", and both have a solid 5.15VDC present. (AC_DET next to the "ST 5V" also has 5.15VDC). The rest of the connector pins read 0VDC.

[Wrog]

Re: LG LCD power supply

Update again: Shorted on/off on P201 with power applied and the relay clicks accordingly, so it looks like the power supply is working fine. Time to start investigating other boards!

[momaka]

Re: LG LCD power supply

Excellent work!
The fact that the PSU is working and that standby power (ST 5v) is present but the TV is not even trying to start the PSU (no relay clicking) indicates that perhaps the fault is indeed within one of the other boards on the TV. I'm assuming there should also be a light somewhere on the front of the TV that should turn on when the TV is plugged in (although, that probably depends on the model). If there is such light but it's not turning on, time to inspect other boards indeed.

See which board is connected to ST 5v standby power and try to trace which components ST 5v goes to (if possible, since I'm pretty sure that board will be multi-layer). Perhaps there might be a fuse or voltage regulators on there that are bad. Post some pictures of that board.

Also just to be clear, this is the LG 32LC2D 32" LCD TV were talking about here, right?

[Wrog]

Re: LG LCD power supply

ST 5V makes it all the way through the mainboard to the IR board where the power LED lives, with a twist. Each input pin on connector P101 reads properly (5.1VDC on ST5V, IR, KEY1, KEY2) except for LED_G and LED_R which read 0VDC, and KEY1/KEY2 5.1VDC is passed through P102 to the side button board. PA101 (IR sensor) reads 5.1V for both VCC and Vout. Since LED_G and LED_R read 0VDC, the standby light does not illuminate. Pressing the power button on the remote or the power button on the side button board does not turn the TV on or change the LED status. Remote is verified working OK using a different IR sensor.

I've tried to attach the service manual PDf (just over 7MB, the limit is almost 10MB for PDFs) but it keeps failing, so I'll try it from home later. I didn't see any fuses, but I did some regulator testing last night and it seemed a little strange that the center pins almost always read 0, while the outer legs read 3.3V / 1.8V (most of these are marked EH16A which should be SOT23 1117-33 equivalents). There are two sets of electrolytic caps near the 5V ST connector on the mainboard: 2x 1000uF 35V 12.5mm/20mm and 2x 470uF 25V 10mm/16mm that don't appear to be bad. I think I have these on hand so I may change them out to see if that changes anything.

I've attached photos of the IR sensor board, mainboard and subboard (inputs) to see if you (or anyone) can spot anything out of place or can identify test points. You are right, this is for the LG 32LC2D. I've tried looking for a factory reset, but it seems to be a part of the OSD (which obviously doesn't do any good). Someone on another forum mentioned a hard reset switch on the back, but I didn't notice it. I'll definitely be checking for that later. Thanks for any and all help.

[momaka]

Re: LG LCD power supply

Each input pin on connector P101 reads properly (5.1VDC on ST5V, IR, KEY1, KEY2) except for LED_G and LED_R which read 0VDC

Well, it looks like whatever controller is responsible for controlling the PS ON signal and/or the LED is not working properly (or not getting power).
Perhaps you can follow the LED-G and LED_R wires and see which board and components they go to. Or maybe follow the PS ON signal from the PSU board to the logic board and see what it connects to.

I didn't see any fuses, but I did some regulator testing last night and it seemed a little strange that the center pins almost always read 0, while the outer legs read 3.3V / 1.8V (most of these are marked EH16A which should be SOT23 1117-33 equivalents).

Most voltage regulators usually have their tabs as the output. Try measuring between tab and ground.

Someone on another forum mentioned a hard reset switch on the back, but I didn't notice it.

There's a small push-button switch, labeled SW101(?) on the main board. Perhaps this is it?

....

At this point, I'll have to admit that this is a bit over my head, so sorry if the above suggestions are not helpful. Hopefully PlainBill or Wizard will see this thread because they have *a lot* more knowledge than I do.

[Navicura]

Re: LG LCD power supply

Wrog, were you able to fix your LCD TV? I'm having the same issue as yours. I have not done as much testing as you have, but I have tested multiple components, i.e. diodes, capacitors, resistors, standby transformer, etc. by removing them and haven't been able to find the culprit.

A while back a measured 5V ST before, and I get a reading too.

No LED flashing in the TV.

[Wrog]

Re: LG LCD power supply

Was never able to nail it down. There were a lot of regulators that didn't test well, and even after replacing them it didn't work properly (still no power LED). I finally gave up and replaced the mainboard (from eBay) and all seems to be well. There seems to be several versions of the mainboard though, make sure it looks like the one you have (some have the mainboard and tuner connections all on one board, mine did not).

[Navicura]

Re: LG LCD power supply

Call me crazy, but I put everything back together, except for the cover, and placed a jumper between the 5V ST and ON. The I plugged the TV into the AC outlet and the front LED came on. Then I pressed the Power button, and the TV actually came on.

Any thoughts? Do you think I still have a bad mainboard? Perhaps any hints on circuits to check? No bulged capacitors found in the mainboard.

[Wrog]

Re: LG LCD power supply

Still think it's the mainboard. It sounds like what you've done is bypassed the standby circuit so you're completely powered all the time. Do you still hear the relay clicking as you turn the TV on/off?

[Navicura]

Re: LG LCD power supply

Yes, I hear the Relay. Upon further research and testing, I believe my problem is the PSU. Some posting I found states that both the AC Detect and 5V ST should measure 5V. My AC Detect is 0.01V. As I don't have a schematic for the PSU board and I don't know much troubleshooting, I'm relaying on the internet and support from different people, like yourself, to help me out.

If you have any thoughts on parts in the PSU that I should test, please feel free to share.

Thanks!!!

[Navicura]

Re: LG LCD power supply

Actually, upon re-reading your message, I'll have to get back to you about the relay. I can't remember if I heard the relay clicking upon On/Off. Will get back to you tonight.

[Wrog]

Re: LG LCD power supply

I can check the voltage measurement on the AC Detect pin tonight, I still have the set (on the dining room table). If you want to post top/bottom of the power supply board, we can do a visual inspection to see if anything stands out. The issue may be the same I ran into from the start: those "H" caps don't have datasheets and they are oddly sized, so it may be difficult to find suitable replacements.

Edit: Oops! Post #7 in this thread shows that I've already tested this and do get 5VDC on the AC_Detect pin.

[vinceroger69]

Re: LG LCD power supply

Hi i have just been given a lg lc32db to repair it uses a psu:6709900016 LGp2637n psu all im getting with only the 240 volt lead in is a quite ticking noise if i connect all the other boards the relay on the psu clicks constantly so any ideas guys where to start fault finding the psu i will upload pictures shortly camera batterys on charge now.

[vinceroger69]

Re: LG LCD power supply

Hi just changed the 3.4 volt capacitor as was bulged also running some tests the 5vst is shorted to ground any ideas guys where to start fault finding.

The psu also has stamped on it lglp2637hep