LG 50PC55-ZB stays in stand-by

[laszlo31]

Hi All,

Our 50" LG plasma TV with 50X4P PDP panel doesn't power on. When hitting the power button in stand-by mode (red LED) the PSU relay clicks, it starts the process (LED goes green) but then after 7 green blinks it goes back to stand-by with red LED. I read something about "protection mode" so the PSU powers down to protect components. Having some XBOX and laptop fixing experience, I dared to open the belly of the beast and took a look. All caps, fuses and connectors seem fine on all boards. PSU connectors have all the correct voltages. Before the TV goes back to stand-by, the upper part of the Z-SUS board gives some chirping noise, then the relay clicks and I am staring at the red stand-by LED again... If I disconnect the cable that leads from the Y-SUS to the Z-SUS, the TV powers on but of course there's just some funny shadows on the right side of the screen. I checked the Z-SUS, no sign of any obvious malfunction. Under the upper heatsink there are 4 RJH3047 IGBTs and 2 GF2030FC rectifiers. Before I start desoldering the heatsink / caps and check the components, I wanted to ask some more experienced forum members about the following:
- should I start checking the Z-SUS or would you recommend some more testing of the PSU or other boards?
- could this be the much mentioned LG plague of AC detect issue?


I attach a few pictures of the suspicious board.
Any opinion, advice is highly appreciated.

Laszlo

from Hungary

[tom66]

Re: LG 50PC55-ZB stays in stand-by

Did the problem occur suddenly or has it gradually got worse?

[laszlo31]

Re: LG 50PC55-ZB stays in stand-by

My wife says that for a few times she had problems switching the TV off. It came back from stand-by to power-on mode after hitting the power button. I thought the remote had some issue. But ever since last week it clicks back to stand-by and we can not watch it. Any suspicion?

[tom66]

Re: LG 50PC55-ZB stays in stand-by

Possibly the AC detect issue. Do you see a pin marked AC-DET or AC-D on the power supply?

[laszlo31]

Re: LG 50PC55-ZB stays in stand-by

Thanks Tom for trying to help.
I do see the AC-DET pin in the bottom left connector, very bottom pin. See picture. Should I try to measure whether the voltage drops? I also remember you suggested to find the opto-coupler and soder it if indeed that's the issue. Can I measure even if after hitting the power button the TV almost immediately goes back to stand-by?

[tom66]

Re: LG 50PC55-ZB stays in stand-by

You can measure it even if the TV goes into standby.

Set your meter to DC 20V or DC volts (if autoranging.) Connect the black probe to an exposed screw hole (you can just put it in there for convenience) and touch the red probe on the exposed metal part of the pin (it will be just visible behind the wire.)

First, check in standby: it should read 3V or 5V.
Then unplug the TV, it should drop immediately to 0V or close to it even if standby LED is still lit (the LED should still be lit for a few seconds due to bulk capacitance, there is also usually a power LED on the main board.)

Then, plug TV back in, and get someone to turn it on again. Monitor the voltage closely. If it drops to 0V just before clicking off you have found the problem.

[laszlo31]

Re: LG 50PC55-ZB stays in stand-by

I tested the AC-DET pin. If all boards and connectors are put back to place, this pin measures 4.69 V to the GND. When trying to power on my DMM indicates 5.15 for a brief second than it drops back to 4.69. There's no indication of dropping to lower voltage or 0 unless the inertia of the Digital Multi Meter.

[tom66]

Re: LG 50PC55-ZB stays in stand-by

I tested the AC-DET pin. If all boards and connectors are put back to place, this pin measures 4.69 V to the GND. When trying to power on my DMM indicates 5.15 for a brief second than it drops back to 4.69. There's no indication of dropping to lower voltage or 0 unless the inertia of the Digital Multi Meter.

I am seeing a lot of these newer LGs do not have this problem, it is one thing to eliminate. Next, try unplugging the Z-sustain. You will get a dim picture (usually ghosting/blurry too) but does it power up properly?

[laszlo31]

Re: LG 50PC55-ZB stays in stand-by

Yes, if I disconnect the Z-SUS the TV powers up properly with blurry forms and shadows on the right of the screen but at least it doesn't go back to stand-by and it properly responses to the remote buttons.

[tom66]

Re: LG 50PC55-ZB stays in stand-by

Great. Set it up to play sound. (For example over a DVD player on the composite inputs.) Turn it off, wait 30 seconds and reconnect the Z-sustain, and disconnect the Y-sustain - opposite board to the Z-sustain.

Now does TV shut off? (You won't get a picture - but you will get sound so you can hear if it shuts down.)

[laszlo31]

Re: LG 50PC55-ZB stays in stand-by

Hi Tom! I used the family camcorder connected to the composite inputs of the TV. When I reconnect the Z-SUS (that is by the way receiving all voltages from the Y-SUS, I guess) and disconnect the Y-SUS power connectors, the TV stays on with black screen and after pressing few times the input button I could find the camcorder and listened to the sounds of our family vacation. The TV stays on and I can hear the sounds in the TV speakers.
Your expert opinion on this?

[tom66]

Re: LG 50PC55-ZB stays in stand-by

My guess is that you have some issue with the power supply. Since it doesn't shut down with the Z-sustain connected but no light appearing on screen (except perhaps dim glow.) Remove the power board and inspect for cold joints. Also check for any burning or browning.

[laszlo31]

Re: LG 50PC55-ZB stays in stand-by

My eyes start hurting as I really scrutinized the Power Board looking for cold joints or any physical sign of malfunction. None found :-(
I also tested (without desoldering them out of the board) the MOSFETs, I couldn't find any shorted one. All caps are SAMWHA, no visible bulging...would you advise me to take them out and measure with a C-meter?

[retiredcaps]

Re: LG 50PC55-ZB stays in stand-by

My eyes start hurting as I really scrutinized the Power Board looking for cold joints or any physical sign of malfunction.

I use one of these Jeweler Loupe

http://www.ebay.com/itm/110936028667

I got mine for 99 cents + free shipping. It works great.

[laszlo31]

Re: LG 50PC55-ZB stays in stand-by

Mine is less professional but certainly more expensive ;-)

[laszlo31]

Re: LG 50PC55-ZB stays in stand-by

I am half way through taking out and measuring the caps on the PSU so far all of them are withing specs.
Could you please, the experienced ones, try to advise me with other common issues with this type of plasma PSU so I could try to check before I start ordering replacement board(s). Thank you in advance for any attempt to help me here.

[tw2005]

Re: LG 50PC55-ZB stays in stand-by

Just another opinion to offer, I personally would go back and check the ZSUS for shorted FETs or shorts, any blown fuses.
You have already stated that the ZSUS receives it power via the YSUS. You said once the ZSUS is isolated, the set turns on but dim screen and unusual.This sounds normal with a dead ZSUS.
You also stated that a chirping noise come from the ZSUS just before shutdown(protection mode)
You disconnected the YSUS(which powers the ZSUS) I assume by removing power and get it to turn on but absolutely no activity on the screen. This sounds normal.
For me this replicates your first step of ZSUS isolation + now the YSUS is also disconnected.

The common thread I see is that if any power is fed to the ZSUS you get a shutdown(protection mode).

Chances are the PSU is good.

[laszlo31]

Re: LG 50PC55-ZB stays in stand-by

Thanks for caring, tw2005. After several days of testing, de-soldering and re-soldering I also tend to believe that the PSU might not be the problem. Yesterday I turned my attention back to the ZSUS. There are 2 big heatsinks on it. Next to the upper one there are 2 signs: SUS_UP and SUS_DN while next to the bottom heatsink there are ER_DN and ER_UP signs. Pictures are in my opening post. Sorry my ignorance but I don't know what these marks mean nevertheless I noticed that the chirping sound before the PSU shuts down (protection mode?!) comes from under the upper heatsink so I checked the voltages on the P1 connector and maybe found a clue to my mystery.
On this connector there are Z0_SUS_UP and Z0_SUS_DN pins (see picture) and I found that while the SUS DOWN goes from 0V up to around 5V during the powering up process before the TV goes back to stand-by but the SUS UP pin goes up to only 0.28V before the shut down. I checked a few cycles and the results are always the same.
Under the upper heatsink there are 4 RJH3047 IGBTs and 2 GF2030FC rectifiers.
My questions:
- what are SUS_UP and SUS_DN and should their respective pins' voltages act similarly?
- can these IGBTs and rectfiers be checked on-board or must I de-solder them?

[tw2005]

Re: LG 50PC55-ZB stays in stand-by

At best I can only offer you my best guess as my knowledge and experience is not extensive enough. If I was at home there may be a LG training manual on file I could look at.

I just fixed a 50" PDP50X3 panel, I'd say the generation before yours and it uses those large IPMs that fail. Anyway I do recall seeing those markings (SUS_UP,SUS_DN )next to each IPM so my best guess is one is the Z waveform for the top half, the other for the bottom half of the screen.

As far as checking the FETs etc, I have successfully repaired a samsung ysus with banks of fets and had no trouble testing in cct the shorted FET(s).

The only thing I found was I could only determine which one and how many by removing out of circuit.
I found if one shorted then it shorted the remaining FETs in that bank. On the first board all 5 FETS were short but ended up being only one faulty.
The next board i worked on had the same bank shorted and ended up with 3 dead FETs.

The beauty of these forums is the collective knowledge and experience. Just like a jigsaw, every little piece counts.

Good luck, hope to see a positive result posted.

[laszlo31]

Re: LG 50PC55-ZB stays in stand-by

After some troubles I managed to take out the upper heatsink of the ZSUS with the 4 Insulated Gate Bipolar Transistors (IGBTs) and 2 rectifiers on it. Both rectifiers checked out OK (with DMM in diode mode) while 3 out of the 4 IGBTs failed the test !!!!! (I used this test I found on the net: http://www.youtube.com/watch?v=AfV7gYChILc)

If you were me would you also take out the bottom heatsink and check the 2 IGBTs and 6 rectifiers on it before I make an order for the replacement components? It's quite a struggle to operate out the heatsink...