LG 50PK550 turns on then off (Not the resistors)

[canadaboy25]

Hey guys,

I was given an LG 50PK550 plasma that tries to turn on then turns off right away. The guy before me bought a new power supply board which did not fix it so he included the spare board with the TV.

When I power up the TV, the relays click, then there is a quick flash of grey on the screen and at the same time I can hear the panel prime but then the panel goes out. The TV stays powered up for a short time, during which I can hear a faint ticking from one of the transformers on the power supply board. After a few more seconds the relays click and the TV shuts down. Pressing the power button again repeats the cycle.

I tested Vs, Va, and 5V going to the Y-SUS, all of which are good until the TV shuts down fully. I checked the 5W ceramic resistors on the power supply board and they are all good. I also checked all of the other large resistors on the power supply and Y-sus board.

Disconnecting the Y-sus from the power supply causes the TV to stay powered up until the power button is pressed. I reconnected the Y-sus board and removed both of the buffer boards. The TV shuts down right away again. Problem must be on the Y-sus.

I hooked the scope up to the Vs rail to be sure the voltage wasn't dropping out and it was solid. I then checked Va. Va is solid while the panel tries to prime but then has downward spikes corresponding to the ticking from the power supply board. I assume this is due to there being no real load on the rail and it is just having trouble regulating. This cannot be the issue anyways since the Y-sus does not use the Va rail.

I scoped VSC on the Y-sus and captured a waveform. The blue trace is VSC and the yellow trace is Va. You can see VSC come up to the rated 150V but die out, spike back up, and die completely. VSC dropping out corresponds to the panel going dark after the initial flash. You can also see the downwards spikes on Va, but they happen after the problem with VSC has already occurred so I do not think they are relevant.

Does anyone know what could cause VSC to drop out? Or any other ideas what to check?

Thanks!

[canadaboy25]

Re: LG 50PK550 turns on then off (Not the resistors)

Any ideas?

[Biruslapio]

Re: LG 50PK550 turns on then off (Not the resistors)

There's another important DC voltage at Y-SUS, -VY, it's right next to VSC resistor TP, and ZBIAS at Z-SUS.

Just because the tv does not shut down when Y is disconnected, doesn't outright mean it's defective, the logic board only does a full power up if it receives correct signals from both Z and Y boards, then the whole assembly will work.

Last time I had a LG Plasma flashing the screen at me, it was one bad Y-buffer, test the Y-buffer boards for shorts, then on the Y-SUS board the IGBTs, those 2W 2R2 resistors in series with IGBTs, the same on Z-SUS.

I don't know if this motherboard in your photo has the same notorious BGA problem as other 50PK550 models sold worldwide with the big heatsink had, but since it did not shut down during your testing, it probably is good, an AUTO-GEN test can rule this board out.

[canadaboy25]

Re: LG 50PK550 turns on then off (Not the resistors)

There's another important DC voltage at Y-SUS, -VY, it's right next to VSC resistor TP, and ZBIAS at Z-SUS.

Just because the tv does not shut down when Y is disconnected, doesn't outright mean it's defective, the logic board only does a full power up if it receives correct signals from both Z and Y boards, then the whole assembly will work.

Last time I had a LG Plasma flashing the screen at me, it was one bad Y-buffer, test the Y-buffer boards for shorts, then on the Y-SUS board the IGBTs, those 2W 2R2 resistors in series with IGBTs, the same on Z-SUS.

I don't know if this motherboard in your photo has the same notorious BGA problem as other 50PK550 models sold worldwide with the big heatsink had, but since it did not shut down during your testing, it probably is good, an AUTO-GEN test can rule this board out.

Fair enough, it may not be the Y-sus based on that test.

Attached are waveforms of -Vy and ZBias. The yellow trace in both waveforms is Va for reference. Both signals have exactly the same behavior as VSC did. They come up to voltage, and then die out right away.

Since Vs is stable the whole time, the logic board must be dropping some enable signal to the Y-sus and Z-sus boards. Do you know of any way to tell if this is due to a fault with the logic board or the logic board seeing some signal it doesn't like from the Y-sus or Z-sus?

When you say motherboard are you referring to the logic board that runs the display signals, or the main board with all the video inputs on it? I'm not sure what the AUTO-GEN you mentioned is.

[diif]

Re: LG 50PK550 turns on then off (Not the resistors)

With the TV unplugged unplug the cable between the mainboard and the logic control board.
There are two jumper pints on the logic board labeled AUTO-GEN, jumper them then plug the TV in. This removes the mainboard and lets the logic board run through a image test sequence.

[canadaboy25]

Re: LG 50PK550 turns on then off (Not the resistors)

With the TV unplugged unplug the cable between the mainboard and the logic control board.
There are two jumper pints on the logic board labeled AUTO-GEN, jumper them then plug the TV in. This removes the mainboard and lets the logic board run through a image test sequence.

Ah ok. I did not know LG's had a self test mode.

I removed the LVDS cable and shorted the AUTO_GEN pins together then plugged in the TV. Exact same behavior as before.

[Biruslapio]

Re: LG 50PK550 turns on then off (Not the resistors)

The main board needs to be disconnected from the power supply too, this way the PSU will auto start as soon as AC voltage is present.

If that doesn't work then proceed to probing the boards with a multimeter.

[canadaboy25]

Re: LG 50PK550 turns on then off (Not the resistors)

The main board needs to be disconnected from the power supply too, this way the PSU will auto start as soon as AC voltage is present.

If that doesn't work then proceed to probing the boards with a multimeter.

I repeated the test with the main board unplugged from the power supply as well and there was no change. Exactly the same symptoms as with the main board connected to the TV.

I'm not exactly sure what to be probing for. I have checked all the transistors and diodes on the large heatsinks in diode mode and they all seem fine. I have also previously tested all of the large resistors including the 2.2 ohm ones. All tested fine. I'm not sure what to check next as I don't have much experience with the operation of the Y-sus and Z-sus boards...

[Biruslapio]

Re: LG 50PK550 turns on then off (Not the resistors)

I have also no experience with LG's PK series, but since the screen flashes it can be assumed that the sustain part is partially working and nothing is shorted on Y and Z boards, and some fault/protection is shutting it down.

What about the buffer boards?

[canadaboy25]

Re: LG 50PK550 turns on then off (Not the resistors)

I have also no experience with LG's PK series, but since the screen flashes it can be assumed that the sustain part is partially working and nothing is shorted on Y and Z boards, and some fault/protection is shutting it down.

What about the buffer boards?

Yes, I agree with your logic. Something must be shutting the TV down due to some error being detected.

I read on several other threads that this TV will power up with no buffer boards connected. I tried this and there was still no change in the behavior. So I highly doubt that the problem is with the buffer boards.

Perhaps probing the signals on the ribbon cables going from the logic board to the Z-sus and Y-sus may show something? However, I have no idea what signals to be expecting on those cables...

[Biruslapio]

Re: LG 50PK550 turns on then off (Not the resistors)

This is a training manual for an older LG model, but the plasma voltage pulses are similar.

From page 43 and forward it details how the signal is delivered to the plasma, don't take it as face value since it's from a different generation of TVs, and no need to mess with the trimpots if they weren't adjusted before by someone else.

Look caurefully for bad solder joints, test if the transformer windings have continuity, you can also disconnect the VA cable to take that out of the equation, the panel will still power up but only display black, if nothing changes plug it back.

[canadaboy25]

Re: LG 50PK550 turns on then off (Not the resistors)

This is a training manual for an older LG model, but the plasma voltage pulses are similar.

From page 43 and forward it details how the signal is delivered to the plasma, don't take it as face value since it's from a different generation of TVs, and no need to mess with the trimpots if they weren't adjusted before by someone else.

Look caurefully for bad solder joints, test if the transformer windings have continuity, you can also disconnect the VA cable to take that out of the equation, the panel will still power up but only display black, if nothing changes plug it back.

I previously found the training manual for a 50PK750 which seems to be the same as mine. All of the circuit boards are the same as the ones in the manual.

I checked the FG5V, FG10V, FG15V, and FG24V on page 73 and they were all good for the second that the TV is trying to start. None of the solder joints look questionable either.

I also probed the floating ground and captured some waveforms. The first 4 waveforms show the floating ground in blue, and Va in yellow. The last waveform shows the floating ground in blue and VSC in yellow. When VSC is at it's proper level, the floating ground signal looks good. So the generation circuitry must be ok.

There has to be some signal going to the Y-sus board to kill all of the generation circuitry. Page 77 shows the pins of the ribbon cable going from Y-sus to the logic board. Pins 1 and 2 are labelled Error and CTRL_EN. The manual says Error should be 13.57V while the TV is running. I get 0V. The manual says CTRL_EN should be 0.09V while running. I get a 5V pulse before the TV tries to turn on and then it is 0V the rest of the time. So the CTRL_EN signal looks ok. It seems like the error signal is incorrect though. Maybe this indicates that there is in fact an error on the Y-sus?

[Biruslapio]

Re: LG 50PK550 turns on then off (Not the resistors)

Nice, I searched for 50pk550 training manual and found nothing, pk750 is similar enough.

I wound't even bother with these Y>CTRL board signals because we don't know what they do.

You can measure the waveform on the Y-buffers and Z-Drive test points while the panel flashes to see if they are correct, described in pages 64, 96 and 101.

If the set-up or set-down do not match the manual there's a problem in that section, as ZBIAS VSC and -VY are working properly.

[Biruslapio]

Re: LG 50PK550 turns on then off (Not the resistors)

Actually, make sure it's not the power supply, add a external load to VS, usually a 90W incandescent light bulb, that would be enough, since the Y, Z or control board have no way of communicating to the PSU to shut it down, the only thing that could be then is overcurrent or the PSU is cutting down when there's significant load on VS.

[Davi.p]

Re: LG 50PK550 turns on then off (Not the resistors)

Hi, there was a brand and a model that had a problem in the gnd plane conduction between ysus and chassis ground, if i remember was an LG, check screws for continuity..

[canadaboy25]

Re: LG 50PK550 turns on then off (Not the resistors)

Nice, I searched for 50pk550 training manual and found nothing, pk750 is similar enough.

I wound't even bother with these Y>CTRL board signals because we don't know what they do.

You can measure the waveform on the Y-buffers and Z-Drive test points while the panel flashes to see if they are correct, described in pages 64, 96 and 101.

If the set-up or set-down do not match the manual there's a problem in that section, as ZBIAS VSC and -VY are working properly.

The 4 attached waveforms are of the Y-scan signal. Page 63 of the manual shows this signal at different timescales and my signal does not seem to match it. I get the same sort of signal but I am missing the group of 3 pluses at the beginning of each burst. The third waveform is of the gap between the two bursts where the 3 peaks should be. I have one peak like the one shown on page 64, but nowhere is there 3 consecutive peaks like is shown in the manual. Could this point to the issue?

Actually, make sure it's not the power supply, add a external load to VS, usually a 90W incandescent light bulb, that would be enough, since the Y, Z or control board have no way of communicating to the PSU to shut it down, the only thing that could be then is overcurrent or the PSU is cutting down when there's significant load on VS.

I will have to get some lightbulb sockets in order to do this test. However, it seems unlikely that that supply is at fault because the supply only turns off well after the panel has already tried to prime and has given up. The Vs rail is rock solid while the panel is trying to prime which suggests the supply is not causing the problem. I can still try to find some sockets to confirm this if you still think it could be the issue.

Hi, there was a brand and a model that had a problem in the gnd plane conduction between ysus and chassis ground, if i remember was an LG, check screws for continuity..

Yes, I have heard about issues like this. I have had the Y-sus board removed from the TV a couple of times and the connections between the board and the screws look very clean. I also make sure to tightly install all screws before powering it up or doing any testing.

[Davi.p]

Re: LG 50PK550 turns on then off (Not the resistors)

I was thinking about those 2 signal pins of control board, they're important and i think worths some thoughts, thinkng about that error pin could be Output pin from Control bd, ctrl-en instead it's maybe a sign that ysus is initialised with correct supply tensions and Control can begin to operate.. this is my thought but never read about these things, so a ctrl board change can be a step, or watch on it if there is a flash memory spi type, it can be tried to freeze it when reading it with programmer then watch inside dump with hex editor, if there are some holes of deleted data, if present retry reading and reprogramming this data on the same chip..

[canadaboy25]

Re: LG 50PK550 turns on then off (Not the resistors)

I was thinking about those 2 signal pins of control board, they're important and i think worths some thoughts, thinkng about that error pin could be Output pin from Control bd, ctrl-en instead it's maybe a sign that ysus is initialised with correct supply tensions and Control can begin to operate.. this is my thought but never read about these things, so a ctrl board change can be a step, or watch on it if there is a flash memory spi type, it can be tried to freeze it when reading it with programmer then watch inside dump with hex editor, if there are some holes of deleted data, if present retry reading and reprogramming this data on the same chip..

I took a closer look at the ERROR signal on the Y-sus side and the pin from the connector goes straight to an unpopulated resistor on the board. So they are obviously not even using the ERROR signal on this TV. The CTRL_EN signal was behaving as expected from the manual so I don't see any problem there.

It is really difficult to track down exactly what is causing it to shut down, especially with no schematics available.

[Biruslapio]

Re: LG 50PK550 turns on then off (Not the resistors)

I suggested the VS test because I've read on this forum that sometimes the PSU VS ceramic resistors fail partially, let's say when there is load they stop working correctly, but if VS is dropping after VSC or -VY then the power supply is working correctly.

Seems like the control board is causing this, or the SET_UP, SET_DN sections of the Y-SUS are bad, and measuring the very first plasma power on pulses on the oscilloscope isn't what the manual intended, in the training manual it's while the TV is on for at least 10 minutes(heat run) on white wash (full white signal), so they are different.