Power but no picture Samsung plasma 42" PS42C450B1WXXU

[Milasmith]

H is anyone experienced with this model and able to help me troubleshoot the issue please? I have a little experience with electronics and can use a multi meter.
The TV powers on with the Samsung melody and the two relays click on but the screen never lights up. There are no obvious bad capacitors.
What is likely to be the issue? Thanks

[angelos]

Re: Power but no picture Samsung plasma 42" PS42C450B1WXXU

You need to measure the Vs and Va voltages. They are marked on a label on the back of the tv. In your case Va = 54V DC and Vs = 207V DC. My experience on these sets are dry joints that occur on the Y-SUS board (The board on the left that plugs into the long thin board)and then you have a short circuiting of Mosfets and then you have the symptom your'e talking about. Firstly you need to check whether these voltages come up and stay up.

[Milasmith]

Re: Power but no picture Samsung plasma 42" PS42C450B1WXXU

Thanks for helping! The Va and Vs voltages are initially on power on at the correct values but after a second or two they slowly drop away to almost nothing.
I unplugged the y main and the voltages remained steady at 206V and 55v.
I tried again with the Y plugged in this time but the X disconnected but same thing happening with the voltage dropping.

[sordids999]

Re: Power but no picture Samsung plasma 42" PS42C450B1WXXU

Sound like the ysus has become faulty but you will have to test the buffer board connected to the ysus for shorts also

[Milasmith]

Re: Power but no picture Samsung plasma 42" PS42C450B1WXXU

Thanks, I've looked at the vid on YouTube about testing y buffers and I've looked at mine and am unsure of the pin to use as the common end while running the other down the buffers? Also can it be tested in situ or does it need removing?
Thanks

[Milasmith]

Re: Power but no picture Samsung plasma 42" PS42C450B1WXXU

Also, do you think it likely that the X Main will have been damaged by it and need replacing also?

[tom66]

Re: Power but no picture Samsung plasma 42" PS42C450B1WXXU

Thanks, I've looked at the vid on YouTube about testing y buffers and I've looked at mine and am unsure of the pin to use as the common end while running the other down the buffers? Also can it be tested in situ or does it need removing?
Thanks

Post a pic of your buffer if unsure. Clear and in focus as we need to be able to see it.

I would also test Vsc on your Y-main with and without your buffers connected. Normally around -190V DC. If it is low when your buffer is connected then you almost certainly have a shorted buffer.

You only need to replace the X-main if the Y-main has shorted and you can't be certain that the X-main survived.

Finally, check the panel is not cracked in any places. If it is cracked, it will not work.

[Milasmith]

Re: Power but no picture Samsung plasma 42" PS42C450B1WXXU

The panel is not cracked. Uploaded photos below.

[Milasmith]

Re: Power but no picture Samsung plasma 42" PS42C450B1WXXU

Something strange happened when I tested the Y main with buffers disconnected.
I disconnected the buffers and left the y main screwed to the chassis so it was still properly earthed. Also disconnected X Main power.
I had the volt meter ready on the VS on the power board. As soon as I powered up the TV with the remote and the relays clicked on, the voltage rose rapidly to 360V! I was about to unplug it as it seemed to be climbing higher and higher but then it started dropping away and fell to a low voltage. Is that wierd?

[Milasmith]

Re: Power but no picture Samsung plasma 42" PS42C450B1WXXU

I tried it again and can't repeat the results. Just drops like before. Same result measuring Vs on y main side of power cable.

[angelos]

Re: Power but no picture Samsung plasma 42" PS42C450B1WXXU

Be very careful! If your power supply is faulty and has an intermittent fault where the Vs goes high it will most certainly damage both the X and Y-main boards. You can test the power supply by connecting a incandescent globe across the Vs supply to give it a dummy load and check for stability. For this test you need to disconnect both Y and X boards.
I am not too sure whether it's one or two incandescent that you must connect in parallel. Perhaps someone in this forum can clarify? Also please check your power supply with a lens or eyepiece. Check for dry joints around all the Mosfets that are connected on the heat sink. OI have had dry joints on coils and Mosfet transistors causing instability on the PSU.

[tom66]

Re: Power but no picture Samsung plasma 42" PS42C450B1WXXU

My guess would be measurement error, some multimeters are affected by the interference of a plasma TV.

If Vs did go up to 360V then capacitors and transistors would have popped!

One 100W incandescent at 230V rating is sufficient for testing Vs under load. Va can usually be left unloaded.

[Milasmith]

Re: Power but no picture Samsung plasma 42" PS42C450B1WXXU

I'll have to get an incandescent bulb. In the meantime, I've tested every output of the power supply and all seems fine now. It could have been my multi meter as it has been acting up a bit recently.
Regarding the photo I posted, which pin should I use to test the buffers?
Thanks

[tom66]

Re: Power but no picture Samsung plasma 42" PS42C450B1WXXU

Test from any pin on top connector to either of the bottom two pins on the middle connector.
https://www.badcaps.net/forum/attach...8&d=1536088232

When the set is powered up there should be roughly 140 - 160V between these.

When off and disconnected from Y-main there should be >10kohm resistance here.

[Milasmith]

Re: Power but no picture Samsung plasma 42" PS42C450B1WXXU

Tested all the buffers and all seemed fine.
Did the test you mentioned by checking continuity between one of the pins on the top connection and one of the bottom 2 pins on the middle connection (with the buffers disconnected) and got no continuity (1M)

[tom66]

Re: Power but no picture Samsung plasma 42" PS42C450B1WXXU

did you get ~150V between these when powered up?

[Milasmith]

Re: Power but no picture Samsung plasma 42" PS42C450B1WXXU

Just got home and checked. With the buffers connected the voltage rises quickly between the pins you specified on startup to around 70v then drops away like the vs does. If it wasn't for whatever error is causing the power supply to cut the power then it might have reached 150v.

[tom66]

Re: Power but no picture Samsung plasma 42" PS42C450B1WXXU

So you have a fault on your Y-main or X-main. Your buffers are fine.

You would need a scope to know which unfortunately but you can try first by checking for short circuits on each board.

[Milasmith]

Re: Power but no picture Samsung plasma 42" PS42C450B1WXXU

Thanks, would that be checking the continuity across pins in the power input (e.g. pin1-vs to pin3-gnd)?
Or checking individual components?

[tom66]

Re: Power but no picture Samsung plasma 42" PS42C450B1WXXU

Individual parts. You don't have a Vs short because Vs is present briefly.
I had a tricky fault with a PS43E450 a while back. No obvious shorts but a small ceramic capacitor was shorted on one of the many regulated voltage rails. No way to trace it down without an oscilloscope.