samsung 50" plasma, no picture, no obvious short or blown fuse.

[mph8783]

Hi,

This is a new thread for my specific problem of no picture, and no obvious short, originally started in https://www.badcaps.net/forum/showthread.php?t=18542.

I'm struggeling to narrow down the fault with my samsung 50" plasma - and any help is appreciated!.

It is a PS50C96HDX/XEU built in 2007.

I was sat watching it when all of a sudden the picture disappeared. There was no bang, smoke or anything else and the TV sound and remote continued to function.
The way the picture went off was more like the broadcasters satallite link failed but alas it was the TV. I.e video source unplugged or psu cut. It took just a couple of frames to go completely blank, ans didn't go pink first.

I have checked all PSU rails (except the -192V Vsc as I cant find where it lives) and all fuses. There are no bulging caps, black components or other suspect parts.

The screen is completely black, no glow or snow.

I suspect it is the X or Y main boards but as the service manual is lacking schematics or any other usefull fault diagnostics not too sure how to isolate which one it is.
However the y-main board doesnt appear to have anything more than 14V on any of it's FET outputs (despite 207V going in) - could it be this or just because I am testing with no input picture and a DVM?
The only other suspect board would be the logic board but im not sure if this has the capability to disable the drive voltages or not.

I would prefer to trace to a component if possible, rather than spend lots on new board.

Any ideas?

[tom66]

Re: samsung 50" plasma, no picture, no obvious short or blown fuse.

Can we have a picture of the Y-main and X-main boards?

[mph8783]

Re: samsung 50" plasma, no picture, no obvious short or blown fuse.

Board Info and Pictures

Overview of whole TV

PSU Board

Y-Main Board (LJ41-05120A)

X-Main Board (LJ41-05118A)

[mph8783]

Re: samsung 50" plasma, no picture, no obvious short or blown fuse.

I have just made some progress!

It appears that that the iCoupler (digital opto-coupler) U5704 on the Y-Main board does not have any power on its secondary side.
I assume that this is the link from the logic board control signals to the power electronics, so no control = no drive = no picture.

It looks like it is powered by a 7805 reg - which also has no power, but the +15V Vcc is being supplied to the board.

So hopefully it is just a case of tracing the route from supply to regulator.

[tom66]

Re: samsung 50" plasma, no picture, no obvious short or blown fuse.

Test with the Y-buffer disconnected too. Could be a short on the Y-buffer.

[mph8783]

Re: samsung 50" plasma, no picture, no obvious short or blown fuse.

looks like the small switch mode PSU on the Y-Main card (bottom right below heatsink) isn't starting up. It appears to take the 200V and convert to something under 25V (i'll guess at 24V) for all the power drive stuff (it supplies the 7805 and also a 7815) . Ive replaced the associated electrolytics, but still no luck. Thinking it is eaither an opto-coupler or the MR4710 control IC (is there a non chineese equivelent that is easy to obtain?)

I have also noted that the power LED on the front of the TV flashed 4 times when switching on, is this normal or a fault code?

[tom66]

Re: samsung 50" plasma, no picture, no obvious short or blown fuse.

Power LED blinking is normal.
There is a 15V fuse on this board that doesn't look like a fuse, have you tested it?

[mph8783]

Re: samsung 50" plasma, no picture, no obvious short or blown fuse.

Yes - it is OK.

[mph8783]

Re: samsung 50" plasma, no picture, no obvious short or blown fuse.

looks like it is a couple of the FET's on the -Vsc side of things. Not sure why I didn't catch on the first pass (must have been half asleep!).
It looks like they are not switched in initially but when they are the switch mode supply on the Y-Main board shuts down protecting things.
Interestingly when power to the TV is then switched off the switch mode supply comes back up again and holds it's 19V rail for about 20 seconds!

[tom66]

Re: samsung 50" plasma, no picture, no obvious short or blown fuse.

They are probably switched on by the control board. And the SMPS runs off Vs which has a few thousand uF in bulk capacitance, so it can run for some time.

[mph8783]

Re: samsung 50" plasma, no picture, no obvious short or blown fuse.

Ok, so I have replaced the 2 blown FET's plus the 4 others in parrallel and driving the blown ones. I have also replaced the top buffer IC on the top buffer board.

The TV now switches on and the screen comes to life, even showing the channel number OSD.
But after a couple of seconds the screen shuts off, but sound continues.

Is this likely to be a bad cap I haven't found or something else?

[tom66]

Re: samsung 50" plasma, no picture, no obvious short or blown fuse.

Try plugging in a source -- perhaps the lack of a signal is causing it to shut off.

[mph8783]

Re: samsung 50" plasma, no picture, no obvious short or blown fuse.

It had a source plugged in.

Should the led on the y-sus be on or off during normal operation?

[jose6326]

Re: samsung 50" plasma, no picture, no obvious short or blown fuse.

hello im new to the site and i am having a problem with a 50 inch plasma tv as well. i dont want to take over this thread unless it is ok or should i start a new thread?

i know some forums dont like new threads being started if they are similar to other threads

[tom66]

Re: samsung 50" plasma, no picture, no obvious short or blown fuse.

hello im new to the site and i am having a problem with a 50 inch plasma tv as well. i dont want to take over this thread unless it is ok or should i start a new thread?

i know some forums dont like new threads being started if they are similar to other threads

Start a new thread. Include the make and model of your set and full pictures of the chassis and major boards inside the unit.

[mph8783]

Re: samsung 50" plasma, no picture, no obvious short or blown fuse.

Some progress...

The TV is capable of working - it just shuts down the Y-Sus after a period of time.

The time period is 2 seconds - i.e. as soon as it tries to drive a picture and not just black most of the time.
However I have had it working for a few minutes before it shut down.

I have also discovered that by disconnecting any two of the Y buffer flexi's to the panel it will stay on, or even the x-sus board (obvioulsy very ghostly picture then).

I presume that a current limit is being reached or overload of a supply - any ideas where to look?

I have also noticed that Vs is measuring 209V, while the label on the panel says it should be 207V. Should I try adjusting this back down two volts?

I feel it is nearly there......

[tom66]

Re: samsung 50" plasma, no picture, no obvious short or blown fuse.

Try checking Vset, Vscan and Va. Seems like any significant load overloads the power supply then?

[mph8783]

Re: samsung 50" plasma, no picture, no obvious short or blown fuse.

VScan is -189V (OK)
Va is 63V (OK)
VSet very briefly starts at 189V, then drops to around 110V - What shoud it be?
VSet also appears to vary a little depending on the picture.

[tom66]

Re: samsung 50" plasma, no picture, no obvious short or blown fuse.

Vset should be Vs +/-30% or so.

[mph8783]

Re: samsung 50" plasma, no picture, no obvious short or blown fuse.

Problem solved!

While testing I had only put the Y-Sus PCB in with one screw (to ensure it had a ground).
It appears that it needs more than one - in particular the one in the top left by the VSet circuit.

The TV now works - and VSet has settled at around 110V w.r.t. chassis. Unless anybody thinks this is a problem I will leave VSet as it is.

The general picture quality has also improved from before the failure, less noise and better blacks - clearly some of the caps I have replaced were ageing.