Samsung plasma PN51D550 bad PSU or bad logic board?

[Locutor13]

When I got this 3D TV it was dead, I opened the back cover and found the fuse was blown. After doing quick inspection on the PSU, I found apparently no shorted components in the heat sinks and the capacitors looked ok. I replaced the fuse and plug the TV and for my surprise the fuse didn't blown.
When I plug the TV I can hear the relays, the red LED blinks 6 times and the LED will flash in response to the remote control, despite that the TV will remain dead.

When I do some voltage testing with everything plugged I don't get any VS or VA, I get PS-ON=0.18V, VS-ON=0V and all other low voltage seen to be ok (STY, D15.V, 5V, etc.}

When I try to run the pattern test doing the jumper (PS-ON to GND), jumper 3 &4 pins and disconnecting the LVDS to the main board nothing happens, the only change I see is that the PS-ON goes from 0.18 volts to 0 volts.

Note: The VS-ON is always 0 volts and I can get the PS-ON to 3.3 volts only when I disconnect the main board without doing putting the jumper (PS-ON to GND). No matter what, the green LED on the logic board never blinks.

I have attached some pictures.

I am not sure if there is something wrong with the PSU, logic board, or both, if it is not something else?

[R_J]

Re: Samsung plasma PN51D550 bad PSU or bad logic board?

On these sets the you need to monitor the vs or va right when the tv is turned on. If there is a fault (and I suspect a shorted buffer ic ) The voltage will only come up for a sec. and shut down.
I found disconecting the buffer board from the Vsus resulted in the same problem as the vs and va would not stay up. On mine I found one shorted buffer ic, replacing the complete buffer board restored the tv to working condition. If you remove the buffer and check with your ohm meter from the ground on the buffer (usually pins on the white plugs connected to the large ground trace) and to any of the pins on the connectors that go to the panel.
If you read any shorts from the panel connectors to ground, that buffer ic is shorted.

[awyatt2]

Re: Samsung plasma PN51D550 bad PSU or bad logic board?

there are some diodes mounted to the heatsink to220 case 3 legged, check them for open or short. if one or both go bad there will be no high voltages. should be near vs, va testpoints if I remember right. only one has to be bad to cause this. I have a post on a pn50a550 somewhere on here with the replacements listed.there were some revisions to the d model powersupply but should still have the diodes.

[Locutor13]

Re: Samsung plasma PN51D550 bad PSU or bad logic board?

R_J the TV won't response to the control remote or the power button, a tried VS and VA when I plug the TV and they didn't show-up at all. I have tested both X and Y and they are not shorted. I have tested to GND all the pins of both buffer and I am getting normal readings.

[Locutor13]

Re: Samsung plasma PN51D550 bad PSU or bad logic board?

Awyatt2 there is a heat sink between the VS an VA test points, there are 3 diodes and 1 transistor. I have tested the 3 diodes and I am getting normal readings, voltage drop of aprox. 420 on way and open the other from the side legs to the middle legs of each diode. Not sure if worth testing them out of circuit?

Do you know if there is a way to test the PSU without having it plugged to the logic board?

[Locutor13]

Re: Samsung plasma PN51D550 bad PSU or bad logic board?

I thing I forgot to mention was that with everything plugged the PSU will do a power cycle like every 10 minutes.

[tom66]

Re: Samsung plasma PN51D550 bad PSU or bad logic board?

What you can try and do:

Disconnect cable between main board and power board.

Disconnect LVDS between main & control boards.

Place a jumper across pins 3 and 4 of the test pattern connector on the control board, this is a little 2x2 header.

Plug the TV in... do you get test patterns?

You -might- have to connect PSON to GND on the connector between main & power.

[Locutor13]

Re: Samsung plasma PN51D550 bad PSU or bad logic board?

Tom66, yes I have already tried that and I didn't get the test patterns neither the VS/VA voltages.

[Locutor13]

Re: Samsung plasma PN51D550 bad PSU or bad logic board?

Please let me know if someone has any literature for this model or similar like service, training manual manual or sepcs diagrams, etc. Thanks

[tom66]

Re: Samsung plasma PN51D550 bad PSU or bad logic board?

1st result:
http://www.toms-service-manuals.com/...h=Submit+Query

[Locutor13]

Re: Samsung plasma PN51D550 bad PSU or bad logic board?

Thanks Tom66, I think I might have found something.

No matter what I did when I was trouble shooting the TV: I wasn't able to get the 3.3V of the VS-ON on PIN 6 of the connector CN 802 (the one that connects the PSU to the Logic Board) in order to turn-on the TV.

So I took my multimeter, unplugged the CN 802 connector from the PSU and got what it looks like normal resistance readings to GND on PINS 1-5 but on PIN 6 I got only 9.98 k-ohms which might be too low and might be also what is causing the VS-ON to stay at 0 volts.

Also just underneath the connector in the Logic Board I found a small SMD fuse with white body and golden metal plates on each side. It is labeled as F2000 LF5A (see the picture), which is open and is somehow connected to the PIN 6.

If someone have it handy, it would be great to verify the fuse and the PIN6 to GND resistance readings in a working logic board.

[tom66]

Re: Samsung plasma PN51D550 bad PSU or bad logic board?

VS-ON will be driven from the main CPU on that board, which will turn it on or off, so I wouldn't expect to see significantly low resistance to either supply rail.

Are you saying the fuse is blown?

[Locutor13]

Re: Samsung plasma PN51D550 bad PSU or bad logic board?

Tom66 yes, it is blown. I tested in continuity/diode mode and got an OL.

[tom66]

Re: Samsung plasma PN51D550 bad PSU or bad logic board?

I would try replacing it, see if it blows again, sometimes fuses fail for no obvious reason.

As a quick test you could find a car fuse of approx the right rating and solder that across the existing fuse with some wires.

[Locutor13]

Re: Samsung plasma PN51D550 bad PSU or bad logic board?

Thank you Tom66, accidentally while I was doing the troubleshooting I have blown two transistor from the PSU which I will have to replace first before trying the quick test on that fuse. I’ll get back.

[Walfischdreck]

Re: Samsung plasma PN51D550 bad PSU or bad logic board?

Hello Locutor13,

I have with my samsung 51 inch plasma tv a very similar problem to yours, so I would like to ask you how you got this problem solved if you did so till now

Best regards, Philipp