Samsung PN50A450 - No Power

[steveh_131]

This is my first question on these forums although I have done hours of research.

I bought this plasma TV on craigslist for $50. The owner said it was hard to turn on and he didn't know what was wrong with it. I figured I would take a stab at repairing it. I did a lot of research and found that there are a few bad capacitors on the power boards of this model, and Samsung actually did a recall.

I brought it home and plugged it in and it does nothing. No red standby light, no clicking, nothing. This isn't really consistent with the problems addressed in that recall. I ordered a new power board for it anyways, thinking that must be the source of the problem. It was used but supposedly tested and functional. I replaced it and still nothing.

I checked the three fuses on the power board, and two glass fuses on two other boards and they all show continuity.

It gets 110V AC at start of the power supply, and I see a higher DC voltage at some of the other power supply fuses.

Here is an overview of the layout:



Any ideas would help, or even some troubleshooting steps. Thanks!

Steve

[tom66]

Re: Samsung PN50A450 - No Power

Can you get a picture of the label near the connector between the main board (with HDMI ports) and the power board?

[steveh_131]

Re: Samsung PN50A450 - No Power

Here's a few:







Do those help? I guessed you were talking about the label directly to the right of the power board, but I wasn't sure so I took a few.

Thank you so much for your help, Tom!

[newtothis]

Re: Samsung PN50A450 - No Power

Should be this one Tom

[steveh_131]

Re: Samsung PN50A450 - No Power

Or this is printed on the actual power board by the connector to the main board:



ETA: Snapped a better one

[newtothis]

Re: Samsung PN50A450 - No Power

If you look on the PSU, look at the connector the with the cable that goes to the main board... you will see near that connector a reference for the voltages of the connector. Take a photo of that

*** Nevermind You posted while i was writing that LOL! ***

[steveh_131]

Re: Samsung PN50A450 - No Power

Does that show everything you need? I'll gladly take any more that would help.

Steve

[PlasmaRon]

Re: Samsung PN50A450 - No Power

It's hard to see from the picture, but do a couple of caps at the top of the Z board have bulges? (Top right of picture)..

Would you expect the standby power supply might be bad or loaded down?

[newtothis]

Re: Samsung PN50A450 - No Power

DMM set to 20VDC, black probe to chassis (screw hole works well) red probe to the STBY PIN, So in orientation with that picture.. 2nd down from top right.

Whats the reading?

[steveh_131]

Re: Samsung PN50A450 - No Power

I don't see any bulging caps. I'll look again more closely.

If I'm reading the STBY pin correctly, it gets 5V.

That is with the cable unplugged, right?

Steve

[newtothis]

Re: Samsung PN50A450 - No Power

Yeah that's fine, so we have the standby voltage.

Next put red probe to PS_ON, Same orientation, Top right... watch the reading when pressing power switch.

Mainboard plugged in this time, You can put the probe into the top of the connector at the wire... it will touch the contact

[steveh_131]

Re: Samsung PN50A450 - No Power

It stays steady at 3.7V whether I'm pressing the button or not.

It has these weird buttons that I assume are touch sensitive? They don't actually press in. I tried the one on the tv and I tried the remote as well, neither seemed to change that voltage.

Steve

[newtothis]

Re: Samsung PN50A450 - No Power

So PS_ON has a constant ON signal.

To me that indicates a short on the main board... Tom66 and Budm have more experience with plasma's than me so I will let them chime in on this and see if they agree.

Can you check each pin on that connector and post back voltages. ta

[steveh_131]

Re: Samsung PN50A450 - No Power

Interesting. So when the main board is unplugged from the power supply board, the standby is on. Then when I plug in the main board, the standby goes to zero. This does suggest an issue with the main board I suppose.

I'll test the rest of them in a bit and post the voltages.

[steveh_131]

Re: Samsung PN50A450 - No Power

The PS_ON and STBY were the only two pins that seemed to have voltage. 2-3V.

I had a really hard time getting the probe in to the top row of pins, so I don't know how accurate those measurements would be, but they all seemed to be zero.

[newtothis]

Re: Samsung PN50A450 - No Power

Without the PSU in the ON state they should all read zero except the STBY pin... Once the PSU received the on signal from the main board the other voltages should become present. Generally the PSU will send a voltage >2.5v to the PS_ON pin to initiate the PSU... as it stands the voltage is constantly >2.5 indicating a fault.

[steveh_131]

Re: Samsung PN50A450 - No Power

Without the PSU in the ON state they should all read zero except the STBY pin... Once the PSU received the on signal from the main board the other voltages should become present. Generally the PSU will send a voltage >2.5v to the PS_ON pin to initiate the PSU... as it stands the voltage is constantly >2.5 indicating a fault.

Ok. That makes sense. So does the main board need to be repaired or replaced? Any troubleshooting steps to take beyond this?

I really appreciate all the help so far!

[newtothis]

Re: Samsung PN50A450 - No Power

Will at this point have to pass you to Tom66 and budm.

[tom66]

Re: Samsung PN50A450 - No Power

Could be bad cap on standby circuit. One way to test.

Unplug power to main board connector from main board end.
Wire jumper between PS_ON and any GND.
Wire jumper between VS_ON and any 5V STBY.
Follow steps here: https://www.badcaps.net/forum/showpo...61&postcount=2

(LVDS runs between main board and the control/logic board.)

You should get a cycle of test patterns on the plasma panel if the only fault is the main board.

[steveh_131]

Re: Samsung PN50A450 - No Power

Let me clarify before I run this test, if you don't mind. Steps:

1. Unplug power-to-mainboard connector AT the mainboard.

2. Wire jumper between PS_ON and GND - On the power board? Or the main board? Should I do this on the end of the cable that was unplugged from the main board?

3. Jumper between VS_ON and 5V STBY - Where is VS_ON? I don't see it on the list from the power-to-mainboard connector.

4. Unplug the LVDS

5. Turn on tv

Thanks again for your help!