Samsung Plasma PN50B450B1D Dim picture, black pixels

[Glitcher]

So I just got a newer model Samsung plasma. Been reading around trying to narrow down if its a panel issue or not but I concluded it isn't hopefully.

Once the tv is on I can see the picture in the background however it is very dim lit. and there are random black lines and pixels. Another thing interesting is where it displays the "no signal" it is completely whited. When I open the menu some of the words seem to be missing.

I checked the fuses on the y and x main and they are good. I measured all the voltages, all are in spec except for Vscan which is around -150v when the sticker reads -198V.

Any ideas which board could cause this problem?

[tom66]

Re: Samsung Plasma PN50B450B1D Dim picture, black pixels

X-main 99.9%.
Check the Ve voltage, you might find it low.

[Glitcher]

Re: Samsung Plasma PN50B450B1D Dim picture, black pixels

I was suspecting the X-main at first also.
Not sure if its my meter but its been acting crazy lately, might need to get a new one.
Anyway im getting crazy fluctuations while measuring Ve now.

So I guess I should check the fets on the x-main. Are there any common components that usually fail with this model?

[tom66]

Re: Samsung Plasma PN50B450B1D Dim picture, black pixels

You don't have a short on Vs since the Vs stays steady and the Y-main does work (hence you get -some- image on the panel.)

I would therefore look at the Xe FETs, you will probably find that the Ve voltage is not getting applied properly. A scope does help, but you can do some short/resistance checks. It is possible, but unlikely, that the Xg/Xs FETs have failed but in a non obvious manner without shorting or the driver circuit for them has failed.

[Glitcher]

Re: Samsung Plasma PN50B450B1D Dim picture, black pixels

Alright so far I desoldered and measured every FET/diode on the heatsinks on the x-main.

I found 2 FETS that read unknown/faulty component on my DCA so im sure those are the problem. FGPF50N33

The diode labeled RF1501 TF3S. There are three pins on it but the one on the middle is shorter. Not sure the purpose of the middle pin is. If I measure the pins 1 and 3 itll measure normal diode junction. but if I measure all three pins it shows shorted between pins 1 and 2. Is it bad?
found it on ebay with a diagram below also.
http://www.ebay.com/itm/3x-RF1501NS3...item3cd4332c47

[tom66]

Re: Samsung Plasma PN50B450B1D Dim picture, black pixels

Are you using the diagram in the datasheet for pin layout, or physically? A short between the outer two pins on those diodes is present by design. The FET could cause your issue. Was it labelled on the board as "Xe"?

[Glitcher]

Re: Samsung Plasma PN50B450B1D Dim picture, black pixels

In my photo the diode, pins 1 and 2 (the left and shorter middle pin) are measured as being shorted.

The bad FETs I found were labeled on the board as "Xf" and Xr" I didn't see an "Xe" on there. it also has Xg, Xs, Xb

[tom66]

Re: Samsung Plasma PN50B450B1D Dim picture, black pixels

Ah on your board Xb = Xe, for some reason Samsung stopped calling it Ve for a few years and started calling it Vb (Panasonic also calls it Ve, perhaps some patent/design proof thing?) But then later they started calling it Ve again. Not sure why!

I suppose those FETs being bad could cause your issue though I would expect the fuse to blow.

[Glitcher]

Re: Samsung Plasma PN50B450B1D Dim picture, black pixels

found out my meter was low on battery, no wonder its been going crazy.

Hopefully the diodes are good so ill put those back. The fuses on the X-main are still good. The bad FETs don't seem shorted but more like its resistances increased to near open circuit? I don't seem to get any readings on resistances on the bad FETs.

Just searched it up on digikey and they have some. I think these are it.
http://www.digikey.com/product-searc...0N33&x=11&y=11

[Glitcher]

Re: Samsung Plasma PN50B450B1D Dim picture, black pixels

so bad news, I just got the new fets in. I tested them with my DCA and they also read unknown/faulty component... So I guess the DCA doesn't have any data on these parts. So i'm guessing its probably not the problem with the x-main.

[tom66]

Re: Samsung Plasma PN50B450B1D Dim picture, black pixels

The DCA doesn't need data on the components, it tests voltages/currents only. It will detect IGBTs as Digital NPN (which is reasonably close) and FETs correctly, as long as they can conduct at least 1mA at Vgs=4.5V, which MOST can. To be sure check the datasheet.

[Glitcher]

Re: Samsung Plasma PN50B450B1D Dim picture, black pixels

well just placed back the x-main board and the problem is still there! Ve is still low and fluctuating.

I was unplugging things and I found that if I unplugged the ribbon cable leading to the control board, Ve measures a steady 90V which is on spec.

So im guessing the control board is bad? I did suspected the control board since the OSD was a little weird.

[tom66]

Re: Samsung Plasma PN50B450B1D Dim picture, black pixels

No probably not due to that, I'd expect that instead the CTRL stops sending the signal which is clipping Ve low but it's still a bad X-main. With a scope you should be able to see exact waveform.

[Glitcher]

Re: Samsung Plasma PN50B450B1D Dim picture, black pixels

guess ill have to pull out my old scope and probes. Where would I have to clip to test the waveform?

[tom66]

Re: Samsung Plasma PN50B450B1D Dim picture, black pixels

Use 10X probe.
On the end of the blue cap on the X-BUFFER (X extender) marked OUT.

[Glitcher]

Re: Samsung Plasma PN50B450B1D Dim picture, black pixels

Heres what I got, I have no idea what it means though.

Measured on 5 volts per, 1 ms per, on 10x probe.

[tom66]

Re: Samsung Plasma PN50B450B1D Dim picture, black pixels

Very unusual - you have the Ve portion (the 100V parts) but it's got little "dips" in it for some reason. You also have the SUS parts. (the large blocks peaking to 200V in parts.)
I'm not sure what to make of that. Try checking Ve with 2nd probe, do you see any dipping corresponding with the SUS part?

[Glitcher]

Re: Samsung Plasma PN50B450B1D Dim picture, black pixels

Where do I place the second probe for the Ve?

[tom66]

Re: Samsung Plasma PN50B450B1D Dim picture, black pixels

Wherever you are testing Ve.

[Glitcher]

Re: Samsung Plasma PN50B450B1D Dim picture, black pixels

Do you mean with a regular multimeter? I'm getting a readings randomly fluctuating from 0 to 25v when measuring VE

I also noticed that my Vscan is also low at -170v when it's suppose to be at -193v. I did the same thing back unplugging the ribbon to the control board and Vscan went back to specs at -195v?