Samsung PN50B650 plasma dark picture (better than what I started with)

[teryaki]

Picked this up for $30 off CL, had sound but no picture. Probed with the multimeter and found a shorted IC on the Y buffer. Replaced that board because I wasn't able to desolder the IC without damaging the board (goddam RoHS). On first startup with that board replaced, there were brief flashes of the TV operating perfectly, but now consistently the picture comes up dark with dancing pixels everywhere, and requires voltages be way out of spec to clear up purple areas near the top.

Also the VSC pot was jumpy, so I replaced that.

Currently have all voltages set per panel sticker and need suggestions on where to go next. Buy an ESR meter and test the caps? Give up and part out this display? Roll for diplomacy?

[teryaki]

Re: Samsung PN50B650 plasma dark picture (better than what I started with)

So when I said I had the voltages set per sticker... well, I certainly had sustain voltage set to what I thought was close enough. Panel sticker calls for 209v and I was getting 205v and figured that must be within tolerance, right? Probably my Home Depot multimeter's not precise enough to tell anyhow.

So I got the advice that it was probably sustain voltage being too low, so I measured. 205v. Turned it up, and can get a 207v measurement with the pot fully open.

[tom66]

Re: Samsung PN50B650 plasma dark picture (better than what I started with)

Check Ve voltage as well. It's normally around 100V and if it's low you could get this symptom too.

[teryaki]

Re: Samsung PN50B650 plasma dark picture (better than what I started with)

Check Ve voltage as well. It's normally around 100V and if it's low you could get this symptom too.

Panel
Va 56
Vsc -200
Vs 209
Ve 92

Current values according to my multimeter

Va 57
Vsc -162
Vs 209
Ve 94

I can't get Vsc to go any higher, 200k pot is turned all the way. Other way takes it to -54.

[tom66]

Re: Samsung PN50B650 plasma dark picture (better than what I started with)

Did you replace the -Vsc pot with the right value?
The low -Vsc is likely the cause of the issue. It will cause underdiffusion in the panel.

[teryaki]

Re: Samsung PN50B650 plasma dark picture (better than what I started with)

Original was marked 202, that should be a 2k ohm pot. Replaced with same.

Thanks for chiming in, BTW. Just going through the stickied stuff here, you're a legend.

[tom66]

Re: Samsung PN50B650 plasma dark picture (better than what I started with)

above, you say 200k, do you have the original to check?
Worth checking the Vsc with the buffers disconnected from Y-main too.

[teryaki]

Re: Samsung PN50B650 plasma dark picture (better than what I started with)

above, you say 200k, do you have the original to check?
Worth checking the Vsc with the buffers disconnected from Y-main too.

Sorry, I'm usually pretty tired when I write these. Should definitely be 2k, not 200k. 20 + two zeros after that by marking conventions.

Sure I still have the original, I can check them against each other. And I'll pull the buffer and see where Vsc goes.

[teryaki]

Re: Samsung PN50B650 plasma dark picture (better than what I started with)

Definitely a 2k pot. Stock pot is 2.05k ohms tested, 2.03k for the replacement.

Vsc is the same with the Y buffer board removed, -165v.

[teryaki]

Re: Samsung PN50B650 plasma dark picture (better than what I started with)

So Tom, at this point... start pulling and testing caps and transistors from the Y board? I don't know what else I can try.

EDIT: Looked up the board on ShopJimmy, they're indicating that the most common failure with that board isn't the caps: https://www.shopjimmy.com/samsung-bn...repair-kit.htm

[teryaki]

Re: Samsung PN50B650 plasma dark picture (better than what I started with)

SO! I checked all the big heatsinked diodes and transistors on the Y sustain board and they seemed consistent. So I started tracing circuits, and got to an optocoupler near a DIP-8 IC, both just upstream of the Vscan trim pot.

Diode test across the LED side of the optocoupler reads both ways (.774 anode to cathode, .788 cathode to anode. Does it sound like I've found my missing voltage?

Photo for reference, the optocoupler is U5013, in the far left of the image: https://i.imgur.com/aqHTFxX.jpg

[tom66]

Re: Samsung PN50B650 plasma dark picture (better than what I started with)

Opto could be bad, it could also be the TL431 or one of the resistors in the circuit, it is hard to tell for sure. Do you have a scope?

[teryaki]

Re: Samsung PN50B650 plasma dark picture (better than what I started with)

Opto could be bad, it could also be the TL431 or one of the resistors in the circuit, it is hard to tell for sure. Do you have a scope?

No o-scope, no. Also it's not the opto, the reading was because there's a 2k ohm resistor across the LED side of that opto.

But I think I've got something! I probed Vs again yesterday probing at different points, and I've got a sharp voltage drop! On the side of the power supply board near Z-sustain, it's reading 212v at the test point and at the connector to Z-sus board. But on the other side of the power supply board, at the pins connector for Y-sus, it's dropped to 202v. At the pins on the Y-sus board, it's dropped further to 192v, just across copper wire with no resistors inline.

[tom66]

Re: Samsung PN50B650 plasma dark picture (better than what I started with)

That's unusual, but it could be because your multimeter is picking up interference from the TV. A lot of cheap meters can suffer with this effect but even some higher quality ones are known to suffer.

[teryaki]

Re: Samsung PN50B650 plasma dark picture (better than what I started with)

I tried with a second Y-sus board, and Vs seems to get worse week after week. Seems like something on the PSU is dying?

[tom66]

Re: Samsung PN50B650 plasma dark picture (better than what I started with)

Could be. Try checking the Va and the other voltages.

[teryaki]

Re: Samsung PN50B650 plasma dark picture (better than what I started with)

Ve and Va are perfect and stable. I've still got that dropoff on Vs as I measure closer to the Y board (can get 209 over by the X sustain board), and Vscan still fluctuates between -140 and -180, with spec being -200. The dropoff is more pronounced now, around 35v lower instead of the 20v dropoff I measured previously.

At this point both Y sustain and buffer boards have been replaced (Y sustain is going back since the old one worked just as well).

[tom66]

Re: Samsung PN50B650 plasma dark picture (better than what I started with)

So if the Vscan is varying that much then I would suspect there is still something wrong with the Y-sustain. It could be an issue with a capacitor or the converter on the board.

[teryaki]

Re: Samsung PN50B650 plasma dark picture (better than what I started with)

So if the Vscan is varying that much then I would suspect there is still something wrong with the Y-sustain. It could be an issue with a capacitor or the converter on the board.

Two different Y sustain boards, though. And a replaced Y-buffer.

[tom66]

Re: Samsung PN50B650 plasma dark picture (better than what I started with)

Boards could have the same failure, though - especially if they are used. It's hard to tell w/out a scope, unfortunately