Samsung pn50b860y2f smps board problem

[sberri]

I have a Samsung pn50b860y2f plasma TV that runs perfectly until the smps board (part # bn44-00277a) warms up. All voltages are dialed in as per the label and are stable until the smps warms up (roughly 5 to 10 minutes after power up) Vs and Va measure a stable 207v and 56v respectively. Once the smps is warm however, the two voltages drop to 182v and 50v and the screen become darker and has temporary ghosting/image retetion issues. I assume this is from the drop in Vs and Va voltages. See images for screen shots of image problems. If left on, the TV will display ever darkening/smuged images and will shutdown after 10 to 15 minutes of operation. If left to cool off, the entire process repeats. X, Y and both buffers have been swapped out with verified working boards and I've eliminated them as possible faults. I have a verified logic board as well but am reluctant to install it for fear of firmware issues and because its location makes swapping it out a ridiculously difficult task. However, I will swap it out if advised to do so. I've checked temperatures of caps, transformers, heatsinks, and pretty much all of the obvious components and nothing rises above 85-90 degrees Fahrenheit. I've also removed all heatsinks and reinstalled them, verifying proper contact with each component. Whatever is "overheacting" is doing so at what appears to be sub 90ish degrees Fahrenheit.

If I leave the back cover of the TV off and use ridiculously high cfm 120mm fans to cool the area around heatsink 3 ( see attached schematic) , I can keep the issue from occurring if the ambient room temperature is under 72ish degrees Fahrenheit. I've checked the transistors and caps in this area and all appear OK with no line voltage applied to the smps. As far as tools for diagnostics, I have a DMM and a m12864 LCR-T4 ESR tester at my disposal. Unfortunately, I haven't taken the plunge and purchased an oscilloscope yet. Any and all advice is welcomed.

Scott

[ReeceyBurger123]

Re: Samsung pn50b860y2f smps board problem

Possibly a X/Polly cap failure these become heat intermittent, got images of the board ?

[sberri]

Re: Samsung pn50b860y2f smps board problem

Here are some images of the board.

[Nick's Tvs]

Re: Samsung pn50b860y2f smps board problem

It is very possible that your power supply is not the issue but in fact one of your sustain boards causing the voltage to drop. In the past, I have noticed that a lot of problems occurring after 5-10 minutes of the tv being on are due to bad solder joints. I would let the tv sit unplugged for a while then take out the SMPS and the sustain boards and check all solder joints on the back side of them. Usually transistors, transformers, even little blue ceramic caps will have cracked joints.

[sberri]

Re: Samsung pn50b860y2f smps board problem

@Nick thank you for the suggestion and insight, but unfortunately, I have already done that with the original X, Y, and their respective buffers as well as their replacements that were taken from from an identical fully functional TV. I wish my 125lb dog hadn't made a chew toy out the smps I removed from the working TV to remedy this entire issue. Before you ask, I had placed a bag of jerky on top of the board and ran to answer the front door. Boomer ate the jerky, its bag, and chewed up the board I guess because it smelled like jerky all in a span of less than 5 minutes while I signed for my UPS delivery. Damn dog. Lol.

[Nick's Tvs]

Re: Samsung pn50b860y2f smps board problem

There are only 2 things I can think of. The first is to freeze spray some of the IC's some of the transistors, and diodes and see if you get a little longer run time before it starts screwing up. It may not have an affect however.
The second would be to disconnect everything and force turn on the SMPS and wait until something really starts overheating and or smoking. But that second one could potentially do some extra damage obviously. I don't have much experience with this SMPS so I can't help you really at this point.
Good luck.

[sberri]

Re: Samsung pn50b860y2f smps board problem

Nick, I've pretty much arrived at the same two conclusions. I've not performed the "wait for magic smoke" test yet because I know I'll end up cooking something else. That's just the kind of luck I have. I'm going to hold off a day or two before resorting to that. Hopefully the TV God himself,Tom66, will chime in with some ideas. I have access to replacement poly and electrolyte caps, I think I'll replace all the caps on the secondary side of the board in the mean time. I have two smps boards left for this set, the one in use with a bad secondary and the original one that has a bad primary. I might be able to hodge podge together one working unit out of the two as long as I don't need to move any surface mount components. Soldering is a breeze for me, but I draw the line at tiny SMD components. Too damn difficult without a hot air station. Do you think this MacGyver idea is plausible?

I'll try the freezing test this morning/afternoon while you think my idea over. Off the top of your head, what temperature range should the components on a working plasma smps board fall into? I have a fairly highend DMM that takes very accurate temperature readings via a K coupler probe and it also has an ambient room temperature sensor . Thanks so much for your input.

[sberri]

Re: Samsung pn50b860y2f smps board problem

I finally had some free time to troubleshoot this further. I've found that mosfet QS801 is overheating. If I keep it cool by a fan or by freezing it, VS and VA are stable and the picture is wonderful. If cooling is removed, VS and VA begin dropping within a minute or two and my picture quickly turns into the examples in my original post. I've cleaned and reapplied new thermal pads and have tried artic silver thermal paste ( without the pads or course) , neither solves the problem. I've also changed out the MOSFET and the new one overheats as well. I'm at a complete loss here. I've attached the schematic and have circled the MOSFET that is overheating. It is on page two, titled VS BLOCK. Any and all help is greatly appreciated.