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ibloomfield

join:2012-09-17
Golden, CO

Philips 52PFL3603D/27 backlight inverter testing

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RDENC2544TPZF
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RDENC2545TPZF
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RDENC2542TPZF
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RDENC2543TPZF
Hi,

I'm new to these forums, so please excuse any convention breaking or similar new guy mistakes.

I have a philips 52PFL3603D/27 that lost its picture last year, and finally got around to fixing it. There is still sound and I can see the picture if I shine a light directly at the screen. After a little bit of research, I narrowed the problem down to the backlight inverter board(s).

There appears to be four inverter boards in this TV; I have uploaded pictures of all the boards. Two of them have obvious blackening, the other two have less obvious discoloration. I am trying to determine which need replacement.

My question is this:

Can I tell which are bad just by looking at them? If so, how can I do this?

If not, what is the best way to test if these boards are working?

Thanks in advance for the help. If clearer pictures are needed, let me know

-Ian


leibold
Premium,MVM
join:2002-07-09
Sunnyvale, CA
kudos:10
Reviews:
·SONIC.NET

said by ibloomfield:

After a little bit of research, I narrowed the problem down to the backlight inverter board(s).

It would help if you provided some details on your research and how you determined that the problem is with the backlight inverter boards. While that is indeed a possible cause, the failure of one inverter board wouldn't necessarily cause all backlights to be off.
said by ibloomfield:

Can I tell which are bad just by looking at them?

Unless you see a component that is physically defect (bulging or exploded capacitor for example) then no, just looking at a PCB will not tell you whether or not it is in working condition. Board 3 has quite a bit of heat related discoloration but that doesn't mean it is defect.

Are there any components on the backside of those boards ? I don't see any switching transistors which I would expect on an inverter board. Perhaps the active components of the backlight inverter are on a different board ?

If you have the service manual with schematics there is a lot you could check. If not you can start by checking whether the fuse on each board has continuity and with the TV powered on you can check from each fuse to ground whether the boards receive power. Since it is unlikely that all 4 boards failed at the same time keep the possibility in mind that the power supply to those 4 boards has failed.
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ibloomfield

join:2012-09-17
Golden, CO

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RDENC2544TPZF back
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Close up
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power board
The research I did consisted of searching "tv no picture but sound" and reading a few forums that suggested that if the backlight was out, a probable cause was the inverter boards. I made a few key assumptions that could very well be wrong:

1. The board with the most discoloration was definitely bad.
2. If one board went, they would all stop working (since they are all hooked up together).
3. The power board is fine because the sounds works as well as the lcd panel.

I have attached two pictures of the backside of rdenc2544tpzf, the one with the most discoloration. I believe at least one of the transistors (?) is bad. I took a close up shot so you could see what you think.

Is assumption 3 correct? I didn't realize there could have been an additional power board just for the backlight inverters. I have also attached a picture of the power board to show how it connects to the inverters.

I can provide additional information on how the boards interconnect if that would help.

Thanks for the help,

Ian


mackey
Premium
join:2007-08-20
kudos:8

said by leibold:

Board 3 has quite a bit of heat related discoloration but that doesn't mean it is defect.

While you're right it's not a 100% reliable indicator, when a device isn't working and one of 4 pretty much identical boards is burnt (that's not "quite a bit" of discoloration, that's burnt) then you've most likely found where the biggest problem is.

said by ibloomfield:

I made a few key assumptions that could very well be wrong:

1. The board with the most discoloration was definitely bad.
2. If one board went, they would all stop working (since they are all hooked up together).
3. The power board is fine because the sounds works as well as the lcd panel.

Those additional pics do prove #1 is correct. The air gap in the middle and molten metal at the bottom of the chip means that chip's toast.

#2 may or may not be true, depending on how the thing's wired. If the failed board didn't damage anything else when it went out, you could have lost just part of the backlight.

#3 is not true. While it's part of the same board, the pic of the power supply shows it's almost certain that the backlight boards are driven off a separate supply then the rest of the display.

The discoloration on the other 3 boards most likely means one of 3 things:
1) The voltage regulation on the power supply board went out and fried the inverter boards.
2) The CCFL tube(s) went out and took the inverters with them.
3) The parts on the inverter board are underrated (or under heatsink'd) and eventually overheated and failed.

Even if you replace the boards, if the problem is #1 or #2 the new boards will just be taken out again. If it's #3 and the replacement boards are the exact same as the old ones then they'll just buy some time before going out again (probably after approx. the same number of hours on them).

Unless that set is $$$$$ or the parts are dirt cheap I'd just toss it. Best case is it was #3 and the new boards were redesigned to eliminate the problem. Worst case is #2 as you'll now need to replace the tubes as well. #1 is fixable with additional troubleshooting and possibly a new power supply board; if you don't know what it is *supposed* to be putting out however it becomes much harder.

/M


mackey
Premium
join:2007-08-20
kudos:8

Hmmmm, those boards are going for <$20 on ebay, so it would probably be worth it to pick one up and see what it does

/M



leibold
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join:2002-07-09
Sunnyvale, CA
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reply to ibloomfield

The backside shows the power mosfets that I was missing and Q520 is a good example of what I meant with a "physically defect" component.

Is the fuse on board 3 still intact ? There is usually a lot of current involved when a semiconductor fails in such a spectacular fashion.

The power board provides several output voltages and it would still be defect even if only one of them is out of spec (too high, too low or missing completely). The fact that other parts of the TV (pixel drivers, audio) are still working is unfortunately not a sufficient indicator that the power board is 100% ok.

It could be that the power board has electronic overcurrent protection and that board 3 with Q520 as a dead short triggers that. This would explain why all backlights are off right now. If you are lucky, replacing board 3 might be all it takes. mackey See Profile has already given you some ideas as to what else may need fixing.

As for the heat discoloration (also visible to a lesser degree on the other boards) it is not clear what is cause and effect.
There might be a problem (power or timing) that causes the mosfets to be driven beyond their design limits (2W for the RSS070) and therefore they get unusually hot and ultimately fail. On the other hand they may have been operating as designed but because airflow (bottom to top through the TV chassis) was somehow obstructed they were getting too hot. TV sets don't have cooling fans and rely on convection cooling. This makes it very important that top and bottom air vents are clear so that the air can move freely.
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IllIlIlllIll
EliteData
Premium
join:2003-07-06
Hampton Bays, NY
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reply to ibloomfield

all of this is moot, because, for the inverter boards to get charred like that, means the CFL's were drawing heavy current, which would mean they were operating at full potential, which means the CFL's could be worn down severely as a result of being operated at "full blast", which causes more current to be drawn on the inverters as the CFL tubes get weaker, which eventually causes the inverters to fail.
point is, the CFL's *may* be bad in addition to the inverters.
you could find out if they are worn if you could disassemble the panel (very fragile and very difficult) and look at the ends of the CFL's for dark black (just like fluorescent lamps).
some CFL tubes are interchangeable from other similar sized, similar dated LCD panels.
all of these philips/magnavox sets, i have yet to see a "backlight adjustment" in the customer menu.
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IllIlIlllIll
EliteData
Premium
join:2003-07-06
Hampton Bays, NY
kudos:7
reply to mackey

said by mackey:

Hmmmm, those boards are going for <$20 on ebay, so it would probably be worth it to pick one up and see what it does

/M

then i would say its worth it to spend $20-$40 for the boards and "risk it", maybe the CFL tubes still have life left, its not known unless they are physically looked at or new inverter boards are installed.
its unlikely the power supply itself is bad, these philips sets all have immense "feed back" circuits, if a single voltage line from the SMPS is down, the set wont operate at all, it will just flash "morse code" in the power indicator which refers to a specific failure.
the inverter boards dont utilize "feed back" though, so the set will still operate.
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ibloomfield

join:2012-09-17
Golden, CO
reply to mackey

Thanks for all the responses, everyone's been very helpful. I will order a replacement board and go from there. That being said, I do have a few more clarification questions.

1) Is there any way to test the power board? Unless I knew the expected output voltage (which I don't), I have no idea how to test this. Do you guys have any suggestions?

2) Although I am still ordering a board, would have replacing the two bad transistors potentially repaired the burnt board? Or do the burnt transistors suggest that other components inside or on the board are bad as well?

Thanks again.

Ian



mackey
Premium
join:2007-08-20
kudos:8

1 recommendation

1) Kinda. There's no way to prove it's working fine, but a few basic tests can see if it's definitely bad. If it's producing something, and that something is reasonable then we know it might still be good. Reasonable in this case most likely means something between 5v and 90v 45v (MOSFET rating), with 12v-36v being the most probable.

2) Maybe, maybe not. MOSFETs do go bad, and if one fails "on" then the shoot-through in an H-bridge would most definitely make something go POP! However, judging by the serious discolorations of the board, in this case something else most likely overstressed the MOSFET to the point of failure. Then again, like leibold See Profile said, if this monitor was stuffed into a hot, enclosed space then the discolorations may be from simple overheating until one failed "on" and the shoot-through made it go POP. So now do you spend $12 (by the time you add handling and shipping charges to your $0.12 part, assuming you can even find it in qty 1) hoping it's just a bad MOSFET, or do you spend $18 (shipped) for a known working board? It's actually the same for the monitor as a whole; do you spend $18 hoping it was just a bad board, or $900 and replace the whole thing? The difference in those numbers is why I said scrap it unless the parts are cheap; $18 is no big loss if it doesn't work out, but once you get into $300-$500 replacement boards you're not sure will even fix the problem it's best to just cut your losses.

/M


ibloomfield

join:2012-09-17
Golden, CO

Cool. Question 2 was out of pure curiosity. I'll see if I can test the power board, and when the new inverter boards gets here I'll cross my fingers.

Ian


ibloomfield

join:2012-09-17
Golden, CO
reply to ibloomfield

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Backlight inverter pins (on power board)
Quick update guys:

I tested the two connectors that feed the backlight inverters with a dmm and I am getting no voltage output. Next to the connectors, it states a range of pins is "+24VI" and the next range is "SGND3" (each range has 4-5 pins in it).

I am hoping that this means the power board has some kind of sensor and shuts off power when nothing is connected. Could this be true?

Additional theory question: When does general lcd design end (lcd panel, power board, mother board, backlight inverters) and specific design begin?


leibold
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join:2002-07-09
Sunnyvale, CA
kudos:10
Reviews:
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SGND appears to be Signal Ground and +24V the supply voltage. The fact that both are distributed over several pins on the connector is an indication for the large current expected here.

?_Sel, ?_DIM, ?_ON, BOOST appear to be control signals. Without the service manual and schematics we can only speculate whether they are signals from the inverter boards to the power supply or vice versa.

What is clear is that under normal conditions (with the TV running, backlight inverter boards connected) you should be able to measure 24V between the +24V contacts and the SGND3 contacts. There is also no doubt that without that +24V supply the backlights won't work.

It is certainly possible that without a Sel (select ?) or ON signal on connector CN2 the power supply won't generate the +24V.
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IllIlIlllIll
EliteData
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join:2003-07-06
Hampton Bays, NY
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2 edits

1 recommendation

reply to ibloomfield

DIM_SEL (dim select)
BL_DIM (backlight dim)
BL_ON (backlight on)
BOOST (backlight boost)
i would test with the meter set to ohms and the set uplugged, for resistance between +24VI & SGND3 on both the power supply and inverter PCB's (looking for a short)
i concur with leibold, the smps turns on the back light (BL_ON) since that connection is on the same cable as the 24 VDC but what tells the SMPS to turn the back light on comes from the SSB module (aka main pcb).
i would also assume there has to be a smaller cable connected to one or both of the inverters that is not shown in the picture, possibly a 3 or 4 pin cable used to provide circuit operation feedback.
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mackey
Premium
join:2007-08-20
kudos:8

said by IllIlIlllIll:

I would also assume there has to be a smaller cable connected to one or both of the inverters that is not shown in the picture, possibly a 3 or 4 pin cable used to provide circuit operation feedback.

I'm not seeing another cable anywhere. It looks to me like some of the wires on the right side of the power supply board may be looping through that board to get to the inverter board. Either that or those backlight signals go to some logic on the power supply board and are muxed together with some other info.

/M

ibloomfield

join:2012-09-17
Golden, CO

1 recommendation

reply to ibloomfield

Well guys, I received the new inverter board yesterday and hooked it up in place of the old one. To my surprise, the TV turned on without a problem. It is now fully functional.

I am still trying to figure out the most reasonable explanation as to why the inverter board fried. I am hoping it was just faulty manufacturing on the board itself and not out-of-tolerance voltages coming from the power supply board.

Either way, I'm glad to have the thing running. Thanks for all the help.



IllIlIlllIll
EliteData
Premium
join:2003-07-06
Hampton Bays, NY
kudos:7

said by ibloomfield:

Well guys, I received the new inverter board yesterday and hooked it up in place of the old one. To my surprise, the TV turned on without a problem. It is now fully functional.

I am still trying to figure out the most reasonable explanation as to why the inverter board fried. I am hoping it was just faulty manufacturing on the board itself and not out-of-tolerance voltages coming from the power supply board.

Either way, I'm glad to have the thing running. Thanks for all the help.

like i previously posted before, the inverter is more than likely operating at "full blast".
there is little you can do to reduce this other than turn the contrast & brightness down (this has a small effect in reducing the duty cycle of the inverter circuit) and make sure the set is well ventilated and the set is not located in an already hot environment.
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cowboyro
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join:2000-10-11
Shelton, CT
Reviews:
·AT&T U-Verse
reply to ibloomfield

said by ibloomfield:

I am hoping it was just faulty manufacturing on the board itself and not out-of-tolerance voltages coming from the power supply board.

These days almost everything is made as cheap as possible. That means the cheapest components that will do. And if it breaks in 3-4-5 years then it's time for an upgrade...
Unfortunately nothing is made to last anymore...