Philips 47" TV 47PFH4109/88 repair - Page 1
Forum section: electronics / repair

author: soldar
author_rank: Super Contributor
timestamp: February 01, 2024, 11:09:02 pm
content: Blank screen. No picture displays. I know nothing about repairing this type of tv.I start by checking the power supply board, marked 715g6163-pof-000-0020, of which I have no schematic, and I measure 12V going out to the main board, which seems correct, and 31 volts around the part going to the LED panel (the screen itself).The 12V to the mainboard looks correct to me but I have no idea if the 31 volts to the LED panel is correct or plausible or not. If it is then I would have to continue with the main video board and I would not know where to even begin there.What say you?

author: inse
author_rank: Super Contributor
timestamp: February 02, 2024, 05:38:25 am
content: What is happening when you turn the set on?Standby LED? Reaction to remote control? Audio?Shine a flashlight on the screen to see whether only the backlight is missing.Is the main processor running (getting warm)?

author: soldar
author_rank: Super Contributor
timestamp: February 02, 2024, 10:33:50 am
content: Quote from: inse on February 02, 2024, 05:38:25 amThanks. It does look to me to be a backlight problem. Let me elaborate.The red standby led turns on. Then I can "turn on" the set and audio, channel change, etc, everything except picture works. When I first turn it on I see the Philips logo flash on the screen for a split second and then darkness. It is like the backlight tries to start but fails. Everything else works correctly.I have done many experiments and once the picture did come on and I can see the backlight is on. Once the picture comes on it stays on indefinitely. I measured the voltage at the "+31V" diode, which feeds the backlight, and it is now 50 V (with the meter, I have not seen with the 'scope). So I suspect that part of the power supply is responsible. I guess I should review every component, track and solder in that part of the power board.

author: kripton2035
author_rank: Super Contributor
timestamp: February 02, 2024, 10:42:35 am
content: schematic of the power supply is here :https://www.abcelectronique.com/forum/showthread.php?t=101188it seems it could be a bulging capacitors problem.

author: soldar
author_rank: Super Contributor
timestamp: February 02, 2024, 02:08:19 pm
content: Quote from: kripton2035 on February 02, 2024, 10:42:35 amAs much as I searched I could not find the schematics. Thanks!The TV belongs to a friend of mine. When it stopped working I recommended a guy I know who repairs TVs and he did repair it by replacing the bad caps on the power supply board. My friend took it home and it worked for some time but then failed again. He took it back to the repair guy who repaired it and did not charge him anything. My friend took it back but after some time it failed again. He says he feels it is just old and will continue to fail and he said he would just dump it but I asked him to let me give it a try.So the caps have already been replaced and everything works when it works. It looks like some bad connection , solder or something like that.I gave it a good inspection but found nothing suspicious. Still, I redid a few solder points and reassembled the set.Now it is working OK but who know how long it will last.One thing is that, in my experience, once it starts it keeps going and does not fail. When it fails it is only at the start.For now I have it under observation.

author: soldar
author_rank: Super Contributor
timestamp: February 02, 2024, 03:28:08 pm
content: Well, it is failing again. Backlight flashes for a split second and then goes off. Definitely the backlight.I don't know what else to do unless just replace the entire PSU board but it seems they are problematic and I might get another bad board.

author: kripton2035
author_rank: Super Contributor
timestamp: February 02, 2024, 04:34:19 pm
content: I would check the esr of all capacitors (electrochemical) . I replaced a lot of capacitors like this, and sometimes, when you're not sure of the quality of the "new" capacitors, they fail again...

author: inse
author_rank: Super Contributor
timestamp: February 02, 2024, 04:59:26 pm
content: We need to find out whether it’s the power supply, the LED control or the LEDs itself

author: soldar
author_rank: Super Contributor
timestamp: February 02, 2024, 05:21:01 pm
content: The PCB layout image does match the actual PCB but the schematic diagram does not. Not even close. So I am not sure how it is supposed to work.Some years ago I had a similar problem with a computer monitor a friend asked me to look at. Screen facing down, on the bench, it would work without fail. Standing upright it would work for some time. As long as it started it would not fail but after days or weeks it would not start. Back on my bench with the back cover off it would work again. I never resolved that mystery. This one is the same.Since I have no idea what I am looking for or what to test my next idea is to try applying hot and cold temperature and see if that makes any difference.The only certain thing is that it is the backlight not coming on.

author: kripton2035
author_rank: Super Contributor
timestamp: February 02, 2024, 05:48:53 pm
content: what about this schematic ?https://elektrotanya.com/philips_715g6163-p0f_psu.pdf/download.html

author: soldar
author_rank: Super Contributor
timestamp: February 02, 2024, 06:32:42 pm
content: Quote from: kripton2035 on February 02, 2024, 05:48:53 pmWhile I cannot vouch that it is exact it does look much more like it and is probably the one. Thanks!I have great difficulty following this kind of diagram which shows electrical connections more than functions. If you already know how the circuit works then this is helpful but if, like me, you do not know how it works, just seeing lines and non-descript boxes does not help much. I also remember, back in my time, schematics would often show waveforms and voltages at certain points when the circuit was working correctly and that was of great help when repairing.Looking at this diagram I see there is an on/off signal and that is the simplest and first thing to check... except that Q8107 and associated resistors are SMD on the lower (back) side of the PCB and I do not even know how to locate them. I need to be able to do testing on the upper side, on the larger components, to which I have access.Any help regarding the first and easy things to check?

author: kripton2035
author_rank: Super Contributor
timestamp: February 02, 2024, 08:27:40 pm
content: again, as said before, in a power supply, that does intermittent things, first things to check are cold soldiers, and cap's esr...

author: soldar
author_rank: Super Contributor
timestamp: February 02, 2024, 08:54:42 pm
content: Quote from: kripton2035 on February 02, 2024, 08:27:40 pmWell, I have already checked as far as I can. I did redo a few solder for through hole components just in case but the back side of the PCB is SMD with extremely narrow tracks and I do not have the equipment or practice to get in there. Same regarding ESR. I just have to trust the caps are performing as advertised.I think the only way forward for me is to try to deduce the cause or to just replace the entire PSU but I think that carries a problem because the ones I see for sale for under 50 euro are from junked TVs and there is a risk that they may fail too. An internet search reveals these boards fail a lot. I have not found how much a brand new and guaranteed board might cost but it is probably not worth the cost.I might do some temperature testing. I have found out in the past that temperature is often the cause and the solution to many problems. I cannot think of what else I can do.

author: kripton2035
author_rank: Super Contributor
timestamp: February 02, 2024, 09:26:13 pm
content: an esr meter may help you a lot in any electronic repair ... there are some easy to build yourself, see the link in my signatureor you could buy a cheap transistor multi tester that will also give you the esr of caps (but you need to desoldier them)also a thermal camera is an invaluable tool when it comes to electronic repair... but it's more expansive.

author: DavidAlfa
author_rank: Super Contributor
timestamp: February 02, 2024, 09:37:19 pm
content: Power off, connect DMM to the led terminals, power on.You might need several tries, it's very fast and the DMM might not be able to pick it in time.If you see a large voltage spike and then it drops to those ~31V, then it's the leds failing open, typical problem in LED TVs, you need new led strips.Sometimes you see the backlight flash because the voltage increases so much that the burned leds might conduct partially, but the overvoltage protetcion will quickly trip over.Not easy, you need to dismantle the panel with extreme care to not damage any of the ribbon cables or the panel itself. Last year I repaired plenty of them, there was a TV epidemic at workYou can use a power resistor there to bypass the OVP, a 5W resistor (For short runs) should give enough time to see if the LED voltage is good.Edit:Checking the schematic, the LED driver is a PF7903B, there's no proper datasheet, so no way of calculating the voltage based on the feedback divider...I only found this, in chinese but the translator does a good job, thought it brings nothing new.https://www.520101.com/html/haier/1154367432.htmlI found some info about the PF7909here(In spanish, and hidden, but can be seen in the site source code):QuoteIf any similar to the PF7903, then assuming a feedback voltage of 1.8V after the 164K / 7K5 divider gives a working voltage of 41V.

author: soldar
author_rank: Super Contributor
timestamp: February 02, 2024, 09:47:48 pm
content: I just had an idea. It seems the TV works fine when I have it on the workbench with the back cover off and it fails when I have it all covered up and put together again.I cannot imagine the plastic back cover would make any difference but it occurs to me that when the TV is mounted on the base / stand it probably is under very different mechanical stresses and I wonder if that might cause the PCB to be stressed mechanically.So I am thinking I am going to have the TV off the stand, just sitting on the shelf directly. If this solves the problem I can always leave it like that or I could not tighten all the screws that hold the PCB; tighten the two top ones and leave the rest a bit loose so that the PCB will not be subject to mechanical stresses.

author: DavidAlfa
author_rank: Super Contributor
timestamp: February 02, 2024, 10:15:09 pm
content: I've seen those leds f**ng around for weeks/months until they completely burn down. Sometimes works, sometimes not, or not working when cold, but suddenly start working after letring it warm up for 10 minutes...It might work after aligning the TV 10.5º east, 192mm from the furniture, and singing the secret unlock sequence below, but eventually it will fail completely.

author: soldar
author_rank: Super Contributor
timestamp: February 03, 2024, 08:54:22 am
content: Well then, I think it is probably not worth it for me to spend much more time and effort on this repair. For now it is working. When it finally fails I will just chuck it.

author: DavidAlfa
author_rank: Super Contributor
timestamp: February 03, 2024, 09:01:10 am
content: When it fails, you'll have nothing to lose, so I'm pretty sure you'll open it up!The leds are cheap:https://www.aliexpress.com/item/1005001952913250.htmlJust check some YouTube videos before opening it up.https://youtu.be/-GUodaLwEic

author: soldar
author_rank: Super Contributor
timestamp: February 05, 2024, 04:22:30 pm
content: Well, that lasted much less than I hoped. It failed in under 24 hours. No, I am not going to open it up again. At least not for now. I have too many stalled projects already. I am going to see if I can find anyone who will take it.

author: Messtechniker
author_rank: Frequent Contributor
timestamp: February 05, 2024, 06:23:02 pm
content: Sure that your Philips TV is running on the latest firmware?Sister had a Philips with a blank screen problem. Can't remember any specifics.Philips provided a firmware update. All is well since then.

author: soldar
author_rank: Super Contributor
timestamp: February 05, 2024, 09:03:33 pm
content: I am pretty sure it's a backlighting problem.

author: DavidAlfa
author_rank: Super Contributor
timestamp: February 05, 2024, 09:44:26 pm
content: Try the Freddy method.. Eeeeeooooo!!!Don't scrap it yet, you can fix it for just 20€... you won't buy another 47" TV for that money!Where are you from? I could help in the (un)lucky case we were anywhere close, I've replaced a lot of backlights.

author: soldar
author_rank: Super Contributor
timestamp: February 05, 2024, 10:32:55 pm
content: 20 euro, plus many hours and the risk of making things worse. Well, I suppose I can't make it worse since it is not working.A couple years ago one of my monitors was failing and I knew it was the PSU and I opened the back and repaired it easily. Changed the capacitors and I was done. Except when I was reassembling the screen, front bevel, etc my hand with a screwdriver slipped and broke the screen and that was then end of that repair and that monitor. What I am trying to say is that no matter how easy a repair appears to be there are always risks even if small.It is not really my TV. It belonged to a friend who gave it to me because he was getting rid of it. It is way too big for my tiny apartment so I have little to no interest in putting much more effort into this project. This TV is way too big for me to work comfortably in my tiny apartment. I was hoping to give it back to him if it was an easy repair but he already bought another one and says I can keep this one. If I managed to repair it the only use would be to sell it or give it away.For now I have parked it out of the way while I think what to do next. (Knowing myself what I will do next is, most probably, "nothing").Many of my repair projects take so long that they get to a point where they are not worth completing. Procrastination is a great way to save labor. But so frustrating ...

author: DavidAlfa
author_rank: Super Contributor
timestamp: February 06, 2024, 12:05:46 am
content: It only takes about 1-1.5h, not "many hours"!Fix it, sell it...

author: soldar
author_rank: Super Contributor
timestamp: February 07, 2024, 03:39:13 pm
content: 1.5 hrs in normal people's time is 6 hrs in my time. I would still give it a try if it were smaller but the thing is huge and I cannot disassemble it and have in the way. The way I like to do my projects is one day I carefully open the thing up, inspect it, plan a course and leave it there. Two hours right there. Another day I change the LEDS, another two hours. Another day I reassemble everything. All very relaxed. I do not like working under pressure. And, as I say, I have no use for it myself.And as for selling it, if there is something I hate is trying to sell used things. People are a pain. They stand you up, they will try to bring the price down, they will argue, etc. I hate it. And if I try to sell it as-is I do not think it would attract any buyers even though I am in Madrid. I think I would rather give it away or trash it.For now I think I will just put it aside and give it some thought. Diogenes is my middle name.

author: soldar
author_rank: Super Contributor
timestamp: March 05, 2024, 01:22:06 am
content: I just had an idea but I do not know if it makes sense. To recap: the led backlight stops working and we are assuming it is driven by a constant current PSU and some LEDS shorted and the voltage does not rise enough so the PSU shuts down. Right?So would it be possible to insert in series a resistor that would raise the voltage enough for the PSU to work?I mean, the LED backlight turns on briefly so it does work. Rather than replace it maybe I could do a shoddy repair by just inserting a resistor?I am even thinking I could try testing the LED backlight with an external PSU. IIRC it was something in the order of 35V and I could gradually raise the current until it looked good.What are the chances of that working?

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 05, 2024, 05:25:20 am
content: That won't work. It blinks briefly because the voltage goes too high.Adding a resistor will keep the voltage under control, but at the same time the damaged led strip has a much higher threshold voltage than normal, so it won't light up.Think it like a 9V zener that drifted to 12V, causing PSU error.After adding load resistors to keep the voltage at 10V, no current will pass through the zener.

author: soldar
author_rank: Super Contributor
timestamp: March 05, 2024, 03:05:49 pm
content: Wait. Let me think about this. The backlight power supply is a (switching) constant current source with upper and lower limits.I am assuming the LED strip has one or more LEDS that failed shorted ... because if they failed open the LEDs would not even blink. So the result is a strip with some shorted LEDs which will work at a lower voltage than the PSU will tolerate and that is why the PSU shuts down.If I put in series a resistor, or even some regular diodes, to bring the total voltage up again then the PSU will not complain and will work. Right?Aside: I am working on installing some shelves in my workroom and the room and bench are unusable for now but once it is all finished and usable again I am thinking I could just open the back of the tv set and connect the LED backlight connector to an external PSU limited in current and raise the voltage gradually and see what happens. I think somewhere before I get to 50 V the backlight will light. Then I could even provide an external PSU for the backlight totally separate from the TV set.In summary: If the backlight lights up for a moment it should be possible to keep it lit. Yes?

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 05, 2024, 05:59:30 pm
content: The problem is not the shorted leds, but then open ones.Every TV I fixed had several shorted leds, but often only one was the culprit, completely burned off, opening the circuit.It's not a perfect "cut", so the higher voltage might be able to partially conduct over the burned stuff.The backlight does basically like this:

author: soldar
author_rank: Super Contributor
timestamp: March 05, 2024, 07:55:13 pm
content: Well, I guess that dashes my hopes. Still, because the backlight is just a simple two wire, +/-, circuit, I might try pushing some current through it and see how it behaves. I am not quite ready yet to disassemble the whole thing and replace the backlights but it looks like it may be the only real solution..