Philips 47" TV 47PFH4109/88 repair

[soldar]

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?

[inse]

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)?

[soldar]

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)?

Thanks. 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.

[kripton2035]

schematic of the power supply is here : https://www.abcelectronique.com/forum/showthread.php?t=101188
it seems it could be a bulging capacitors problem.

[soldar]

schematic of the power supply is here : https://www.abcelectronique.com/forum/showthread.php?t=101188
it seems it could be a bulging capacitors problem.

As 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.

[soldar]

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.

[kripton2035]

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...

[inse]

We need to find out whether it’s the power supply, the LED control or the LEDs itself

[soldar]

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.

[kripton2035]

what about this schematic ?
https://elektrotanya.com/philips_715g6163-p0f_psu.pdf/download.html

[soldar]

what about this schematic ?
https://elektrotanya.com/philips_715g6163-p0f_psu.pdf/download.html

While 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?

[kripton2035]

again, as said before, in a power supply, that does intermittent things, first things to check are cold soldiers, and cap's esr...

[soldar]

again, as said before, in a power supply, that does intermittent things, first things to check are cold soldiers, and cap's esr...

Well, 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.

[kripton2035]

an esr meter may help you a lot in any electronic repair ... there are some easy to build yourself, see the link in my signature
or 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.

[DavidAlfa]

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 work 

You 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.html

I found some info about the PF7909 here (In spanish, and hidden, but can be seen in the site source code):

The protection is triggered below 1V (Short circuit) and over 2.5V (Open circuit).

If any similar to the PF7903, then assuming a feedback voltage of 1.8V after the 164K / 7K5 divider gives a working voltage of 41V.

[soldar]

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.

[DavidAlfa]

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.

[soldar]

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.

[DavidAlfa]

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.html

Just check some YouTube videos before opening it up.
https://youtu.be/-GUodaLwEic

[soldar]

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.

[Messtechniker]

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.

[soldar]

I am pretty sure it's a backlighting problem.

[DavidAlfa]

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.

[soldar]

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 ...

[DavidAlfa]

It only takes about 1-1.5h, not "many hours"!
Fix it, sell it...