LCD TV Power Supply Board question

[Crystaleyes]

First off,

Happy New Year everyone!



Someone recently asked me to look at their Philips 42pfl4508g/78 TV which had stopped working.

A quick visual check of the PSU revealed a ceramic cap with a hole burnt through it, and that the output voltages were wonky. The 5.2v Standby was only 3.2v. The 24v - 19.1v and the 12v - 11.2v. We found a cheap 'guaranteed working' replacement PSU online, which they bought and installed but it still the TV didn't switch on, and when I went back to test the new PSU board, it showed the same kind of low output voltages.

Anyway, I took the original (715G5778-P02-000-002M) board home with me to change the damaged cap and to see if that would resolve the low output voltages (which it didn't ) and have been slowly checking components whenever I've had a free moment.
So far, I've checked all the TO-220 Fets and diodes, all the electrolytics for values and ESR, all the 1W and 3W low value resistors and various other transistors, diodes and resistors for shorts or open circuit.


What has become obvious is that I need a deeper understanding of 'how' these boards function.


For example, I had been under the impression that when connected to the mains supply, that the only DC voltage present in the secondary section should be the Stand-by, which when connected to the Stand-by pin, switched on the other voltages, in this case 24v and 12v.
We are all used to seeing (and hearing) a relay used in the Stand-by circuits, this board however has no relays, so I imagine that the Stand-by is activating the controller IC?
More importantly, the 24 and 12 volt rails are permanently 'on' once the mains is connected.

Surely this isn't correct?


Attaching the service pages if anyone is interested?

[lotas]

Do you have PFC (Power Factor Correction) working? What is the voltage on the power capacitors in operating mode? It should be about 400V.

If you are testing a power supply and checking the voltage on the table (without the TV itself) at the 5.2V output, you need to load the output on it, that is, hang
at the output is a car incandescent lamp, a resistor...

[Crystaleyes]

The voltage on the filter caps is 360vdc across the bridge rectifier is 140vdc - The mains AC supply here is 127v.

I'll stick a resistor across and see what happens...

[Diah]

this TV need its PSU always active. because some app and function need to be Online connected... its early system after Philips changed to Android and the PSU remain the same too always active. only BL could be turn off and 24V, this one reason the ceramic cap exploded .

you said you have trusted replacement PSU which installed but TV didn't work..and have same issue... then you need the MB taking it to home with PSU. to check the PS_ON DC to DC on MB if its had damage

[Crystaleyes]

In the end, I put a 12v bulb onto the 12v output and sure enough, once mains connected, the lamp lit up.
However, the 24v output dropped to 18.1v.

Surely this can't be correct?


Also, when the PS/On pin is bridged to the Stand-by pin, then the 12v lamp goes off.

Surely this is wrong? It's like the stand-by circuit is connected in reverse. When you close the circuit, the TV goes off, open the circuit and it turns on.... ?????


As for when I said 'trusted replacement', that is only what the seller stated. This is Brazil. You have to think 'Nigeria' in terms of blatantly shameless lies and scams. That said, I do actually have the main board here with me although I didn't bring the connecting cables, but I can have a look for sure.


Back to the PSU, one thing that I noticed is that on the Drain pin(s) of IC9301, the voltage is fluctuating. I had been under the impression that at this point the voltage should be over 300v and stable? This Drain is obviously connected directly to transformer T9301, pin 3, yet pin 1 has the stable 360v.
I can't help but suspect a fault in IC9301 but being some kinda PWM controller/ FET, I have no idea how to test it, so maybe I'll remove IC9301 and see if the Drain trace voltage goes up at all?

[Crystaleyes]

this TV need its PSU always active. because some app and function need to be Online connected... its early system after Philips changed to Android and the PSU remain the same too always active. only BL could be turn off and 24V, this one reason the ceramic cap exploded .

you said you have trusted replacement PSU which installed but TV didn't work..and have same issue... then you need the MB taking it to home with PSU. to check the PS_ON DC to DC on MB if its had damage

The exploded cap was C9877.

Looking over the main board, nothing 'looks' out of place and all the electrolytics have been checked for high ESR. The one thing that is clear is that the BGA chip has been running hot and clearly scorched the board through to the other side. Nothing extreme, but the heating is clear.

I'm still far from convinced that the PSU board is working properly.

[Crystaleyes]

A quick update.

Found a video of some tech with the exact same TV and identical voltages on the PSU output.

He also noticed that the BGA chip on the main board had overheated so switched the main board and the TV came back to life. So, have ordered a new board and I'll confirm the result when it arrives.


👍