TV died, suspect power board. Need help figuring out where to start

[jimmy85]

Hey fellas.

So my TV didn't want to turn on, it is a Philips 65OLED973, and since it is five years old, the service technitian I talked with recommended me scrapping it instead of fixing it.

As it is a nice TV, I would prefer giving it a try to repair it, so I opened the TV and took out the power board.

Finding the service manual for the TV and schematics for the powerboard is easy by searhing for 715G9106, the full partnumber is 715G9106-P0G-000-00G0
https://www.scribd.com/document/443143653/Philips-50PUS7303-TPM18-1E-LA-Service-Manual

Symptons:
-Does not turn on. Only sign of life is a flickering red light on the front. Note that it is faintly flickering and it seems like it would blink an error code (2 blinks maybe), but it is flickering heavily and low
-There is a faint, faint clicking sound coming from the power board-area.

So what I have found so far;
-Opened up and taken out the power board.
-I measure DC-voltage through the rectifiers, so the fuses are not blown.
-I have problems measuring any reasonable voltage. On the 12v output I measure around 8-9v, but nothing more noticable
-There is a very low ticking noise coming from somewhere on the board. I have located it near the small transformers, but can not guarantee that it is one of them that are making the noise.

Any idea where to start measuring and what I should be looking for? I see nothing obvious burned on the board, nor does it smell of burned electronics, so there might be an internal failure or short somewhere..

The ticking and flickering red light points me towards something unstable or changing. At one point I also manged to measure a voltage that constantly changed, but can't say where I measured it.

Adding some schematic pictures:
https://pasteboard.co/XhmaBPxDtLm7.png
https://pasteboard.co/ZtHKWCLZ3JuZ.png
https://pasteboard.co/LFQhENzKm63f.png
https://pasteboard.co/WkyKfwcrP1oR.png

Hopefull for any help 

[fzabkar]

What is the resistance between 12V and ground? Do any of the other outputs have a low resistance?

Check C9117 and C9151 (pin 2 of U9101).

[jimmy85]

Thank you for the pointers! I will measure the resistance between 12v and the capacitors you mention in a couple of hours from now!

Do you think it matters if the power board is disconnected from the rest of the TV, or should the measurements here be a good point when the board is not connected to anything else except the power outlet?

[jimmy85]

Allright, so I have done some measurements.

So please note that the board has been taken out of the TV, so it has no connected control board or load, if that could have an effect on the measurements.

Resistance:

Measured resistance between pin 1 and 13 on connector CN9101:
Difficult as it fluctuates a lot, but no dead short atleast.
After being disconnected from the mains for a good hour, it settles at ~510 Ohm

C9117 -
Positive lead on + : open
Negative lead on + : ~516 ohm

C9151 - 516 ohm
Positive lead on + : open
Negative lead on - : ~516 ohm

Voltage, measuring DC:

Measured voltage between pin 1 and 13 on connector CN9101:
Jumps around 8.5-8.9v. Should be 12v?

C9117 - Jumps around 12-14v
C9151 - Jumps around 12-14v

[MathWizard]

Usually there's a 5V standby rail, 1 of the little transformer type things and some chip or 2. The clicking could be relays, or the inductors when it tries to start up. Sounds like some chip is sensing a problem, and shutting down, or trying to restart every so often.

IDK what voltage rails are in this type of TV, but for instance in some LED TV I troubleshot, there was the 5V STBY, some low voltages like 3.3V for big processor type chips, then something like 12V as Vcc for control chips like buck regulator chip's. Then there was some step down and then step up DC-DC stages, 2-3 stages total, for the main rail for the LED's.

Look for the chip's near all the switching transformers, look up their datasheets, see if they get Vcc and if they are running, they might be being told to stay shut off.

If you're new to all this, watch out for dangerous voltages. Usually if I have to go working on a PSU, and time is no issue, I'll check a lot of stuff without power 1st. Then solder on some wires at key points, for easier and safer probe attachment. It's way too easy to short pins together on IC's when probing live.

[magic]

Relays are quite loud, while quiet ticking may be caused by transformer vibration due to the PSU turning on and off.

If there are no obvious shorts on the output, it may be something stupid like a small capacitor which stores charge for the PWM controller chip on the primary. Look up the controller, find its datasheet, find where the GND and VCC pins are and look for an electrolytic between them. If it's obviously bad, replace instantly. If not, connect a second one in parallel on the bottom side of the board and see if it helps. Then replace the old one if it does. Or, dunno, just replace it right away - it's a relatively common failure point.

Remember that primary "ground" is really the negative output of the rectifier, so it carries full mains voltage during negative half-cycles. Be very careful if measuring anything on the primary while plugged into power.

[fzabkar]

Here is the datasheet for U9101:

https://www.semicon.sanken-ele.co.jp/sk_content/ssc3s910_ds_en.pdf

The IC starts operating when Vcc rises to around 14V (typical) and stops when it drops below 9V or so. Even though C9151 and C9117 are quite large (100uF and 22uF), it might be worth testing them with an ESR meter.

[p.larner]

just measure the electrolytic caps for capacity but mostly there esr,if you cant just replace them,ps,measure them out of the pcb!.

[jimmy85]

I have now measured the pins on U9101 and found the following:
GND means Pin 10 on the chip.

Pin 1 - GND : 2.6v
Pin 2 - GND : jumping 12-14v
Pin 4 - GND : 5.3v
Pin 5 - GND: 2.8v
Pin 12 - GND: 10v
Pins 14/15/16 - GND : jumping 10 - 80v
Pin 18 - GND : 320v

The voltage over C9151 and C9117 also jumps between 12-14v

Any clues on where to continue the hunt?

[Psi]

For powersupplys.
Bad caps, Bad diodes and Dry joints are the first things I look for.
Also any large value resistors like 100k+ that look a bit discolored. Bootstrap resistors failing from the high temp at startup are common.

[thedoc298]

Did you already change/test all the caps on the ps..?

[Paceguy]

I agree with some others who have said that it could be filter caps on the power supply board. First look for bulging caps, measure the ESR of all the caps. Most of the sets that I have fixed had bad caps.

[MathWizard]

Do you have another TV to watch ?

If time is no concern, I'd start making a map of the circuit, at least around the chip. The typical circuit layout is in the datasheet.

With a magnifier of some sort looking at PCB's, it's like exploring another world sometimes.