vizio tv repair - 19.5V instead of 12V?

[mouserat]

Was gifted a broken 65" vizio tv, seemed that the backlight had failed. While first trying to explore/understand the TV, observed a few things:

* 4 main PCBs: PSU, 2x identical backlight drivers (?), logic board
* both backlight driver circuits coming off the PSU are outputting 10V, which is correct according to the silkscreen.
* there's no standby LED that i can find -- or possibly there is and I can't find it or it doesn't work
* the power button on the tv has no observable effect
* the logic board powers up, and responds (based on powering the logic board with a bench power supply and watching the current draw) to what looks like an on-PCB reset button

Ideally, I'd like to repair the TV but other than "don't be an idiot the high side mains voltage can hurt or kill you. the caps stay charged even after you unplug it. they will hurt too" I'm honestly not sure what I'm doing.

My immediate problem (maybe) is the silkscreen says is supposed to be a 12V rail from the PSU to the logic board (at the logic board connector) is outputting ~19.5V. Trying to follow it back from the header, the relevant transformer is outputting at the same 19.5V. I don't know if the transformer is supposed to be doing that, and if so, where along the way the power is supposed to get dropped down to 12V.

None of the capacitors appear to have any damage or bulging (don't busted caps usually cause a voltage drop?), and nothing on the PSU board seems to visually be amiss.

Passives aside, there is an IC AS78LXX (labeled G78F) that the datasheet says is a "100mA POSITIVE VOLTAGE REGULATOR" - but 100mA seems way too low to drive the entire logic board. It might instead be there to power a nearby SP6018E mosfet driver(?). This IC might in turn be driving a K20A20D mosfet?

I'm not sure if the 19.5V is a problem I need to be chasing, or if I'm wasting my time. Images of the PSU pcb attached for reference.

This project is probably (certainly) beyond my skill level, but any suggestions would be appreciated.

[fzabkar]

Circuit diagram for PSU (715G7732):

https://elektrotanya.com/715g7732-p01-000-0030_philips_psu.pdf/download.html

[mouserat]

Thanks for the circuit diagram. No idea how you found it, but thanks!

It's definitely very close to my board, maybe close enough? The logic board power header is slightly different. My board has:

* only one of CN9101/CN9102
* on CN9101 only pins 12V, 12V_A, GND, and PS_ON (no DIM, ON/OFF, 3D, or +5V. All of these pins are NC)
* all of the 12V and 12V_A lines are connected together - or at least, the multimeter says there's continuity between them. On the schematic, they look like two separate but completely equal (go through the exact same value inductor and capacitor) rails fed by Q9104. Not sure why they're separate or why one is called "_A"

The schematic says the transformer (T9101) is supposed to be putting out 12V. I don't know if it's the subtle differences from the schematic to the board, or something is broken. Separately the schematic says the backlight driver is supposed to be 24V, but on this board the silkscreen (and the output) is 10V which is what makes me question.

[fzabkar]

What are the values of R9120, R9115, R9116 and R9128 on your PCB? These will determine the intended output voltage on the "12V" rail.

The values in the circuit diagram result in 9.0V on 12VS when DV5 is low, and 12.4V on 12VS when DV5 is high.

[mouserat]

R9120 - measures unreliable? the longer it is probed, the higher it goes. Starts off around 2K but very quickly climbs through 5K and beyond. marked 2.2k. I guess this is because something else on the board is throwing off the measurement?

R9115 - 2.7K (marked 3k)
R9116 - 178 (marked 180)
R9128 - 13K (marked 13k)

If the transformer is putting out 19.5V, the combination of R9120, R9115, R9116 should make 12VS 11.5V when DV5 is high? Showing my work:

12VS * (R9115+R9116) / (R9120 + R9115 + R9116)

19.5V * (3000+180) / (2200 + 3000 + 180)

11.5V seems too low to be usable.

Having a hard time understanding how to read the schematic if DV5 is low. Truth be told, I'm having a difficult time understanding the schematic more generally. Somewhere in this sort of sub-circuit, there's an input from the transformer at 19.5V and somewhere there's an eventual output of 11.5-12.4 after going through these resistors?

I assume DV5 switches the MOSFET Q9105, which changes what resistors the current flows through, changing the voltage of 12VS.

Sidenote: I think the schematic symbol for Q9105 is wrong? It shows an n-channel depletion MOSFET, but 2N7002's symbol is an enhancement. Maybe it doesn't matter.

Sorry for being an idiot, really am trying to understand what I'm looking at.

[fzabkar]

I'm assuming that R9120 is 22K, not 2.2K.

The REF pin of U9104 is set at 2.5V.

https://www.diodes.com/assets/Datasheets/AS431.pdf

Therefore I calculate an output voltage of 19.8V. Otherwise I only get 4.2V if R9120 is 2.2K.

Code: [Select]

V = (R9120 + R9115 + R9116) /  (R9115 + R9116) x 2.5
  = (22 + 3.0 + 0.18) / (3.0 + 0.18) x 2.5
  = 19.8V

R9128 (13K) in parallel with R9115 + R9116 (3.18K) gives an equivalent resistance of ...

Code: [Select]

13 x 3.18 / (13 + 3.18) = 2.555K

So, when DV5 is high ...

Code: [Select]

V = (22 + 2.555) / 2.555 x 2.5 = 24.03V