Philips 288P6LJEB/27 monitor PSU outputting 8V instead of 12V

[logocar3]

Hello,

I am having problem with fixing Philips 288P6LJEB/27 monitor PSU, board number 715G6457-P01-000-0010.

Data:
- Schematic is attached to post
- Photos can be found on flicker: https://www.flickr.com/photos/198547898@N03

Problem:
- PSU board have power outputs at 5.2V, 12V and 24V - board is correctly outputting only 5.2V (correct), and not 12V (without load only 8.16V) or 24V (without load 0V)

State of PCB / monitor:
- Bought defect monitor from a company auction
- There were some attempts to fix the monitor, as I saw a changed capacitor and screwdriver marks on the enclosure.

Troubleshooting steps:
- There is no short to ground from 12V (approx 3kOhm) or 24V (approx 3kOhm) - I guess there is no problem on the low voltage side
- Vsin after full bridge rectifier looks OK (325V DC with DMM) - I guess nothing wrong until after full bridge, marker Vsin in schematics
- Some voltage regulation works:
  - Voltage on 12V rail says approx the same after applying 230Ohm resistor to ground (8.16V before, 8.10V after)
  - Checked all diodes with DMM (all diode voltages looked OK), checked all capacitors for shorts, checked all resistors (all resistance looked OK). I am not 100% sure I did not miss some OR I measured wrongly.

Troubleshooting actions:
- Replaced STF24NM60N Q9801, as it looked damaged (nothing changed)
- Replaced C9807 (nothing changed)

First guess for the problem:
- problem in current sensing circuit for SSC1S311 IC (VCC marker in datasheet - without load DC=13V, AC=0.26V, with load DC=16V AC=0.012).

While I have some knowledge about electronics, I don't know how Quasi-Resonant Off-Line Switching Controllers (SSC1S311) work, so got to the knowledge wall I can not pass, as I can not pinpoint which part of the circuit is faulty.

Any ideas about what could be wrong - feel free to share, because I am out of them:)

Thank you in advance for all your answers,
Gregor

[m k]

Page 3/5, low right,
5.2V is independent and sourced from 12VO.
Same page, top center,
so 12VO must be ok and after that the route is pretty straight to 12V.

[MathWizard]

So it sounds like the PFC/boost converter stage is working, and then there's problems. Do you have an oscilloscope ?

That controller chip SSC1S311, senses a few things like over/under voltage. Is it getting it's standby/startup power ? When it's running properly, it gets it's Vcc power from the smaller winding of T9101. I'd start checking there, then see if any of the dual purpose pins are being held low or high, to mean a fault.

And of course see if the gate drv is trying to run or not, and what sort of voltage is on the opto, it could be something half burnt out in the feedback path. Or even all the output caps are worn out, and just can't hold a charge.

You'd really want an oscilloscope to see stuff, a DMM will just see the low frequency average.  Even of of the $20 ebay scopes would see if the gate drive is really running or not.

Here's a link for anyone to the SSC1S311 chip
https://datasheetspdf.com/pdf-file/837887/Sanken/SSC1S311/1

See what's on pin 2, the FB from the optocoupler, and make sure the opto reads like a 1V diode on 1 side, and a closed Collector/Emitter on other.



Some problems, should make the chip turn off,/back into stby mode. In which case there wouldn't be anything on the outputs.


I have a 750W PSU I'm working on, but I have a hand injury, and so I can't really go soldering or probing stuff yet. I never found a single problem with the PSU unplugged, so IDK what to expect. But I made a full schematic of it already.