Philips 288P6LJEB/27 monitor PSU, the problem with primary side

[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 is 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) - 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 multimeter (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.

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

First guess for problem:
- problem in current sensing circuit for SSC1S311 IC (VCC marker 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 konowledge wall I can not pass, as I can not pin point which part of the circuit is faulty.

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

Thank you in advance for all answers,
Gregor

 

[Mkop]

I had a similar problem, after replacing the 50v 47uF capacitor, the behavior is the same, under load it seems to go into protection without switching on. I didn't change anything except the capacitor, but I noticed that it looked like garbage between the legs of the STF24NM60N transistor, but I assume that these are single crystal structures that grew out of the solder, after removing them and tinning the legs of the transistor, the unit turned on, everything works stably. As a result, only one capacitor is damaged and the legs of all STF24NM60N are pierced. https://flic.kr/ps/46n1By