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

 

[Nigel Goodwin]

Have you checked the 12V rail with an oscilloscope?, is it pulsing? - it's also a good idea to check the input to the 12V rectifiers (D9106), as it might be more obvious there, than on the DC side.

You say the 24V rail is 0V, where did you check it?. You should check on the output of D9105, as later on it's switched by Q9107.

Are there any obvious signs of faulty electrolytics?, such as bulging or leaking ones?, and do you have an ESR meter to test them? - although if you have a scope (and as it's running) you can check for ripple on them with the scope.

 

[logocar3]

Thank you very much for the answers!

Have you checked the 12V rail with an oscilloscope?, is it pulsing? - it's also a good idea to check the input to the 12V rectifiers (D9106), as it might be more obvious there, than on the DC side.

With or without the load?

You say the 24V rail is 0V, where did you check it?. You should check on the output of D9105, as later on it's switched by Q9107.

At the connector. I will measure it at D9105 when I get home.

Are there any obvious signs of faulty electrolytics?, such as bulging or leaking ones?, and do you have an ESR meter to test them? - although if you have a scope (and as it's running) you can check for ripple on them with the scope.

No ESR meter, but I have a scope. How much ripple is too much?

 

[Nigel Goodwin]

Thank you very much for the answers!



With or without the load?



At the connector. I will measure it at D9105 when I get home.



No ESR meter, but I have a scope. How much ripple is too much?

If the caps have gone high ESR it's blindingly obvious - instead of a few mV or tens of millivolts, it will be 12V or so on the 12V rail. You need to check on the output of the rectifiers - +12VO and +24VS on the diagram.

But I suspect, as there's only 8V on the 12V rail, that the PSU is pulsing - best shown on the input of the rectifiers, where with a low timebase speed you will should be able to see the pulses starting and stopping.

 

[logocar3]

So the measurments

DC voltage on C9108 (+24VS) is 17.34V
DC voltage on C9135 (+12VO) is 8.2V

I can see pulses on both, 12V and 24V lanes (only one picture, and the second one is similar).

Now the for question - where do you think the error can it be?

P.s.: I found the thread in polish, where they were debugging the same board with same 8V on 12V rail, maybe it will help: https://www.elektroda.pl/rtvforum/topic3665506.html

 

[Nigel Goodwin]

Turn the gain down, and connect the scope to the input of the rectifiers - and slow the timebase down a lot, and see if the pulses are starting and stopping. Why does it say 30.5320KHz at the bottom of your scope?.

 

[logocar3]

Just for info: measurements on scope were done with AC mode.

Input of the rectifiers: you mean look at voltage between ground on low-voltage side and 1 or 3 pin of rectifier D9106 / SBL20U150F?

OK I will slow the timebase and repeat the measurement with the scope when I will have time.

For the frequency 30.5320kHz - no clue, it was cheap scope, I will investigate.

 

[logocar3]

Btw - thanks a lot for helping me!

 

[logocar3]

The pulses do not stop (the blank space in the middle is a problem with scope, not saturation)

 

[Nigel Goodwin]

Turn the gain way down - 2V per division, and drop the timebase slower as well.