Repairing Power supply won't turn on (with "Sense" line ?)

[Mozee]

Hello ,

I have this audio amplifier power supply, It outputs different voltage and has "Power on" line and "Sense" line as shown in the attached picture,
Now the 3.3 and 5 volt rails are not providing any power my guess is that it is due to the "sense" and "power on" lines , I have little knowledge about power supplies so help will be appreciated here. I want to know how does this know of power supplies sense the need to turn itself on,
It reminds me of the ATX power supplies (the desktop computers PS) where we had to add a resistor between the white wire and ground (can't remember exactly how!).

Thanks in advance


Edit:
I uploaded new Images of the PCB, Here you go:
https://ibb.co/mgRaBJ
https://ibb.co/g0Zj5d

[LateLesley]

The P_ON/OFF will probably need jumped to ground to power it up. I'm not suggesting you do this at this point though. The sense wire will be a feedback wire to the regulating circuit to set the output voltage.

I suspect that there will be more components underneath the board. Any chance of taking the board out of the plastic carrier, and getting a photo of underneath? I think it needs a once over before it's powered up.

And remember to short the caps before taking it out, especially on the primary side, so you don't get any nasty surprises. (All with mains removed of course, we don't want it powered yet!)

[lordvader88]

Troubleshooting & Repairing Switch Mode Power Supplies
https://readerspage.files.wordpress.com/2016/07/smpsrepairebook.pdf

[Jwillis]

The power on is just that it switches the unit on.The sense line is used to sense current requirements.When it senses the requirements are low the supply will go to low out put state  for more efficiency.
When the the unit senses a higher requirement it boosts the power as required. This is done through a power management chip or it may be in the PWM it self or can be done with a FET or transistor by detecting a load.  Best to find the datasheets for the individual chips to see what they do.I can see three opto couplers but can make out the 8 pin .Likely the PWM but not sure. And  schematic might be helpful.If you can trace where the sense line leads that may be of help to figure the circuit out. A clear image of the bottom  traces and description of the input output lines there might help as well.
Switch mode supplies can be a little daunting but not impossible to understand.

edit // sorry can't make out 8 pin

[Mozee]

Thank you all , I added new Images of the PCB's back! Couldn't attach them directly due to size limit 

[LateLesley]

Ah ha! And someone's been there before! Can see their handiwork. We need to check out SMIC45, it looks like it has some history. Can we get the number off that component?

[Mozee]

Ah ha! And someone's been there before! Can see their handiwork. We need to check out SMIC45, it looks like it has some history. Can we get the number off that component?

Hmmm, Which one do you me? Which component? sorry for my stupid question 

[Jwillis]

wait sec.Pin 15 (-VP) on the connector. What is that ? Is that a termination or a broken/burned off wire?

[LateLesley]

Nice spot Jwillis. Looks like the wires for pin 14,15 are chopped. Looks intentional.

@OP I mean the black transistor-looking thing, next to 3 electrolytics, nearest the power connectors. If you look at the black writing on the board, you'll see all the components have labels SMC for capacitors, SMR for resistors, SMD for diodes, etc, then a number.  The transistors/FETs/ICs seem to be labelled SMIC then a number. the numbers help identify which component it is.