Several SMPS use a topology that employs an optocoupler to close the control loop for the PWM controller that drives the primary of the ferrite transformer.
However, I'm not sure and I can't find information of what should be the typical voltage in the feedback pin of these controllers.
The feedback pin, also called COMP in some controllers, is grounded by the optocoupler when it's conducing, but I don't see any pull up resistor in them (on the outside).
And I have this TV PSU (A 715G6677) which is not functioning and have a wildly oscillating DC voltage everywhere where it should have a stable 12 V in the "standby" circuit.
It's oscillating form 9 to 16 V while open, and falls to 4 to 7 volts if I put a 200 ohm resistor on the output.
This "standby" circuit also powers the TV motherboard, so it has a reasonable power, just not if compared to the circuit that powers the LEDs.
It uses a LD5532 controller and this is the typical application shown in its datasheet:
On my PSU, the circuit is a bit different but has the same basic topology, just a bit more convoluted than this one.
The points where I should read 12 V that I mentioned before would be equivalent to the positive of the DC output of this scheme here.
The voltage on the VCC pin of the LD5532 (which is fed by a separate winding) is also oscillating wildly, the transformer is not being properly driven and it isn't producing enough voltage (on that separate winding) to power the chip, it's seemingly being powered only by that resistor that comes from the AC mains.
And, what's more, the voltage on the COMP pin is steady at zero like it was grounded.
I removed the optocoupler and tested it, it seems to be operating normally.
I don't know if this reading is normal and I can't find any reference for what voltage should be seen in there, and It seems weird to me that it's steady at zero while the output is oscillating wildly.
Is this normal or should there be something wrong with the controller?
If you want to check it, here's the schematics for my PSU, check on the A03 circuit (standby). The AC Input and PFC are working perfectly as far as I could test.