low-powerpowerpower supply
I trying to understand what problem with next scheme.
I have a power supply which should provide +12V but in my case it's only about 5v in output (checked on 1-2-7-8 and Gnd pins). I have tried to change LNK304P to new one but without result. So seems that problem in harness. I assume that problem with PC101 but Now I prefer to be sure before replacing component. Please help to understand could it be problem?
Most likely there's nothing severely wrong with the power supply.
Your description leads me to believe that the power supply has its feedback on the +5V rail and having zero load on the +12V rail causes it to drift high. In the application (the DVD player) there's always load on the +12V rail which keeps the output closer to +12V.
High-pitch acoustic noise often comes from ceramic capacitors or the magnetic components. A faint noise is generally not a cause for concern - it was most likely always there but when it was enclosed you couldn't hear it.
That being said, it's possible that the power supply can no longer deliver its rated current, or the rest of the hardware is drawing too much current and forcing the power supply to protect itself. You need to check for short-circuits and dried-out electrolytic capacitors on both the power supply and in the rest of the DVD player itself, as well as probe any diodes and transistors you can find. ICs are tricky (especially if the unit is using some custom ASIC).
(Realistically, the way consumer electronics are designed these days, eight years is a remarkably-good run for a device that most likely only came with a 1-year warranty.)
You need to capacitively couple the inputs (try 3.3uF with the + to the chip)- other schematics also show 51K to ground.
I don't know why they don't show that on the reference design, maybe just to make the circuit look simpler. It's obviously required or you'll be outside the common-mode range of the input on a single-supply.
Best Answer
This is, in my opinion, a bizarre buck regulator. It's also rather unsafe because it's a non-isolated mains voltage regulator.
If you want to be sure that PC101 is your issue, I would measure PC101 pins 3 and 4 to check if it's shorted. If it is, then you know that's the issue (or at least the main issue). The other thing to check would be to setup a 10mA current between pins 1 and 2 and measure the resistance on pins 3 and 4. If the resistance drops when you power the opto-coupler, then you know it's still working.
If that turns out to be fine, I would check all of the diodes to make sure they're operating as expected. Use a diode checker on them. They should show an open circuit in one direction and virtually short for the other direction. For the zener's you'll also want to test them for their reverse voltage level. To test that, you need to put a voltage supply with a current limiting resistor on them and voltage probe the diode contacts. You should see approximately the reverse breakdown voltage.