Electronic – DVD player power supply circuit strange behavior. Need help to understand root cause

power supply

I have an old (~8 yrs) Cyberhome dirt cheap DVD player that worked well for the time I was using it. I bought it in 2004 from a Best Buy and was using it regularly till 2010. After we got a new DVD player, it had been lying around packed nicely and securely. I recently brought it out just to connect it to another TV and found that it doesn't power on. I opened it up and was able to isolate its power supply (the pic shows it was very easy to do so). It works from 110 V to 240 V AC input 50-60 Hz. I have been using it in India which has 240 v AC @ 50 Hz usually. The DC out of the power supply circuit is like so +5V-GND-GND-+12V. I have assumed that the +5V was for the decoding circuit while the +12V would have been used by the player motor and maybe also the Class 1 laser.
The +5V out works fine, but the +12V doesn't. The first time I checked it showed +17V and remained steady there. When I switched off the power, it very slowly (over 10-15 mts) ramped down to nearby 0 V. Based on this, I decided to wire up a voltage divider to bring 17 to 12 on a breadboard. Before I connected the +17V to the voltage divider, I checked again to make sure and now it showed ~+12V! It was slowly ramping up and 5 mts later was at +14.30V and has been holding steady there since last 10-15 mts. Switching off the power supply doesnt slowly ramp it down but goes down to 0 V pretty fast.

What is the reason for such behavior? Obviously, something has gone poof in the power supply circuit, and I am interested to know the root cause. Some pointers will definitely help and using those I may try to debug through the board to figure out more. (I plan to wire up the input of the system to +5V-GND-GND-+12V independent of this power supply board to check the player but that's later as I don't have a +12V source handy currently. I tried with +5V and the dvd player powers on fine – but without the +12 doesn't do much apart from that).

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Best Answer

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.)