Industrial SMPS sound

[additive]

Hello all,

I am trying to fix a SMPS from an industrial robot.
Long story short it has 3 x 24V outputs and 1 x 5V output, 24V outputs have MOSFETs at the outputs to turn them off when something is wrong and LEDs to indicate it.
This specific power supply had 24V problem (LED on and 24V outputs measured 0V), so I took it apart, tested all MOSFETs, diodes, some resistors, capacitors and opto-isolators also and could not find any problem. Only the fans needed a gentle push to start spinning.
I only replaced MIP0223SY chip, just in case (It seems to be responsible for generating 5V, 22V and some more voltages for the rest of the board).

Then I assembled it back together and everything seems to work again. No red LED turned on and all outputs measured ~24.4V.
Only problem is this sound that I can not figure out, it does not seem normal for me.

I read https://www.repairfaq.org/sam/smpsfaq.htm#smpsstsm   under "Sounds that SMPSs make" that it might be caused by improper loading, so I tried with 2 x 47 Ohm 11W series resistors at the output. It worked longer, but after ~3 hours of running the sound still returns.

When I probe the circuit with multimeter at the MOSFET, which gate is driven by NCP1563A (power factor correction chip), the sound will change.

Unfortunately I could not find any schematic for this power supply. If needed I can try to explain more from what I have understood after tracing the tracks.

Here is a short video:


Also some images:






Thanks for your time

[coromonadalix]

The sound change is because of the meter impedance  (supposed to be in the 10 megs range unless its an bench dmm who has giga ohms impedance say 0- 20vdc)

Be careful  when you probe, you may toast the smps while doing so, if you make some frequency changes or feedbacks changes while probing the smps may react badly.

[additive]

Thanks for the heads up coromonadalix!

I wrapped some wire around transformers and inductors today and performed measurements with oscilloscope.

CH1 is MOSFET gate driving transformer
CH4 is output transformer
CH3 is around output inductor


5V part:


24V part:


So yeah, this power supply seems far from fixed.
I also found out for sure, which transformer was making the noise. But it does not seem important anymore, because of the pictures above.

It is not worth searching any further I'm afraid.
Thanks for the attention guys!