Laptop power supply failure. Whistle and heat up. What is the problem?

Question

Do it yourself

Hello.
There is an Acer laptop charger. Standard 19v 3.42A.
When connected, it starts charging and after 10-30 seconds it stops charging, and the battery and laptop power indicators blink twice.
Having disassembled, I suggested that it was cut down due to overheating. Since the field effect transistor is very hot.
When connected to 220ac and the plug to the laptop, "DZZZZ" is heard, almost a whistle but not quite an accurate whistle.
At the exit at idle and under load 19v.
There is no short circuit to the cable.
Tried on another laptop, same results.
Changed all electrolytic and ceramic capacitors.
Changed field effect transistor.
Checked all resistors.
There are two more transistors, I checked for breakdown - negative, diodes too. (without load)
I applied thermal paste between the radiator and field worker.
In appearance, the tracks are intact, there are no mechanical damages, I looked under a magnifying glass.
The result is the same.
When connected, a "whistling" begins, and the field-effect transistor heats up (the thermocouple showed ~ 85 degrees when it is cut down). The power and charge lights on the laptop blink twice and stop charging.
How to treat?
I suspect something with the transformer? I'm not much of an artist.
I did not touch the microcircuit and the optocoupler.

Answer 1

The magnetic permeability of the magnetic circuit of a pulse transformer decreases over time, which, if not calculated correctly during design, leads to "planned failure". One of the symptoms is increased heat generation. If this is the case, then rewinding the transformer will help, taking into account the new parameters.

Answer 2

Repairing a switching power supply without an oscilloscope and experience, by replacing parts at random, is a waste of time and money.
Replace the entire block.

Answer 3

It is better to gain experience on specialized forums of iron repairmen, for example www.espec.ws or www.rom.by