After all this years it broke its fan system.
It's summer, and if I want it last other 20 years, I had to fix it somehow.
The very first thing to do is understand how fan subsystem works.
Not the hardest task in the world, you just need to follow the tracks on the PCB from fan backwards to components.
HP4410D: the MOSFET used to command the power 5V circuit
TC648: the fan speed PWM controller.
One of those twos must be the broken one.
Roughly, the fan subsystem should look something like:
Both devices are packaged into a SOIC8 package, people skilled more than I am, are able to remove this package without damage the PCB nor the component using just iron. About me, I'm not that good and I had to use my cheap rework station.
In my case it was 2.4V which is a resonable.
With TC648 removed, the base of the mosfet is exposed to manual drive, I could therefore short Vout to Vdd and GND and observe fan spin at maximum rate, and fan to stop when Vout is put to GND.
This leaded me to the conclusion that the MOSFET HP4410D had to be ok.
As counterproof, test the TC648 was a little more difficult since its package is not breadboard friendly.
I hope someone else could found this article useful.