Let me try to get up to speed here.....

"Now, after having replaces all capacitors, voltages oscillate, without
going over default values... i.e. 5V oscillates between 1 and 4V, 12V
between 6 an 8. 3844 Vcc value is under the minimum value. "

This is not resolved correct ? In that case likely it is the Vcc or Vdd filter at the chopper IC. The big IC by the transformer. If you can't get the pinout you can identify that pin thus; the transformer has two windings on the hot side. One the the output of the chopper and drives the whole shebang. Another is a secondary which supplies the lower source to the IC. This voltage is initially supplieed by a resistor from the rectified AC, or even AC from one side of the bridge, depending, but it still has a resistor and there will also be a diode from that winding going to the same IC pin. The resistor just starts it, but does not supply enough current to run. The cap must hold the voltage until the thing gets started up and supplies itself. The reason it would rune with the secondary filter bad is that there was less inrush current drawn and thus a short on time for the output device.

That is IF I read it right. The reason the negative supplies drop like a rock is probably alot less filtering on them.

"but have only a doubt: can I substitute the LM340T12 with a
MC7812? "

On negative supplies they are 79XX series, 78XX are positive. The XX is the output voltage.