Notebook power supply modification

Modification of IBM notebook power supply

How can I modify the output of power supply from 16.8 to 12 volt?Most of the switched mode supplies are normally provide with a reference voltage to regulate the output, which is the heart of the device.

Identify the voltage divider:

First I opened the case and looked for the feedback circuit and voltage divider.

Then, I tried to locate the two resistors that are responsible for setting  the output voltage.

The stabilization of the output voltage can be archived in the same way as TL431.
TL431  will be used in chip switching mode supplies and you can find many documents about its functionality in internet.

Vo=Vref*(1+R1/R2)

The location these two resistors can be found near the opt-coupler.

Only R1 is interesting for me. I can simply add an additional resistor in parallel to it and reduce the total value.The resistor Rx51 on board as you see in the picture can be a good candidate. I measure the value and it was 976 ohm .
Measure the voltage at the opt-coupler was about 1.9V

On the left side of the board I found a tiny circuit board, which is shown in the picture. I assume its functionality is the same as TL431.Additionally, on the tiny broad I found the resistor R62 with 1.9 K in series to RX51.

I soldered parallel to R62 (1.9K) a new 6.8K resistor.
The result was 1.4K and gave me an output of 11.8 Volt and max 3.4 Amper

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