Lenovo E40 laptop screen darkening repair

Machine model: Lenovo E40

Malfunction: The computer screen is dimmed.

Maintenance process: the screen failure of the replacement test machine remains the same, and it is found that the motherboard has been filled with liquids such as drinks during disassembly inspection. Open the drawing. The backlight EN on the drawing is 42PIN, and the actual measurement is only 1.2V V. Obviously, the backlight on the screen can’t be turned on because VCC is not enough, and normally it can only be turned on at 3 V. Continue to find the next working conditions.

Lenovo E40 laptop screen darkening repair Figure 1.

Lenovo E40 laptop screen darkening repair Figure 2.

Lenovo E40 laptop screen darkening repair Figure 3.

Analysis of the working principle of the above circuit:EN PANEL_BKEN, which is backlit by the graphics card after working, is at 3V high level, which is applied to the 3PIN collector of Q17. Because no VCC of 2PIN B pole is at low level, and B pole is not higher than 0.7V,C of E pole, C pole and E pole are in off state, so that 3V signal VCC is applied to the positive poles of D7 and D8, and Lid 51 # is passed.Turn on the backlight after the signal passes D7.

If the backlight EN has VCC above 3V, one: LID551# sleep signal can’t be low and can’t be pulled low. Second, there must be a backlight EN from the graphics card with a high level of 3V. Third, LCD_BK_OFF can’t be high, and it can work when the above three conditions are met. Actually measured LID551# signal is 3V normal, LCD_BK_OFF backlight off signal is 0v, and 0V,PANEL_BKEN backlight EN is 3V normal.Then why does it become 1.2V at the back of DISPON? There are two possibilities. The first one is caused by Capacitor short-circuit current, but the Capacitor mark on the drawing is not installed, which excludes this possibility. The second one is caused by the damage of Q17 transistor.

Lenovo E40 laptop screen darkening repair Figure 4.

After measuring Q17 C to E, there are more than 400 diode value, and conversely, there are also more than 400 diode value. After replacing an NPN transistor of the same type, the fault can be repaired.