Powers secondary standby standby logic circuits and USB power switches.
3. Netlist / Connectivity • All power pins of MCU connected to +3.3 V net. • UART_RX left floating – tied to GND via 47 kΩ (added TP‑UART_RX).
A resistor or capacitor labeled R102 in Rev 1.0 might have a different value or be removed entirely in Rev 2.0.
If the laptop suffered a liquid spill or a USB port short-circuit, the standby power controller IC often fails, getting boiling hot to the touch. 4. Step-by-Step Troubleshooting Checklist
Check the gate voltage of the first two isolation MOSFETs. If the gate voltage is missing or incorrect, check the feedback resistors and the VCC/ACIN pins of the charging controller IC. 4. Burnt +3VALW/+5VALW Regulator ICs lae791p rev 20 schematic diagram verified
It reads the basic input/output system firmware from the SPI Flash (BIOS) chip using the +3VALW rail.
: Signals ending in # (e.g., PLT_RST# ) are active-low signals.
Essential for locating physical components on the PCB, found at The Tech Stall .
Let’s apply the to real problems.
The is indispensable for any technician working on high-performance Dell laptops. By understanding the power sequence, tracing signals, and identifying components accurately, you can reduce repair time significantly.
A typical LA-E791P PDF schematic is divided into logical blocks that you should familiarize yourself with: A. Power Sequence (Input & Switching)
Solution: Use the schematic to identify the exact SPI Flash pins (VCC, CS#, HOLD#) to safely dump and re-flash the BIOS with a verified working dump. Blown +3VALW / +5VALW Regulator
Powers the Embedded Controller (EC/SIO) and the BIOS chip. +5VALW: Powers various secondary ICs and USB ports. Step 3: EC Initialization & Power Button Signal Powers secondary standby standby logic circuits and USB
The schematic defines specific voltage sequences essential for a successful power-on self-test (POST).
Common issues associated with this board revision include "No Display" scenarios caused by SOC or power management problems. If you are troubleshooting a dead board, focus your search on the Power Sequence
| Tool | Purpose | | :--- | :--- | | | For measuring voltage, resistance, and continuity. Essential for tracing shorts and verifying power delivery. | | Soldering Station | For removing and replacing small surface-mount components. Look for a temperature-controlled unit with a fine tip. | | BIOS Programmer (e.g., CH341A) | For reading/writing to the BIOS chip. This is crucial for fixing "no power" or "no display" issues caused by corrupted firmware. | | Boardview Software | OpenBoardView (free and open-source) is the most popular tool for opening .brd files. | | Documentation | A copy of the LA-E791P Rev 2.0 Schematic (PDF) and its corresponding Boardview File (.brd). |
Since many of these boards use USB-C for charging, the controller chip (TPS65982 or similar) frequently fails due to voltage spikes. 5. How to Use the Schematic for Troubleshooting Identify the Symptom: (e.g., No Power). Check Main Rail: Search for B+ or +19V in the PDF. • UART_RX left floating – tied to GND
The project name contains 57 pages, each dedicated to a subsystem like power, clocks, or I/O ports. It is often distributed with a BoardView file showing the physical location of each component.
The 5V output typically converts to a "run power" at specific IC pins (e.g., pins 8 and 9) after the system triggers a power-on state. Accessing the Schematic