HP Pavilion 15 (hp 15bw504ax) no display repair - LA-E841P main board

In this repair case, a customer brought an HP Pavilion 15 laptop that was completely dead with no power indication. The laptop did not respond when the power button was pressed, and there were no charging or status LEDs.

After opening the laptop and removing the bottom cover, I began troubleshooting by checking the main power input connector on the motherboard.

When the power adapter was connected, the adapter initially supplied 19V, but the voltage immediately dropped to 0V. This behavior typically indicates that the main 19V rail is shorted on the motherboard. When such a short circuit exists, the adapter’s protection mechanism shuts down the output to prevent damage.

Diagnosing the Short Circuit

To locate the source of the short, I performed a standard motherboard diagnostic procedure by measuring the resistance between ground and the main power inductors.

During this test, one of the inductors showed a direct short to ground. Further investigation revealed that this inductor belongs to the power rail supplying the graphics processor (GPU).

This finding indicated that the graphics chip power rail was shorted, which is a common failure in laptops that use dedicated GPUs.

The diagnostic process confirmed that the laptop’s 19V rail was collapsing due to a short circuit in the graphics power supply line. In many cases, this condition is caused by a faulty GPU or a shorted capacitor in the GPU power circuit.

To confirm the source of the short circuit, I removed the power inductor PLV1, which supplies power to the graphics section of the motherboard. After removing this inductor, the short circuit disappeared and the 19V main supply rail stabilized, confirming that the fault existed on the downstream side of the GPU power circuit.

Although the input supply was now stable, the motherboard still did not power on. This indicated that additional components in the graphics power stage could be damaged.

The next step was to examine the MOSFETs controlling the GPU power rail, identified as PQV1 and PQV2 in the motherboard schematic.

After measuring these components with a multimeter, PQV2 appeared to be functioning normally with no abnormal resistance readings.

PQV1, however, showed a short circuit between the drain and source terminals, which is a clear indication of MOSFET failure.

A shorted MOSFET in a switching regulator stage can prevent the power rail from operating correctly and may also cause the upstream power rail to collapse.

To proceed with the repair, I attempted to remove the faulty MOSFET PQV1 using a hot air rework station. However, the removal process was unsuccessful. Because MOSFET drain pad was welded to the copper layer due to the heat during laptop operation. therefore i had to cut down the copper layer to remove the shorted MOSFET.

After referring the schematic of the board, i realized that converting discrete graphics (dis) to UMA may solve the problem and bring board back to life.

How to convert dis to UMA

1. Disconnect power to the graphics chip. Remove PJV1 connection.

 

2. Remove Graphics power driver.

 

3. Shift R65 resistor(10k) into R67 as described in the schematic diagram.

  

4. Connect 56k into R214.

  

5. Connect pull up resistor(10k) here. some board versions does not come with this resistor.

 

After doing all those modifications, i was not lucky. when i pressed the power button, fan spins at full speed and no sign of life. therefore i removed defective graphic chip and its RAM.

  

Success !!!!. I was able to boot up the motherboard. Display was ok. I edited the bios settings and boot order. tried to install windows 10. Setup was smooth in the beginning. But somewhere middle of the installation, machine freezed with the following screen.

 

This issue occurs because the BIOS is configured to recognize a discrete graphics subsystem. Internally, it retains a mapped address space intended for switching and communicating with dedicated GPU hardware. However, since the discrete graphics hardware is no longer physically present, the operating system still attempts to access this predefined address during initialization or driver loading. As a result, the system becomes unresponsive, as there is no actual hardware available to respond to these communication requests.

The solution to this issue is to program the BIOS with firmware that supports only UMA (Unified Memory Architecture) graphics. To achieve this, the BIOS was reprogrammed using firmware from the HP Pavilion 15-BW255G6 motherboard, which is designed to operate exclusively with integrated (UMA) graphics. This approach eliminates references to discrete graphics hardware, and the system operated successfully after the modification.

 

   

Installing a push button in a electrically noisy environment

Installing a push button over a long distance in a noisy factory environment (motors, contactors, VFD drives, welding machines, etc.) requires proper wiring practices to prevent false triggering, signal noise, and unreliable operation. Below are the industrial best practices used in automation and control systems.

1. Use 24V Industrial Control Voltage

For long-distance push buttons, avoid using 5V or logic-level signals. Factories usually use 24V DC control signals because they are more resistant to electrical noise.

Typical wiring : +24V  -> Push Button -> PLC Input / Controller Input

Advantages,

• Better noise immunity
• Standard in industrial automation
• Compatible with PLC inputs

2. Use Twisted Pair Cable

Long signal wires should use twisted pair cable. it reduces electromagnetic interference (EMI), Improves signal reliability and minimizes induced noise from nearby cables

3. Use Shielded Cable

In noisy factories with motors or VFD drives, use shielded twisted pair cables. Important rule - connect the shield to ground at only one end (control panel side). connecting both ends can create ground loop currents.

4. Separate Signal Wires from Power Cables

Signal wires must not run close to motor power cables.

Recommended separation distances

Cable Type                                     Minimum Distance

Signal vs control cables                     10 cm

Signal vs motor cables                     30–50 cm

Signal vs VFD cables                     50–100 cm

If crossing is unavoidable, cross at 90° angles.

5. Use Opto-Isolated Inputs

If the push button is very far away (100–300 meters) or installed on another machine, use opto-isolation. it protects the controller from voltage spikes, eliminates ground potential differences and Improves reliability.

(Industrial controllers often include built-in opto-isolated inputs.)

6. Solution using Relays

Working Principle,

In this method, the remote push button is used to energize a relay coil. When the push button is pressed, the relay activates and its isolated contacts change state. The control circuit then reads the signal from the relay contact instead of directly from the push button wiring.

This approach provides two major advantages,

Electrical Isolation

The relay coil circuit and the relay contact circuit are electrically isolated. This prevents electrical noise and ground potential differences from propagating into the sensitive control electronics.

Improved EMI/RFI Immunity

Since the control system reads the signal through relay contacts, high-frequency noise and interference picked up by long cables are significantly reduced.

Avoid routing near VFD output cables. VFD outputs generate strong electrical noise.

How to wire long distance RS485 cables without communication errors

In industrial environments, long-distance signal wiring such as RS485 must be installed carefully to avoid electrical noise, signal distortion, and false triggering. Factories usually contain motors, VFDs, contactors, and high-current cables that can generate strong electromagnetic interference (EMI). Following proper wiring practices ensures reliable and stable signal transmission.

Below are the best practices used in industrial automation systems.

1. Use Twisted Pair Cables

For long-distance signal transmission, always use twisted pair wires.Twisting the wires helps cancel electromagnetic interference induced by nearby cables. For RS485 systems, cables similar to Cat5/Cat6 network cable work very well.

2. Use Shielded Cable in Noisy Environments

Factories usually have electrical noise from,

• VFD drives
• Large motors
• Welding machines
• Switching power supplies

Using shielded cables prevents noise from entering signal lines.

Important rule - Connect the shield to ground at only one end (usually the control panel side). This avoids ground loop currents.

3. Separate Signal Cables from Power Cables

One of the most important industrial wiring rules is physical separation. Recommended separation distances,

Cable Type                         Minimum Distance

Signal vs low power cables 10 cm

Signal vs motor cables         30–50 cm

Signal vs VFD cables         50 cm or more

If cables must cross each other, cross them at 90° angles. This reduces electromagnetic coupling.

4. Use Proper RS485 Bus Topology

RS485 networks should use a daisy-chain topology, not star wiring. Star connections cause signal reflections and communication errors.

5. Use Termination Resistors for RS485

RS485 lines must have termination resistors at both ends of the cable. Typical value is 120 ohms,

Example:

120Ω ------------- RS485 BUS -------------- 120Ω

Master                                                      Last Device

Termination prevents signal reflections in long cables.

6. Use Bias Resistors (Fail-Safe Biasing)

When no device is transmitting, the RS485 bus may float. bias resistors keep the line in a defined state.

Typical values,

Pull-up resistor → 680Ω to 1kΩ

Pull-down resistor → 680Ω to 1kΩ

This prevents false data reception.

7. Use Opto-Isolation for Long Lines

Industrial environments often have ground potential differences between machines. Using opto-isolators protects communication circuits. this prevents generating ground loops.

Isolated transceivers are highly recommended for large factories.

Following these practices greatly reduces noise problems and communication failures in industrial systems.

600VA DCP UPS repair

 600VA Non-Sinusoidal Basic UPS – Fault Finding and Repair Guide

Uninterruptible Power Supplies (UPS) are widely used to provide backup power for computers, routers, and small electronic equipment. Many low-cost UPS units rated around 600VA use a 12V battery and a non-sinusoidal (square wave or modified square wave) inverter design. These UPS systems are simple, reliable, and easy to repair if the circuit operation is understood.

In this article, we will look at the basic schematic structure of a 600VA non-sinusoidal UPS, explain how the circuit works, and discuss practical fault-finding techniques commonly used during repair.

600VA DCP UPS is taken as the subject.

A typical 600VA line-interactive UPS with a 12V battery consists of several functional blocks:

• AC Input and Battery Charger
• Battery Section (12V Lead-Acid Battery)
• DC-AC Inverter Stage
• Oscillator / PWM Control Circuit
• MOSFET Power Stage
• Step-Up Transformer
• Output Switching and Protection Circuit

These blocks work together to supply AC power during normal operation and switch to battery-powered inverter mode during a power failure.

Main Components - Typical components found in many 600VA UPS inverter circuits include:

• Control IC – 8bit microcontroller
• Power MOSFETs – IRF3205, IRFZ44N
• Step-up transformer – 12-0-12V primary, 230V secondary
• Battery charger circuit
• Protection circuits (low battery, overload)

The oscillator (microcontroller PWM output) generates alternating pulses that drive the MOSFETs, which switch current through the transformer to produce AC output from the 12V battery.

When AC mains power is available,The load is powered directly from mains.The battery charger charges the 12V sealed lead-acid battery.The inverter remains inactive. When mains power fails, The control circuit detects the loss of AC input. The inverter oscillator starts generating pulses. MOSFETs alternately switch the transformer primary winding.The transformer converts 12V DC into approximately 230V AC.

Because the waveform is produced by simple switching, the output is typically square wave or modified square wave, not a pure sine wave.

Common Faults in 600VA UPS Systems

 1. UPS Completely Dead

• No output
• No LEDs or indicators
• No charging

Possible Causes,

• Blown fuse
• Open transformer winding
• Faulty power switch
• Damaged PCB tracks
• Failed control IC

Testing Steps,

• Check the AC input fuse.
• Measure battery voltage (should be around 12–13.5V).
• Verify that 5V or 12V supply rails reach the control IC.

 

2. UPS Works on Mains but Not on Battery

Possible Causes,

• Weak or dead battery
• Faulty MOSFETs
• Oscillator not running
• Open transformer primary winding

 Testing Methods,

• Check battery voltage under load.
• Inspect MOSFETs for short circuits.
• Use an oscilloscope to check PWM signals from the controller IC.

 

3. MOSFET Failure

MOSFETs are one of the most common failure points in UPS inverters.

Symptoms,

• UPS immediately shuts down
• Blown fuse
• Transformer overheating

 

Other common faults are,

• Transformer faults
• Battery Charging Failure
• UPS Beeps Continuously

continuous beep is most common among UPS systems. this may indicate-Low battery, Overload condition, Battery disconnected or AC mains voltage error.

Practical Fault-Finding Procedure

Step 1 – Visual Inspection

• Burnt components
• Broken PCB tracks
• Bulged capacitors
• Loose connectors

Step 2 – Check Battery Voltage

A healthy 12V SLA battery should measure 12.5V – 13V (fully charged). Below 11V indicates a weak battery.

Step 3 – Check MOSFETs

Remove power and test for short circuits. Check gate impedance with source. should be in mega ohm range. test source-drain body diode.

Step 4 – Verify Oscillator Operation

Check PWM signals from the control IC.

Step 5 – Test Transformer

Measure primary / secondry winding continuity

A 600VA non-sinusoidal UPS with a 12V battery uses a relatively simple inverter design consisting of a control oscillator, MOSFET switching stage, and step-up transformer. Understanding the function of each circuit block makes troubleshooting much easier.

Most faults occur in the battery, MOSFETs, charger circuit, or transformer, and can be quickly identified using a multimeter and basic diagnostic steps.

For technicians involved in electronics repair or power electronics servicing, mastering these fault-finding techniques can significantly reduce repair time and improve success rates when servicing UPS systems.

Programmable Timer Design - From concept to board

Programmable Timer Design Using PIC12F629 Microcontroller

As part of my embedded systems development work, I designed a compact programmable timer module based on the PIC12F629 microcontroller. This timer is intended for use in automation systems, industrial control panels, laboratory equipment, and custom electronic projects where accurate and configurable timing control is required.

The device allows users to set alarm timings according to specific application requirements. Once the desired time value is configured, the timer counts up and triggers the alarm output when the preset value is reached. This makes the module useful for tasks such as delayed switching, process timing, equipment control, and notification systems.

To provide a clear visual interface, the timer uses two seven-segment displays (SSD) to show the configured time values. The display allows users to easily monitor the current timer settings and countdown status. The display system is optimized to operate efficiently with the limited I/O resources available in the microcontroller.

User interaction with the device is intentionally designed to be simple and intuitive. The timer settings can be adjusted using two push buttons, which allow the user to increase or modify the time values without the need for complex controls or external programming tools. This minimal interface reduces hardware complexity while maintaining ease of use.

The firmware running inside the PIC12F629 manages display multiplexing, button input detection, and timing control. The design focuses on low component count, reliable operation, and compact circuit implementation, making it suitable for embedded applications where space and cost are important considerations.

This type of programmable timer can be easily integrated into custom electronics projects, automation controllers, sensor systems, and industrial devices. The design also demonstrates how small microcontrollers can be effectively used to build practical control modules with minimal hardware.

Download firmware, schematic diagram and operating instructions using following links.

Operating instructions - PDF file

Schematic diagram - Schematic image

Firmware files - Microchip XC8 project files

PCB files - Eagle 7.6

Need any custom module - GET INSTANT QUOTE

Custom PCB & Electronic Module Design Services – From Concept to Production

Industrial Electronics Design for Automation, Sensors, and Embedded Systems

We design and build custom electronic modules and sensor interface boards for industrial, automation, research, and embedded system applications.

If you have an idea for a sensor module, control board, or electronic device, we can help you turn it into a working product — from circuit design and PCB layout to prototype development and testing.

Modern sensor modules are widely used in automation, robotics, environmental monitoring, and industrial control systems to measure parameters like temperature, pressure, vibration, and humidity.

• Industrial grade designs
• Complete schematic + PCB + firmware
• Small and bulk production support

We develop reliable electronic modules tailored to your application requirements.

Sensor Interface Modules

• PT100 / RTD temperature sensor interfaces
• Thermocouple signal conditioning modules
• Humidity and environmental sensor boards
• Vibration and motion sensing modules
•  Pressure sensor interfaces

Industrial Signal Modules

• 4–20mA signal transmitters
• 4–20mA to voltage converters
• Analog signal conditioning circuits
• Industrial analog input/output modules

Embedded Control Boards

• Microcontroller-based control systems
• Data logging modules
• Wireless sensor nodes
• Automation control boards

Power & Interface Circuits

• Industrial power supply circuits
• RS485 / Modbus communication modules
• Relay driver boards
• Motor and actuator control circuits

Professional Schematic and PCB design with Optimized signal routing ,EMI and noise reduction practices. Complete schematic,PCB, GERBER for production or full working prototype will be provided. 

ORDER VIA FIVERR  = Connect OptimElectronic

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HAAS HFD4 Vector Drive Repair

 

I was requested to visit one of my customer's premises to troubleshoot HAAS mini mill machine. HMI was displaying error alarm 119,177 and machine could not be started. when vector driver was replaced with new one, problem solved. therefore vector drive may have some error and brought it to my workshop.

First of all, all parts of the drive were cleaned.

             

 

the problem was clear. machine sends overvoltage alarm when supply voltage is within safe limits. unit gives the error when on stable DC bus of 312V.

therefore we can directly point to the voltage detect section of the motherboard.

According to the circuit diagram, the opto-isolated op amp circled in yellow(A) is the main part that responsible for voltage measurement.
HCPL-7800A has two supply voltage inputs. both supplies are looks ok and stable 5V detected. op-amp has differential output directly connected to LF411 op-amp(B). output of LF411 measured 4.2V, seems has the fault. because it must be lower the shown value.

Without experimenting much with the circuit, i replaced the HCPL-7800A to check whether it was the actual fault.

After replacing the 7800A, the output of LF411 became 2.8V and looks normal.

I handed over the drive to customer to install into the machine and see the actual result. Ploblem solved.