MediaTek Dead Phone Repair Guide – Fix Buck Voltage Not Generating (MT6365 / MT6359 Power IC)

Modern smartphones powered by MediaTek chipsets are widely used in both 4G and 5G devices. Many technicians face a very common issue while repairing these phones: the phone comes in dead condition or low boot condition.

When technicians start diagnosing the motherboard, they usually check the BPPH voltage. In many cases BPPH voltage is present, but when they check the buck voltages on the power IC coils, none of the buck voltages are generated.

Important: This problem is commonly seen in MediaTek power ICs such as MT6365 and MT6359, which are used in modern 4G and 5G smartphones.

Understanding MediaTek Power IC Architecture

MediaTek power management ICs like MT6365 and MT6359 generate multiple voltage rails required by the smartphone processor, RAM, modem, and other hardware components.

These voltages are generated through buck converters connected to coils placed near the power IC. In many MediaTek motherboards you may find around 8 to 9 buck coils.

Each coil generates a specific voltage rail such as:

High LDO Voltage – around 2.05V
Low LDO Voltage – around 1.3V
CPU Core Voltage – 0.6V to 1.2V
RAM Voltage
Modem Voltage

If these voltages fail to generate, the CPU cannot start and the phone remains completely dead.

Common Problem: BPPH Present but Buck Voltages Missing

One of the most confusing scenarios for technicians is when:

BPPH voltage is present
But none of the buck voltages are generated

Even after performing repairs like CPU reballing or replacing the power IC, the phone still does not boot. This situation requires deeper diagnosis.

Step 1 – Check GR Value of Buck Coils

The first step in diagnosis is to check the GR (Ground Resistance) value of all buck coils using a multimeter in diode mode.

Normal coils will show diode reading
If any coil shows 0.00, it indicates a short circuit

If a buck line is shorted, the power IC will not generate buck voltages.

In this case remove the coil and check which side is shorted:

CPU side
Power IC side

After identifying the faulty section, repair the short circuit.

Step 2 – CPU Reballing

If no short circuit is found on any buck coil, the next step is to try CPU reballing.

Sometimes the solder joints under the CPU become weak due to heat or drop damage. This prevents communication between CPU and power IC.

After CPU reballing, buck voltages may start generating again.

Step 3 – Replace the Power IC

If CPU reballing does not solve the problem, technicians may try replacing the Power Management IC.

Common MediaTek power IC models include:

MT6365
MT6359

However, sometimes even after replacing the power IC, buck voltages still do not appear. This means the fault is somewhere else on the board.

Pro-Level Solution 1 – Check 26MHz Crystal

One hidden reason for missing buck voltages is the 26MHz crystal oscillator.

The CPU requires this crystal clock to start the boot process. If the crystal stops oscillating, the CPU cannot start.

If the crystal fails, the power IC may not generate buck voltages even though BPPH is present.

Solution:

Re-solder the 26MHz crystal
Or replace the crystal

Pro-Level Solution 2 – Check Charging Detection Line

When a charger is connected, the charging IC sends a trigger signal to the power IC.

In older MediaTek designs this signal was called VCDT line. But in newer power ICs like MT6359 and MT6365 this signal is called:

Charging Detection B

If this line is broken or disconnected, the power IC will not start generating buck voltages.

Therefore technicians must check the connectivity between:

Charging IC
Power IC

Pro-Level Solution 3 – Check VC Sensing Pin

Inside the power IC there is a sensing pin called VC sensing pin.

This pin monitors the BPPH supply.

In many MediaTek power ICs this pin is located at M11 ball.

If the M11 ball is damaged or disconnected, the power IC cannot detect BPPH properly. As a result, buck voltages will not generate.

Pro-Level Solution 4 – Check SMPS Line Resistor

MediaTek power ICs use SMPS circuits to generate buck voltages.

Near the power IC you will usually find a 0-ohm or 1-ohm resistor connected to the SMPS line.

This resistor supplies BPPH voltage to the SMPS input.

If the phone experiences:

Water damage
Drop damage
PCB corrosion

This resistor may become open or its resistance may change.

Normally both sides of the resistor should show around 3.7V battery voltage.

If voltage is present only on one side, replace the resistor.

Other Possible Reasons for Missing Buck Voltages

Some additional causes include:

Short circuit in initial rails like VREF or VDIG
RAM voltage short
Fault in external buck converter circuits
Missing fault detection signals

Complete Troubleshooting Process

If you receive a MediaTek dead phone with no buck voltages, follow this process:

Check GR values on buck coils
Repair any short circuit found
Try CPU reballing
Replace power IC if necessary
Check 26MHz crystal oscillator
Inspect Charging Detection B line
Check VC sensing pin connection
Verify SMPS line resistor

Repair Tip: Always perform proper diagnosis instead of randomly replacing components. Understanding the power management circuit will increase your repair success rate.

Conclusion

MediaTek smartphones are becoming more advanced and require deeper understanding of power management circuits.

When a phone shows BPPH voltage but no buck voltages, the problem may not always be CPU or power IC. Often the real fault lies in small components such as:

26MHz Crystal
Charging Detection Line
VC Sensing Pin
SMPS Resistor

Technicians who understand these advanced troubleshooting techniques can repair MediaTek 4G and 5G smartphones more efficiently and improve their success rate.