Base Voltage Rail Short on Polaris GPUs

This page will show where shorts might occur on a Polaris card and how to repair it.

12V_BUS Short
Video example

Symptoms:

 * Sub 100 Ω on the 12V_Bus rail.
 * Computer won't power on or instantly shutting down upon pressing the power button.
 * Fans not spinning (in case of blown fuse).

12V_BUS is used to power the first phase of Vcore, Vmem and VDDCI controllers and mosfets/powerstages, Display Rail, and the 5V rail regulator in addition to their input capacitors.

Symptoms:

 * Sub 100 Ω on the 12V_Ext rail.
 * Computer won't power on or instantly shutting down upon pressing the power button.



The 12V_EXT rail is mostly used to power the mosfets/powerstages for the Vcore rail. Sometimes, vendors use the 12V_EXT to power 5V regulator instead of the 12V_BUS.

Symptoms:

 * less than 100 Ω on the 3.3 V rail
 * Fans don't spin
 * Computer won't power on or instantly shutting down upon pressing the power button.
 * No other minor rail turns on if the computer does not turn off instantly



3.3 V isn't used to feed power-hungry components and therefore is very unlikely to short, but if it does, it's either the Display Rail regulator, 1.8V rail regulator, or the BIOS. it's worth noting that 3.3 goes straight to the GPU core so if it's non of those then it's highly likely the GPU itself is shorted.

Repair:
To find the component that's causing the short you need a lab DC power supply and some freezer spray or isopropyl Alcohol (or a thermal camera).

Step 1: Injecting voltage
Start by applying 1 V through the shorted rail using a DC Lab PSU and measure the voltage across Vcore, Vmem, and 1.8V rails. If the voltage going through is less than 50 mV, then you can apply more voltage to the 12 V rails without worrying about killing the GPU core or memory. However, if the voltage going through is more than 50 mV, you need to remove all the inductors going to the core and memory to protect them from high voltage.

Step 2: applying alcohol or freezer
Apply freezer spray or alcohol on the suspected components while the power is still connected. The faulty component will heat up causing the alcohol or the freezer to evaporate.

Step 3: Removing the shorted component
After identifying the shorted component, remove it and check if the short is still there. If there is still a short, repeat step 1.

otherwise, replace it and test the card.