Vmem Rail on Polaris GPUs

Here we'll talk about Vmem Rail on Polaris series of graphics cards. How it works, what it is used for, and common problems.

The Controller Circuit
Vmem is the voltage for memory chips. The controller for Vmem is often the GS7256 or less commonly used is the UP1666Q (some Gigabyte models). In this page, we'll cover the most common variant, the GS7256.

Depending on the model, VCC for the controller is either supplied by the 5V rail or as in the schematic by the 12V_EXT rail.

Enable is controlled by the PGOOD signal from the Display Rail controller as shown in figure 3. Markings on the schematic and board could differ from GPU model to another but the circuit is almost always the same.

Usage
Vmem, as the name suggests, is the voltage powering the memory chips for the card.

Vmem also goes directly to the core as well, powering memory controller logic.

No Vmem voltage
Start by measuring the EN and VCC pins of the controller, if one of them is missing, the controller will not operate. If they are both high, check if the FB circuit is as it should be as shown in figure 2.

If everything seems fine but Vmem is still missing that could signal a dead MOSFET. MOSFETs tend to short when they fail but sometimes they can just stop working without shorting. This can be confirmed if you have an oscilloscope, simply measure the gate of the MOSFET or the "UGATE" pin of the controller to see if the controller does actually create the PWM signal or not. If it doesn't then the controller is likely dead.

After confirming the above is working but there still no Vmem, this could signal a shorted Vmem.

Short on Vmem
If Vmem measures less than 10Ohms, often times this means one of the memory chips is shorted. You can check that by injecting 1V to Vmem and applying isopropyl alcohol on the memory chips and see if one or more of the memories are heating up, evaporating the alcohol faster than the rest. Otherwise though, if it's not a shorted capacitor somewhere odds are the core itself is shorted.