Our measurements showed 1.78V VPU voltage, but the combination at the input of the chip corresponded to 1.4V. It meant that there was another way for changing the VPU voltage, besides changing the bit combination on the input. I didn’t like the idea of cutting and soldering lines on the PCB and had to read the specifications of the FAN5240 to find another way. According to the specs, pins 17 and 18 of the chip (marked as VCore Output Sense) serve for voltage control as well as for protection and monitoring. The typical connection circuit from the manufacturer differed from what we saw on our graphics card: there was an R1597 resistor with a resistance of 1620Ohm between pins 17 and 18. In fact, pin 17 was connected to the common output. So, I decided to connect a trimming 22kOhm resistor in parallel to the R1597.
I soldered one wire from the trimming resistor to pin 18 of the FAN5240 chip, and the other wire to a small bond area next to and attached to the R1597 resistor.
We took our measurements from the point marked as VCore in the scheme.
Our idea was correct, as the Vcore was growing when we were reducing the resistance of the trimming resistor. We stopped on 1.9V, when the trimming resistor stayed at 4.8kOhm. As a result, we made the VPU work at 500MHz. The card was actually stable at 510-520MHz core frequency, but produced some visual artifacts in 3D, so we made it work at 500MHz in our performance tests.
An additional 120mm 9W fan was installed to blow at the butt-end of the graphics card. It pushed air along the entire PCB and cooled down the graphics chip and memory chips similarly. This solved the problem of heat dissipation, although I can’t say the card was cool. An external thermal sensor reading data from the aluminum sole of the graphics chip cooler showed 55-60°C under workload and 46-47°C in the idle mode. For example, the same temperatures before the volt-modding were 46°C and 40-41°C, respectively. The difference is quite perceptible, considering that our sensor was external and the real graphics core temperature was even higher. Here is my advice: don’t save on cooling when you are overclocking your graphics card. I think a water cooling solution might help. A solution like that is noiseless, while our graphics card was not: you wouldn’t work in such a noise, if you are not completely deaf.