Modifying a Motherboard's Voltage Regulator Circuit

The pdf at the bottom of the page is the guide, not the text here.

BACKGROUND INFORMATION

This guide demonstrates how to modify a CPU’s voltage regulator circuit on a socket 3, 486-class motherboard. Although this document illustrates how to modify the regulator circuit on a Biostar MB8433-UUD v3.0 motherboard, the approach is would be similar for most motherboards which allow for varying a CPU’s core voltage via jumpers or DIP switches. Most motherboards employ an adjustable voltage regulator integrated circuit, whereby the output voltage is controlled by two set resistors. The designers of the MB8333-UUD adopted a Sharp PQ30RV21 voltage regulator. The specification sheet for this is still widely available for download on the internet. It contains much useful information such as pin-outs (4 pins), typical connection diagrams, and some basic equations. A voltage regulator is used to drop, or down regulate, either a 5 V or 12 V signal from the computer’s power supply down to voltages which CPUs typically run at.

For the above mentioned voltage regulator, the output voltage (the voltage that powers the CPU) can be approximated as,

Vout = Vref x (1 + R2/R1), where Vref ~=1.25V.

Resistors R1 and R2 are located on the motherboard and are known as surface mount (SMD) resistors. The MB8433-UUD has jumpers for 3 discrete voltages, 3.45 V, 4.0 V, and 5.0 V, which are controlled by jumpers JP37, JP39, and JP36. These jumpers simply act as path switches to different resistors to control the output voltage in the above equation. The 5 V jumper setting, however, probably does not transition through the Sharp voltage regulator, but comes directly from the computer’s power supply [and direct to the CPU]. 3.45 V and 5.0 V settings are commonly found on socket 3 CPUs so we probably don’t want to adjust those, but 4.0 V is rarely used so this is the voltage jumper setting we will tamper with.

There are a few CPUs which will benefit from running in the 3.45 V – 4.0 V range, namely the Cyrix 5x86 series of CPUs which are very thermally sensitive to voltages greater than or equal to 4 V. Cyrix/IBM 5x86 processors operating in the 120 - 133 MHz range usually require operating voltages of 3.6 - 3.9 V.

The motivation for this document was due, in part, to having achieved great success in overclocking an IBM 5x86C-100HF to 133 MHz on motherboards with a UMC 8881/8886 chipset and running at 2 x 66 MHz. This configuration was deemed stable at a specific CPU voltage of 3.75 V. Further details on this project can be found at,

So you want a Cyrix 5x86-133?
Re: The World's Fastest 486 - Cyrix/IBM 5x86-133

MOTHERBOARD SURGERY

The remainder of this document will guide you through the surgical process pictorially. Refer to the pdf attachment below. Best viewed at 100% scale.