MythTV Part 3: Getting Quiet Enough for the Living Room
Pages: 1, 2, 3

Replacing the north bridge was a complex operation because many north bridge coolers do not attach with screws. Plastic "spears" poke through holes in the motherboard to hold the north bridge heat sink in place; removing the old heat sink required completely dismantling the computer to get at the underside of the motherboard. Figure 4 shows the underside of the motherboard. In the photo, my two fingers are pointing at the north bridge retention holes. By squeezing the plastic spears, the north bridge fan came out easily.

Figure 4: Underside of motherboard

Figure 4 also shows the CPU backplate, which is glued onto the motherboard. It's designed to accommodate many of the CPU coolers on the market, but there's a slight incompatibility with the Zalman. The backplate is glued on to the motherboard, and has two posts that stick through to attach heat sink retention brackets to. Zalman includes with the 7000 the brass standoffs shown in Figure 5 (left photo). With the added height from the backplate posts, however, the heat sink does not clamp tightly enough to the CPU for maximum cooling power.

I initially tried using the standoffs as-is. With the slight additional height, they did not allow the CPU cooler to press down hard enough. In this configuration, the idle temperature of the CPU was 42°, almost four degrees hotter than the stock cooler. At this point, I had to either remove the motherboard again to replace the CPU backplate, or find an alternative.

After searching through Zalman and MSI user forums, I decided that removing the backplate was not for the faint of heart. Removing the stock backplate requires heating the glue and gently prying it away from the motherboard, taking care not to damage any circuit traces on it. I opted for a simpler solution. The stock motherboard heat sink retention bracket comes with two long screws. I used the long screws from the motherboard to attach the CPU cooler, as shown in Figure 5 (right photo). With more pressure applied to the CPU, heat transfer improved enough to drop the idle temperature by eight degrees.

In Figure 5, the cylindrical post from the backplate can be seen rising through the motherboard. The clamp from the cooler must be horizontal to press the cooler tightly enough to the CPU to absorb heat. If the brass standoffs are used, it is not possible to get enough leverage to press the cooler on to the CPU.

Figure 5: Attachment of the CPU cooler. Brass standoffs (left) from Zalman did not work as well as the stock motherboard screws (right).

Figure 6 shows the new CPU heat sink in place. Note the exposed north bridge and empty mounting holes at the right side of the picture. (There is a slight white smear from the small amount of thermal paste used with the stock north bridge cooler.) The CPU heat sink barely fits. On the left side of the photo, it is very close to the memory slots, and on the right side of the photo, it is very close to the video card slot. Although the fit is tight, Zalman's compatibility list was absolutely correct.

Figure 6: CPU heat sink in place

Installing the north bridge heat sink is straightforward. After positioning the clip arms to fit the mounting holes on the motherboard, apply some thermal paste and snap it in. Figure 7 shows the position of the north bridge heat sink. I suspect the reason why MSI chose to use a cooler with a fan is that it allows for the use of full-length PCI cards. The heat sink is quite tall, and will obstruct the use of full-length cards. However, the tuner cards are quite short. If it were necessary to use a longer card, I could easily shift one of the tuner cards right to free up an unobstructed slot.

Figure 7: North bridge heat sink

The Results

Naturally, replacing the cooler is only worthwhile if the temperature can be kept steady (or better, reduced). Table 1 shows the results, unscientifically measured by recording the output from sensors when the system was idle and when it was under load. To create MythTV load, I waited for an hour-long program to finish recording and for the commercial flagging process to reach 80 percent completion, and measured the temperature again. Commercial flagging requires significant processing resources, and always runs the CPU at maximum (2GHz) speed for extended periods of time.

Table 1: CPU temperature

As an extra check of thermal stability, I used CPU Burn-in, which uses processor-intensive floating-point calculations to generate a much more demanding load than MythTV. (The better known CPUburn is written in assembly code, and is not available for the AMD K8 architecture.) After running CPU Burn-in for half an hour, the CPU temperature measured 52, which is still lower than the typical CPU temperature measured with the retail heat sink and fan.

As a final note, the system appears to run slightly cooler all around. The motherboard temperature sensor reads a degree lower (18° instead of 19°), and the third temperature sensor also reads slightly lower as well (28-29° rather than 31-33°).

Matthew Gast works in the Office of the CTO at Trapeze Networks, where he works on product architecture and industry standards. He is a voting member of the IEEE 802.11 working group, and serves as chair of 802.11 Task Group M. At the Wi-Fi Alliance, he chairs the Wireless Network Management marketing task group and the Security technical task group. In 2007, Matthew was a founder of the OpenSEA Alliance, a group which supports the development of open-source network security solutions. He currently serves on the engineering steering committee and on the organization's board of directors.

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