I'm in the process of cooling and quieting a 6100/66 with a 486/66 DOS card. For a first modification, I cut away part of the drive cage that diverts airflow away from the processors. I'm not sure what Apple was thinking here since it doesn't make much sense on the Centris 610 either.

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Trimmed with a rotary tool:

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Test conditions:

• Single 80mm Noctua NF-A8 FLX PSU fan at 12v (no temperature-based control)
• 601 CPU idle, stock heatsink
• 486 CPU, stock heatsink, running Raptor: Call of the Shadows demo
• No CDROM, HDD or floppy, running from MacSD
• Cover installed, empty drive bays taped over
• 486 temperature measured at the heatsink

Improvement: 5.1*C

Not bad for a free mod. Heatsink upgrades are on the way for both CPUs. Depending on how they perform, I may go with a rear 90mm or front 120mm fan.

Next is replacing the 601's heatsink with a 50x50x15mm copper heatsink. I made a custom torsion spring from a piece of nitinol wire to hold the heatsink in place. For those unfamiliar, nitinol is a nickel-titanium alloy with some interesting properties:

• When a section is red hot, it can be easily bent and the shape is remembered as the wire cools.
• At room temperature, the wire is somewhat flexible.
• When heated above a certain threshold (around CPU operating temperature), the wire returns to how it was shaped when red hot.

I used a propane torch and a pair of vise grips to bend this wire. It took about an hour to get it right. After forming it, I deburred with a grinder, added clear heat shrink tubing to the ends (to avoid scratching the board) and twisted it to make it easier to pass through the logic board. The neat thing is, the 601's heat will restore the wire's clamping force.

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Added thermal grease to the die and four dots of thermal glue to keep the heatsink from shifting around.


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Copper heatsink installed:

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Very nice custom heatsink :)

Thanks!

I upgraded the DOS card's heatsink as well. New heatsink (left), vs stock heatsink (right).

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Temperatures with the 80mm Noctua PSU fan at 7V weren't great:


However, with the new heatsink and 7V fan, the 486 runs cooler than it did with the old heatsink and 12V fan.

I cut out the HDD section of the drive cage and test fit an old 120mm Antec fan over the vent. The lowest speed seems to be around 1000 RPM (no tach signal) and CPU temps dropped quite a bit, so I cut a hole in the case floor for better airflow.

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A view of the fan and both CPUs. Fanning out the fins on the copper heatsink reduced temperature around 1-2 degrees:

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There's currently no 80mm fan in the PSU. Removing the monitor power outlet from the PSU creates another exhaust port. There's a steady stream of hot air leaving the rear of the case. I'll trim the PSU case further to improve airflow:

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With the slow moving 120mm fan, I recorded much better temperatures:



I'm happy with both CPUs in the 40s. A Noctua NF-F12 will replace the old Antec fan.

The 6100 is almost done. I took pictures along the way that I'll add later, but this is the result:





Modification summary:

Both CPUs are in the 40s and the machine is effectively silent.


General:

• HDD section removed from drive cage.
• Front Noctua NF-F12 120mm with modified frame runs at 5v / 704 RPM.
• 3D printed, flexible TPU shoes added to the four case feet to lift the case about 3mm, improving airflow significantly.
• 3D printed, flexible TPU fan gasket mounts the 120mm fan and power LED.
• Logic board and PSU recapped.
• Crack repaired in cover.

CPU:

• Copper heatsink added to 601 CPU, attached with torsion spring.
• Aluminum heatsink added to 486 CPU, attached with glue.
• 3D printed ABS duct guides airflow over the 486 and 601 CPUs for improved cooling.

PSU:

• PSU flipped upside down, and mounted to case with 10mm standoffs. It overheated in its original orientation without the 80mm fan.
• Power switch is riveted to chassis.
• 3D printed ABS linkage connects the power button to the switch and is locked into position by the floppy drive.
• 3D printed ABS plate secures power jack and opens an exhaust port where the monitor power jack used to be.
• Main heatsink replaced by a custom heatsink made of a 100x100mm copper sheet cut, hammered and pressed into shape.
• Multiple smaller heatsinks upgraded.
• 80mm wire fan grill added to prevent shock hazard.

Storage:

• 3D printed ABS, modified MacSD bracket attaches it to the rear of the case, allowing card access without opening the case.
• Custom SCSI cable.
• Spinning HDD removed, stock CDROM and floppy drive retained.