G3 CPU Active Cooling Experiments by Brent Basham 9/10/98 This documents the procedures I have used to cool a G3 cpu in order to obtain higher system clock speed. The methods used range from a $12 purchase and installation of a blower to force air across the cpu heat sink to a total $50 expenditure to build and install a Peltier cooler. Do not attempt any of the procedures unless you are confident with your work and willing to accept responsibility should something go wrong as that cost is considerably higher. [This info is for reference only, modifications to your computer will void the warranty and can be hazardous to you and your computer.] I purchased a G3 266 DT in November 1997. By February 1998 I was convinced that the machine was past the "infant mortality" point, and having not read of a single case where overclocking caused irrepairable damage to the cpu, decided it was time to speed things up. With the natural convection (as received) method of cpu cooling I determined that this G3 could operate at 292/83/33 and 300/66/33 without problem. 315/70/35 booted but locked after 3-5 minutes, but 333/66/33 and 333/83/33 presented blank screens at start-up. I then purchased a Gamma 28 blower from Radio Shack. It fit perfectly in the space in front of the PCI card bay. A cardboard duct between the HD and motherboard was used to convey the air to the cpu heat sink. It was not pretty, but the computer ran fine at 315/70/35. Until summer, that is, when occasional lock-ups forced a retreat to 300/66/33. I removed the pad on the heat sink and used thermal compound between the sink & cpu, but there was no improvement. That configuation was maintained until recently when I moved the HD to an upper bay. The cardboard duct was then replaced with a hose to increase air flow across the heat sink. The blower puts out about 8 CFM of air, and was positioned so the upper tray wouldn't restrict air flow when closed. The benefits of the blower at 300/66/33 are shown in the following table using the temperature calculation from SpeedMeter: Temperature Comparison Stock (no fan): 67C (152°F) Fan w/duct: 51C (123°F) Fan w/hose: 47C (116°F) This was better, but 315/70/35 was still not reliable. Then about 2 weeks ago Mike posted a note on Peltier cooling with a link to an electronics firm. I purchased a1.17" x 1.17" Peltier Junction and decided to experiment further by making a 10W Peltier cooler. The results so far have been dramatic! Not only is 315/70/35 entirely reliable, but the machine will now boot at 333/83/33. (333 has thus far been unstable. though. It only boots with extensions disabled - explain that - and locks after 10 minutes or so. ) Although I've read posts that Peltier coolers don't provide any benefits, I suspect that those particular units were either undersized or installed incorrectly. My results with a 10w Peltier cooler have shown substantial cpu temperature decreases are possible which in my case was the principle factor to successful overclocking. To illustrate the point consider the 29°F improvement from the fan and 65°F from 'as received' at 300 MHz with the Peltier cooler: Temperature Comparison 10W Peltier Junction 300/66/33: 31C (87°F) 10W Peltier Junction 315/70/35: 35C (95°F) 10W Peltier Junction 333/83/33: 39C (102°F) With a few more modifications I think a stable 333 will be possible.   Peltier Cooler Assembly Procedure A few items before outlining the assembly procedure. I'm not affiliated with any of the sources that will be mentioned. This is just where I bought materials. Also, a Peltier Junction is essentially a thermocouple run in reverse. There is a cold side and a hot side. The heat must be dissapated or you will fry the cpu. Finally, what worked for me may not for you. Proceed at your own risk. The parts for the assembly are shown in the following picture: 1. This is your G3 heat sink. You will need to remove the pad from the back and remove about between 1/16" and 1/8" from the fins. Tape sandpaper to a hard, flat surface and lap it down. It will take about 30 min. Final thickness of the heat sink was 5/8 inch. 2. The Gamma 28 blower. 3. 1-1/4" plastic hose (Home Depot -$6). Cut the 'flat' section of hose (not shown) so it fits inside the blower outlet duct. Secure with hot-melt glue or tape. 4. Aluminum "cold sink", 1/8" thick. Cut about 1-1/4" square with appx. 3/8" 'ears' on two sides. Aluminum flat from Home depot - $3.50. A cold sink is needed to even out the cooling from the 127 individual points of the PJ. Copper is much better here and on my list as a future improvement when I find some. 5. The Peltier Junction. Cat# PJT-1 from All Electronics (www.allcorp.com), $17 ($5 handling). They also have the Gamma 28 blower, # CF-39, for $7. 6. Power resistors, 10 ohm, 10W, Radio Shack (or AE) In the middle but not numbered, 18 ga. single strand wire to assemble the unit & hold it on the cpu. The first thing to do is find & mark the hot side of the heat sink. Open the computer case and locate one of the power plugs that will be used as the electrical source. 12 volts are obtained between the blue lead (+) and the second black (if the blue is #1, the 2nd black is #3). Hold on to the Peltier junction (PJ) and connect the red lead to the blue wire, the black to black. One side of the PJ will get hot, the other cold. Mark which is which! Coat both sides of the PJ with thermal compound and sandwich between the heat sink and the cold sink. Be extra sure the "hot" side of the PJ that was marked is against the heat sink. Loop a small piece of wire around a heat sink fin and a cold sink ear and tighten. Reconnect the heat sink ground strap. Put thermal compound on the cpu and position the peltion cooler on the cpu. Secure the cooler in place using a piece of wire with a loop at one end. Using the same tabs on the zif assembly that hold the normal spring clamp, put the loop over the tap on the back of the zif , pull it reasonably tight, and loop at the front. Several tests I performed on the cooler assembly convinced me of 2 things - the heat output using 12v is beyond the reasonable capacity of the heat sink & blower (in other words, it got too hot IMO), and the other voltage available, 5v, didn't provide enough cooling. If the resistance of the PJ stays constant the amount of cooling will be proportional to the square of the voltage that's applied, and 7v would give 2X the cooling of 5v, 8.7v would give 3X, 10v, 4X, ect. To the best of my recollection the G3 cpu generates about 5w of heat so I chose to design a 10w Peltier cooler. The PJ had an internal resistance of about 5 ohm , so a resistance load of 3.3 ohm in a 12v circut would drop the voltage to the PJ to just over 7v and yield 10w. The load was made by putting three 10 ohm power resistors in parallel in the circut. The resistors will get quite warm, so were mounted in a "tree" behind the internal speaker so as not to touch anything in the case. The cooler and tree are shown in the following picture. Carefully check if the upper case will swing closed and lock without being forced. Remove the assembly and sand some more if there is any doubt about clearance. The blower was then wired to the same12v source and installed. The end of the hose is secured in position with a small piece of wood that fits into the gap between the motherboard locking tabs. Start-up with the case opened, extensions disabled, and with an alias of SpeedMeter in the Startup Items folder. If you have any indication that the hot side of the PJ is against the cpu shut down immediately! Otherwise, give the temperature a minute or so to stablize, then shift the position of the cooler assembly to get the lowest temperature possible. The overall power draw of this configuration presented totals 19w - 10 from the cooler, 7w from the resistors, and 2w from the blower. I have made no attempt to measure the efficiency of the cooler, and intentionally did not insulate it so the blower air would prevent condensation. I have not seen condensation at either the10w or 13w power level. The cooling can be easily increased by hanging another resistor on the tree or using single resistors with fewer ohms as shown in the table: Power to Peltier Cooler (watts) Voltage across PJ Extra Load, Ohms Total Power (watts *) 10 7.2 3.3 17 13 8 2.5 19 15 8.67 1.9 21 18 9.5 1.3 23 20 10 1.0 24 29 12 0 29 * - Blower power not included in total. If you step up the power to the cooler be sure to check the cpu temp carefully . All additional heat must be rejected through the heat sink so it will heat up. If the cpu temp ever increases when power to the cooler is increased, BACK OFF because the blower/heat sink are at capacity. Good luck, & post those OC results at XLR8YOURMAC.com.

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