HOW I REPAIRED MY NORTHBRIDGE HEATSINK

Tools used:

1 x long arm 3 mm hex key1 x cross-head screwdriver1 x needle-nose pliers (not essential) to ease the push-pins out1 x syringe of thermal paste1 x small plastic scraperTissues, ear-buds and isopropyl alcohol and wipes.2 x plastic, spring-loaded push pins a.k.a. heatsink rivetsFlat surface at comfortable working height with soft cloth cover.

Method

Switch off the computer and disconnect all cables.Remove the side panel by lifting the lever at the rear as shown in Figure 1.Press out the latches on the processor tray as shown at the bottom of Figure 1.

Figure 1: Mac Pro Early 2009, 8-core, with side panel off.

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This is a Mac Pro Early 2009, 8-core, with 16 GB RAM. Some time back I replaced theoriginal video card with an EVGA GeForce GTX 680. The PCI enclosure also holds aStardom SJ20-x8 3G PCI-e 4-port e-SATA Host Adapter and an Areca 12x4 RAID card(which replaced the poorly performing Apple RAID card).Fully open the latches and pull the processor tray complete out. Set it down on theworking surface as shown in Figure 2. Make sure you discharge your static.

Figure 2: Processor tray with front towards worker.Use the hex key to slacken the 8 hex bolts on the CPU heatsink casings and lift off vertically.There is a plug to the processor board at the outer sides of each casing so there will be alittle resistance but they will lift off quite easily. Turn them over as shown in Figure 3.

Figure 3: The inverted CPU heatsink casings.

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As can be seen in Figure 3, the CPU chips come away with the casings, but if theCPUs have been running at normal temperatures, the thermal grease holding themto the casings is soft and the CPUs, with their plastic carriers, can be easily lifted off(always just holding them by the edges) by applying a gentle twist. Keep the CPUsadjacent to their respective casings.

Figure 4: Processor board removed from tray and Northbridge heatsink detached.Using the cross-head screwdriver, undo the 9 black screws holding the processorboard to the tray. Lift the board off and set it down on the work surface.Remove any remaining push-pins on the Northbridge heatsink and invert it asshown in Figure 4. I did not detach the sensor cable but others might want to dothat. It is probably wise to cover the CPU contacts, but I didnt.Figure 4 shows that the thermal paste had cooked on the lower surface. The pasteon the contact part on the heatsink was still soft and could be cleaned off with atissue, but the contact area on the chip had to be cleaned by gently scraping with aplastic scraper then wiped. The end result is shown in Figure 5.

Figure 5: Northbridge chip and heatsink contact areas cleaned.

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At this point, clean the processor board. I used one of the small tools on a vacuumcleaner and then blew it off with a hairdryer on cold and at maximum blow.After that, clean the contact surfaces on the heatsink and the chip with isopropylalcohol and once the vapours have dried, apply a little thermal paste using a cottonbud to spread it thinly on the chip contact plate only, then place the heatsink inposition and press in the push-pins. I dipped a cotton bud into the isopropylalcohol then squeezed excess liquid out before applying. No liquid should run offthe contact plate being cleaned. Press down firmly on the heatsink, but not overly so, so that the paste layer is asthin as possible. (There are many different instructions on the internet about how toapply thermal paste. I put a small, rice-grain-size bead on a cotton bud thenspread it very thinly on the landing plate, making sure it did not go on the chip.)

Figure 6: Heatsink replaced and paste smeared on CPU A contactNext, replace the CPUs one at a time and clean them in the same manner. The CPUsonly fit on one way so if you have forgotten how they came off, no matter. Just lookat the notches on the edges and match them up with the notches on the base andgently place the CPU on to the base. Similarly, the black plastic frame can also onlygo on one way. Gently press that into place.After cleaning the contact surfaces with isopropyl alcohol, renew the thermal pasteon the CPU landing plate. Figure 6 shows the Northbridge heatsink replaced withnew push-pins installed, and the landing contact for CPU A smeared with paste.This photo is before I used a cotton bud to run round the edges of the contactbefore reassembly, to remove any excess paste.

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You will see that the two edge fins on the Northbridge heatsink are pressed in alittle. This was the original situation when I opened the computer, however, beforefinal reassembly, I eased them out so all fins ran parallel to maximise airflow.Finally, lower the CPU heatsink casing down on to the four bolts and press in thesensor connection. Use the long hex key to tighten the bolts evenly and notexcessively. I screwed them down on opposing corners until there was someresistance then give them all a little twitch to tighten. The reassembled unit isshown in Figure 7 (before I adjusted the Northbridge heatsink fins).

Figure 7: The reassembled processor tray.Slide the tray back into the computer and close the latches. Connect only the powercord, display and keyboard and power on.In my case, the temperature of the Northbridge heatsink went from a high of 128Cbefore repair to a high of 79C after repair, with the intake and exhaust fansrunning at the low of 600 RPM. The CPU temperatures were normal.I then used Macs Fan Control to set the intake and exhaust fans to respond to theNorthbridge chip temperature and the computer has settled at a high of 75C forthat chip, with the fans at 900 RPM.I then used Geekbench 3 to run a stress test and everything worked well. As theCPUs warm up during the stress test, the main fans run faster to effect morecooling and that reduced the temperature of the Northbridge down to a low of56C.Finally, since it was my first attempt at applying thermal paste, and being a littleparanoid, I disassembled the CPU casings again and found that the paste hadlevelled to a very thin, transparent layer and there was no run off. I ran a cotton bud

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round the edges of the landing contacts again anyway then reassembled thecomputer. Now, several days later, all is working well.This explanation is about what I did. It is not what would happen in an AppleService Centre or in a reputable repair centre and is not meant to be. If I did itagain, I would cover the CPU contacts before I removed the Northbridge heatsink,but I dont think Id do anything else differently. I tried to disconnect theNorthbridge sensor but it was a bit difficult so rather than force it, I just left itattached.The only items I found difficult to get hold of were the correct sized push-pins. Icould see some advertised on e-Bay in the U.S. and Hong Kong (I live in Singapore),but I couldnt be sure they were the right size. I shopped around locally andcouldnt find any. Eventually I went to the famous Sim Lim Square and a kindgentlemen in a repair shop gave me a heatsink from a scrap laptop motherboard,complete with push-pins in good condition, and they fitted perfectly.My thanks to all those who wrote before and I hope this helps others.

Bruce Ewen20 June 2015

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