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Radical fix for dead iMac center fan Desktop Macs
The Power Management Controller (PMC) failed for the center (hard disk) fan in my iMac Core 2 Duo 3.06 27-Inch (Late 2009). The two outer fans work perfectly, but no center fan gets no power at all.

It took me three tries, but I finally got it right. It's an ugly fix, but it works great.

1. First i got Marcel Bresnik's free Temperature Monitor to read the disk's temperature. It shows the hard disk temperature in the dock. when it started to show close to 130 F, I would shut down the Mac. This was unacceptable.

2. I got a small, square USB fan and attached it over the rear center hole with velcro. This made the heat buildup at the hard disk worse. I figured the new fan was blowing air out of the original fan's intake, so air was moving in the wrong direction. And I couldn't tilt the screen down.

3. So I watched some how-to videos before cracking the case. I had to pull off the motherboard to access the center fan and place a double layer of clear tape over the fan's intake. Then I re-attached the USB fan on the back, and this seemed to cool the disk much better. However, I still could not tilt the screen down. If you are comfortable with the screen not tilting downward properly, you could stop here, but I got better cooling by continuing...

4. I found a short piece of hose from a vacuum cleaner, which even had a small HEPA filter. Then I made a small wooden box for the fan, complete with the air filter.

Next, I used a hole saw and cut a 2-1/8 inch circle out of the stand, directly behind the iMac's center air hole. I installed my custom-filtered fan on the stand with more velcro, and the hose running thru the stand hole directly into the center hole of the iMac. The original fan's air intake is still taped over.

Benefits:
a. My hard disk's temperature has not exceeded 120 degrees in over three years. I keep Temperature Monitor in my Dock to make sure.
b. The two (working) original fans operate normally, without additional software. I think this is important, as I doubt they were intended to run at full speed forever, and running them at full speed pulls dirt into the machine.
c. The new fan makes a very, very small amount of noise, and it's filtered air.
d. The three-inch section of vacuum hose extending thru the stand hole has enough flex that I can tilt the screen easily, while maintaining proper air flow.
e. I already had a hole saw, so for me the only cost went to a cheap USB fan.

Notes: Opening the iMac case is not simple. The hard part is getting just the right parts. You need a suitable USB fan, a fan housing (filter optional), wide cellophane tape, velcro, some kind of 2-inch hose that is flexible, a drill that isn't too fast, and a hole saw available for $10 at any hardware store.

Cutting the hole is scary to think about, but it's a lot easier than it sounds. It does not harm the integrity of the stand (there is already a larger hole in it). The aluminum is soft enough to cut cleanly and easily. Put a piece of paper underneath to catch the filings, and don't allow the aluminum particles to enter the iMac. Mark the center and use the saw's center bit to start the cut and maintain the center position. Do not try to go too fast, or press hard. It cuts like butter!

Do not use a high-speed drill; it will get hot, and you don't want that. Use a medium- low-speed drill and take your time. You might even pause for a few minutes halfway through the cut to avoid heat build up. It will stay cool enough that the silver paint will be completely unaffected. A nail file will clean up any sharp edges.

In the end the only problem is that the fan uses one USB port, and I guess it looks a little strange when you peer around the back, but i don't care about that.

[kirkmc adds: Well, I have to admit, I'd never have done something like that, and I find the solution quite creative. It's certainly a lot cheaper than replacing whatever part of the iMac would need to be changed.]
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Multiple display mirroring on a Mac Pro Desktop Macs
While not everyone has this problem, if you do a lot of video work, you may have come across it:

a) You have a Mac Pro with dual video cards, and each video card can drive two monitors. You use 2 desktop monitors and 2 additional monitors (such as projectors on HDTVs, which is how I use this setup).

b) You want to turn on Display Mirroring, but not the expected way, where all four displays to show the same image. I wanted Monitor #1 to mirror to HDTV #3, and Monitor #2 to mirror to Projector #4. Typically, once all the displays appear in the Displays pane of System Preferences, all mirroring does is display the output of Monitor #1 on all four devices.

After several months of talking with Apple Geniuses, consulting multiple AV companies and purchasing useless video splitter boxes, we accidentally discovered a solution to our problem.

Open System Preferences > Displays. Hold down the option key while you drag any monitor icon on top of any other monitor icon. You will see those two icons layered on top of each other to imply mirroring. Now you can selectively mirror any monitor to any other monitor.

We have successfully used this on Snow Leopard and Lion, on Mac Pros.

[kirkmc adds: I'm a single-monitor guy, so I can't test this. If anyone has four monitors and a Mac Pro, feel free to tell us, in the comments, if it works correctly.]
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Best way to cool your iMac Desktop Macs
My iMac killed its hard drive the second year of use and then the entire screen/graphics card and logic board had to be replaced at the three year mark. It unfortunately began to misbehave again shortly thereafter.

I use mine to run a 54" Bravia HDTV as the extended desktop and use EyeTV and noticed numerous graphics errors lines, and failures to refresh the screen during overheating bouts. I observed that the upper left hand corner of the iMac has some sort of an air pocket that traps extreme heat from the graphics card there. (The rear-viewed 'right' of the top air exhaust slot).

This top left hot air pocket-trap seems to be due to negative cabinet air pressure interfering with natural heat convection on the left side; heat collects there but just can't get out! The fans are part of the problem and just don't help at all.

To verify this, I installed a small temperature probe in the back top left hand (from the front) side of the back air exhaust slot (right side viewed from rear) around 20 cm (9") from edge. The usual temperature reading there runs around 130F (54C!) and often (always during malfunction or addressing error graphics lockup-freeze) exceeds a peak-hold temperature of 140F (60C).

At first I mistakenly thought forcing cooler air into the bottom might reduce this but it does not. There is an air pocket there caused by the vacuum back-pressure of the internal fans, which are all trying to suck air in through limited air inlets, and blowing out the upper right side of the slot so heat just accumulates there and it just runs away

The correct and ideal simplest way to permanently fix your overheating 24" Imac (iMac6.1 iCore 2 Duo at least, and several other models) requires two fixes.

First download and install smcFanControl 2.2.2. Set it up to provide (still quiet) minimum fan speeds of 2200 rpm for the HD and 2800 rpm for the CPU, leave the Optical Drive fan at 600 rpm to minimize dust accumulation (or just tweak it up if/when you will be using it much)

Next obtain a 12 vDC enclosed squirrel cage fan like the San Ace B76 - you need not buy a new one nor this particular (best) model, but any enclosed suction-blower fan of this type (rather than an open blade straight exhaust) is ideal. It shouldn't cost much at a surplus shop. Get and hook it up to any common 9vDC/AC adapter (around 10 vDC) to run it cooler and more quietly than full 12v speeds, since it is now sucking heat instead of blowing cold air.

Tape it (with clear boxing tape) to the rear left corner of the back (3" from the side) so it sucks air out of the hottest point of the back slot and blows it away upwards.

Your iMac will now be fixed - the air slot temps will seldom ever exceed 106F (40C) again (even under heavy graphics use) and your graphics card will stop cooking.

Barring other serious dust or other heat sink assembly issues, your iStatNano display should now (roughly) read (degrees C): (at 26C Ambient room temperature)
  • HDD 42
  • CPU 33
  • GPU 40
  • GPUD 46
  • GPUH 38
  • Amb 26
  • DVD 1 39
Always blow out your system with a vacuum's air exhaust hose by blowing air pressure into the back slots and the small round center air exhaust under the desktop stand (to reverse-flow and force air and dust back out the bottom intakes) with power off once every six months or so to keep the air passages clear. Suction alone (out the bottom) does not do such a good job, and may actually draw dust further into the machine if used from the top at the heat exhaust slots.

[crarko adds: I haven't tested this one. Has anyone else needed to do something like this to prevent over-heating? If so, please post your experience in the comments section.]
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Indicator of iMac sleep status Desktop Macs
On an iMac use the Caps Lock key to indicate whether or not the computer is awake. It is useful for those that use their iMac as a wireless access point to quickly check the status of the computer.

If you leave the Caps Lock active, the little green LED will be glowing when the machine is awake and go dim when the iMac sleeps.

[crarko adds: I tested this, and it works as described. This should work with the built-in keyboard on a laptop, as well. I don't use any of my desktop Macs this way but do use the MacBook as an access point when I travel.]
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Quick access to System Preferences isn't gone Desktop Macs
You can open several frequently used System Preferences directly in 10.5 and 10.6 with a simple keyboard shortcut.

If you have an Apple keyboard (or a third party one with similarly branded function keys) you can bring up various system preference panels without using your mouse at all.

Simply hold down the Option key combined with one of the Apple-branded function keys to bring up the related preference panel. So Option+F1 or Option+F2 will bring up Displays.

On the newer keyboards Apple changed the layout of a few of these function keys so they could include iTunes control keys; Option+ those f-keys used to bring up iTunes (under 10.5 and maybe the early versions of 10.6?) but that does nothing now.

Option+ any of the volume keys will bring up the Sound System Pref, and Option+ any of the Expos function keys will bring up the Expos/Spaces System Pref.

On a related note -- those newer Apple keyboards no long designate F7 as a toggle for display mirroring/extended desktop modes -- but you can still do that toggle without going into the Displays Pref panel; just hit Command+F1.

[crarko adds: I haven't tested this one. I mostly prefer the older style keyboards, and in fact I'd use the old ADB Extended Keyboard if I could.]
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Yet another method of ejecting stuck CD/DVD media Desktop Macs
I had a DVD in my iMac that would not mount, and would not eject. After Googling around and trying various methods, I ended up calling Apple Support, and got this tip that I had not run across in my searches:

Boot the iMac while holding down the Option key to get to the boot menu, where the start up drives are displayed. Press and hold down the Eject key on the keyboard, and the stuck disc should eject. This tip worked great for me; I hope it's helpful to others.

[robg adds: The topic of stuck media has been quite popular here in the past. In an effort to turn today's tip into a one-stop reference for all the potential solutions, here are the methods covered in past hints:
  • Power up the machine and let it sit [hint]
  • Use Open Firmware [hint]
  • Use FireWire Target Disk Mode [hint]
  • Use disktool and System Profiler [hint]
  • Grab it with the shutter from a floppy disc [hint]
  • Put your Mac in a cool location for a few minutes [hint]
  • Sleep then wake the Mac [hint]
  • Flip the machine upside down [hint]
  • Zap the PRAM [hint]
If none of the above methods work, it may be time to break out the screwdriver and begin disassembly! If you've got a method that's not covered in the above alternatives, please post it in the comments...though what's possibly left, I can't imagine.]
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One way to fix an overheating graphics chip in an iMac Desktop Macs
My late 2006 iMac started to freeze and have graphics issues after about 18 months of use. It turned out that this was an overheating problem experienced by a lot of users. smcFanControl made the computer noisy, but didn't stop the problem, especially in the summer. I'd had the iMac sitting (unused) on a desk for a while, and was going to throw it out...but instead, I tried making a few ventilation holes, as seen in this picture.

If you want to do this yourself, you'll have to locate a service manual for the iMac (20-inch Late 2006); try a PDF search on Google. Follow the instructions in the manual, and take almost everything out of the computer. Next, drill some holes in the back with a hole saw -- the important holes are marked in the above-linked picture. Finally, install smcFanControl, and crank up the fans.

This modification makes the computer loud, but the processors consistently run at 35 Celsius, and the computer doesn't crash. I haven't tried turning down the fans yet, but it's been running for weeks without a problem.

[robg adds: I would not advocate this solution myself -- I'd either take it in for service, or donate it to someone else who could have it serviced. However, to each his/her own, and the above solution is basically free.]
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10.6: One possible cause of 'random sleep' in Snow Leopard Desktop Macs
Shortly after Apple issued 10.6.1, my 2009 iMac (2.93 GHz) began to randomly sleep, sometimes quite often, sometimes with hour-long intervals in between. The kernel began logging localhost kernel [0]: Previous Sleep Cause: - 101 after each incident.

None of the three Apple sources I contacted would precisely identity this error code. There was no reference to it that I could find on the internet. The standard reply I got was that errors logged with a negative sign are abnormal, which has been documented, and that although Apple's engineers were aware of it, the exact meaning was closed-source. I was told, however, that this indicated a "partial kernel-panic."

In each case I told them I had no idea what they were or weren't allowed to say, and I didn't care -- I was just trying to fix the problem. I tried every possible method of booting Snow Leopard in both 32-bit and 64-bit modes, but couldn't solve the problem. Oddly, I wasn't able to reproduce the same random sleep with a re-install of 10.5.8.

Still under warranty until May of 2010, Apple made the repair by replacing the Main Logic Board of the iMac. It was part number 661-5133, the total repair cost would have been $1100, and it took the Apple Store in University Village just under one week. So if you're experiencing odd random sleep under 10.6 but not 10.5, it seems you may have a defective logic board, and it's probably worth a service call.
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Use a G5 as headless port/drive extender and MIDI server Desktop Macs
The new Macbook Unibody has about five ports. In my case, all of them are full all the time. Unless you buy a hub, why not use that defunct G5 for additional ports and devices? You may save some energy by not relying on a large number of external devices (keyboard, monitor, hard drives, etc.) or AC power. I was also tired of the issues with a USB powered Oxygen 8 keyboard from M-Audio. Using my G5, the VNC access is actually somewhat tolerable for light operations at reasonable resolutions. If you have your Mac configured with a SATA II card and an internal drive mounting system, you can have access to more drives at decent speeds.

These instructions cover setting up a FireWire 800 link, a MIDI Network Link, VNC access using Vine Server, which is much faster than ARD. Also, you can easily use your G5 as a bridge to connect to two-pin devices (most video-cameras). Latency is minimal and the data path is more than sufficient, if all other interfaces are disabled it is also somewhat secure. These instructions also go over the issues I have encountered, and specific Cocktail features that aid in VNC/file access.
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New logic board for older G5 iMacs with bad capacitors Desktop Macs
Not so much a hint, but a suggestion which hopefully will help someone else. My iMac G5 (1.8 GHz, 2005 model) started having frequent kernel panics which I quickly realized was hardware related, as it persisted when booted from an external drive. When I opened the machine, I saw the dreaded bulging capacitors.

This was a known issue with a defect in the manufacturing of these capacitors, and Apple had a program to replace the motherboards when the capacitors failed. As I learned when I called the Apple rep, the program just ended in January and there was "nothing to be done." As I explained to the rep, I was being penalized for my machine lasting a bit longer than others, which made no sense. In fact, since there was no recall issued, I couldn't get my machine repaired before it failed, so Apple really had no choice in my opinion but to replace the defective motherboards. The rep agreed, and set up an appointment for my free replacement.

Hope this helps someone else who machine fails in the near future.
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