iMac 21.5" mid-2010 Won't Start

TidBITS Talk
1

I have been using an old iMac to run legacy software that won’t run on my newer machine. On Friday I experienced a power failure and the computer will not restart. I am assuming that the PRAM battery has died and once the capacitor drained during the power failure, the dead battery won’t allow it to start. I reviewed the instructions on iFixIt and replacing the battery seems pretty daunting. Is there a way to get the iMac to start without replacing the battery? Does someone know an easier way to get the battery than the process show in the iFixIt document?

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Before trying to replace the battery, try resetting the System Management Controller (SMC). From the Apple Service Manual for the mid-2010 21.5" iMac (pdf):

Resetting the System Management Controller (SMC)
The System Management Controller (SMC) is a chip on logic board that controls all power functions. If computer is experiencing any power issue, such as not starting up, not displaying video, sleep issues, or fan noise issues, resetting SMC may resolve it. To reset SMC on an iMac:

  1. From Apple menu, choose Shut Down (or if the computer is not responding, hold power button for approximately ten seconds until it powers off).
  2. Unplug all cables from computer, including power cord.
  3. Press and hold the power button for 5 seconds.
  4. Release the power button.
  5. Attach the computer’s power cable, making sure power button is not being pressed. Note: If you press the power button while inserting the power cord, the iMac will enter a mode in which the fans run at full speed.
  6. Press the power button to turn on the computer.
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3

jose, is the SMC involved in any way with a scheduled shutdown or startup? My Mid-2011 iMac sometimes doesn’t shutdown or start up on schedule. I always make sure to quit all running applications before leaving at night. I have CCC scheduled to run at 10 PM and the iMac to shutdown at 11 PM long after CCC has finished. I woke up at 1 AM once and found it still running so I manually shut it down.

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I’d forgotten that trick, but, alas, it didn’t work.

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I think it’s possible, though I think it is a relatively unlikely occurrence. Nonetheless, it can cause odd behaviors with powering on or off a Mac. In general, it’s safe to reset your SMC, so you can give it a try if you like.

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Hi Eric.

My slightly-educated guess would be that the machine needs a new power supply.

I have a 21.5" mid 2011 model (the year after yours).

In 2020, I found it wouldn’t power on.

I found an Apple service manual and followed the steps. There are some diagnostic LEDs inside. They were all off. I bought a replacement (recycled) power supply from https://www.thebookyard.com, fitted it, and it has been working ok since… touch wood.

Here’s the service manual one for the 2010 model. http://tim.id.au/laptops/apple/imac/imac_21_mid10.pdf The relevant symptom chart starts on page 31.

Servicing these machines isn’t easy. The bit I always find particularly difficult is getting the glass panel back on without dust specks. But it is possible.

Best wishes.

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