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Model A1312 / Mid 2010 / 3.2 GHz Core i3 or 2.8 & 3.6 GHz Core i5 or 2.93 GHz Core i7, ID iMac11,3

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Testing power supply for working system

I've read a number of messages here about iMac power supplies, but they all seem to be about completely dead systems.

My mid-2010 27" imac powers up and boots, but it's performance is unreliable. It will boot very slowly, and trying to install an OS to it (El Capitan up through a Patched Catalina) will run for a time, but the install will eventually hang anywhere between 20-80% of the way through. Interestingly, the same failure happens with Fedora 39 and 40. I did get Mountain Lion (10.8) to install, but couldn't get it to upgrade to a newer version. Also had LMDE6 (Linux Mint) install but that eventually locked up after 3 hours, after which it would lock up within 5 minutes every time I restarted it.

It has new memory, SSD, I cleaned out the cooler fins and replaced the thermal paste. Currently I have been running it with the LCD removed (forcing it to display to an external monitor) so that I can have a fan blowing on the system boards. Running from a pre-installed Catalina (patched) disk it ran for 15-20 minutes and suddenly dropped power (and booting to a HDD on the SATA connector still took 15 minutes)

Long-winded explanation, but it leads to my question on the power supply. I have read that a failing power supply can cause some of these symptoms. The problem is, I don't want to throw more money at it trying to make wild guesses at what's wrong (the parts I've bought thus far can be used in various other machines). I would like a more determinate means of identifying or eliminating the PSU as the culprit (it could just as readily be the main board or the GPU).

As al alternative to replacing the PSU, is there some way to temporarily adapt an ATX power supply to power the iMac? I would think it would be a good way to test the rest of the system taking the iMac power supply out of the equation.

If nothing else I could do a monitor conversion, but I'd have to find extra-short cables to move the I/O ports to the back of the shell.

Update (05/09/24)

I'd need to pull the board to check the caps (would be a few days). I did upgrade the CPU, although the system was having it's issues with the original CPU. The RAM that was in it when I originally bought it was bad (it wouldn't boot, and the beep code indicated bad RAM). I had initially tried it with RAM from a couple of MacBook Pros I had upgraded (2010, 2012), then bought some Crucial 8G RAM. Again, same behavior before and after the RAM swap. Yes, it would be so much easier if it started failing after I swapped parts. Although I should check that the memory from the MBPs wasn't already aftermarket.

The SSD is a 1TB PNY SATA III (I had thought a SATA III drive merely would be limited to the SATA II speeds). Originally had a Western Digital WD1001FALS.

  • Original CPU: Intel Core i5-760 (2.80GHz)
  • Replaced with: Intel Core i7-870 (2.93GHz)

And I didn't mention that yes, all fans are working.

And I'm thinking that if I'd need to remove the boards to inspect them anyway, I could add an eSATA port on the back to connect to the extra SATA connector. Would be faster than USB2 or FireWire. Has anyone tried that before? Maybe this one from StarTech

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@senileotaku41519 - Well... Yes and No! When SATA III spec was introduced the assumption was people would be mostly transferring their older drives into a new system as drives back then where more expensive than the systems logic board (desktops ruling at the time). As drive prices dropped and SSD's became more common and to add to this drives where getting quite fast! We started to encounter buffer runout issues with SATA drives (which is likely your issue) as the I/O data flows where exceeding what the system could take in.

So the original fixed speed drives where often returned to the vendor as defective when if put in the higher I/O system wouldn't be an issue. Then came Auto Sense technology! You first saw it in networking when 100mbps nodes where mixed within a hub with 10mbps. That same tech was used within SATA with the intro of Auto Sense drives! That allowed people with older systems to use the newest drives.

Now some time has past... The need of Auto Sense has waned as better than 90% of the systems now are SATA III and as costs are always being looked at by the bean counters the tech has mostly been removed by many drive makers! So you need to review the given drives spec sheet to see what it says. As an example this PNY drive is auto sense CS900 Series

Note the Interface line >> SATA-III (6 Gb/s); Backwards compatible with SATA-II <<

You'll need to check out what the drive you have installed is as PNY has both fixed and auto sense drives!


Yes, this is a CS900 SSD (SSD7CS900-1TB-RB). Yesterday I tried this machine with a SATA-II HDD and SATA-II SSD. Ran fine for a while, but occasionally it wouldn't boot from the drive (getting 40-60% of the way through the boot process before hanging). Booting can easily take 10 minutes for it to eventually come to the login screen.

I had built the High Sierra install on the HDD on my 2010 MBP (not optimal I know) so I could get around it's problems with installing.

So still the same thing here; how to determine between the PSU, mainboard, and graphics card which one is failing. No one around here I could try swapping parts with. This is where something like the Pico-PSU would be a good test tool (would have to make a board that split out to the appropriate connectors) or even an outright replacement. (not my field of expertise)


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Assuming you've done the basics like resetting the SMC/PRAM, probing the power supply could be the next thing to try. I'm having trouble tracking down a pinout diagram for your model, but it looks like the late 2009 - mid 2011 27" iMacs all use the same power supply, which means that this post referencing the mid-2011 model should help with the pinout.

It may be hard to probe the power supply rails with the computer running, but I would specifically look out rails that are below or above their specified value (ex: 12V rail reading 11V).

A few other questions that may help debug:

  • Are any capacitors on the logic board leaky or buldging?
  • Does this machine have its original CPU or has it been upgraded?
  • Can you remove / swap around some memory, or install the original memory the computer shipped with? Sometimes these machines are finicky with aftermarket RAM
iMac Intel 27" EMC 2309, 2374, 2390 or 2429 Power Supply Görseli


iMac Intel 27" EMC 2309, 2374, 2390 or 2429 Power Supply


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You made note of replacing the HDD to a SSD who’s did you install? As your issues sound more like a SATA speed mismatch. That’s when the SSD drive is too fast for the systems SATA port.

Here we are talking about using a fixed speed SATA III (6.0 Gbps) drive in a SATA II (3.0 Gbps) system.

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