iMac not powering up after upgrade (RAM, SSD, CPU thermal transfer)

[orvn]

I did a few upgrades to a 2017 21.5" Retina iMac, and unfortunately, upon reassembly, it isn't powering on. Here's a photo of it, display disconnected. In the second photo I've labelled things for reference.

Status:

• In the photo, AC is plugged in
• Of the 5 diagnostic lights, the first one (bottom), is lit up. This indicates that the machine is receiving power
• The others don't LED's up at all (light 2 would indicate the machine is powering on)
• When I push the power button, nothing happens at all. The fan doesn't move, no sign of activity, like a brick.

What I repaired:

1. Switched out the disk with a new SSD (currently blank, was going to install MacOS from USB)
2. Switched out stock 8GB RAM with 32GB from OWC
3. Applied a [graphite cooling pad](https://www.enostech.com/innovation-cooling-ic-graphite-thermal-pad-40x40mm-review/) I had instead of the stock thermal paste on the CPU (I regret doing this because it was really difficult to align it on the non-level iMac surface)

My experience:

• I'm not necessarily an advanced hardware person, but my background is in computer science
• I'm a software developer
• I've worked with Apple products for years
• I've repaired a about 30 different Apple products (various iPhones, iPads, Macbooks, iMacs and iPods, although not too many in recent years)
• It's been a few years, but I don't think I've had a single problem in an Apple repair or upgrade before. I'm careful and I'm familiar with best practices.

Things that could have gone wrong

1. I'm really nervous about the thermal paste replacement, mostly because I've never used one of those graphite pads before. They seem to work fine for everyone though, so I tried it. I cleaned the copper heat sink and top of the CPU chip of all paste with 95% isopropanol. It took me _many times_ to align the stupid graphite pad well, and I can't say for sure that it stayed perfectly aligned when I attached the heat sink, because you're supposed to attach it, then flip it over to screw it tight (I did follow the cross pattern for gradually tightening the corners of heat sink pressure)
   
   
2. Although I looked at the iFixit tutorial beforehand, one of the things I didn't realize was that removing the heat sink from the CPU would also remove it from the GPU and its 4 VRAM chips. These 5 chips had a thick thermal paste (like K5-PRO) on them. I didn't reapply any additional paste when I closed them back up. I think this is normally okay, but now I'm not so confident.
   
   
3. For a brief moment the CPU fell (was stuck to the heat sink), and landed on the aluminum chassis of the iMac from a height of maybe 15cm (6"). I cleaned both sides with alcohol, inspected it. Seemed fine. Aligned it and put it in the CPU socket and continued.
   
   
4. I was really careful with the power supply and made sure that all connectors were fully in place. Actually I was very careful with any junctions or connectors, not touching exposed parts with my hands, making sure they were fully inset to click, etc.. But since the device isn't powering on, I'm a little nervous about the power supply.
   
   
5. There was one weird moment with the PSU. I cleaned everything with compressed air and, for a second, I thought I heard the PSU let out a shock sound when the air "froze" the surface of the PSU board. I didn't think much of it, but now I'm wondering if that is the kind of thing that can bridge a connection on a board and short something? Note that the machine hadn't been connected to power for several hours at that point, and I did try to hold the power button at the start of the repair to fully discharge capacitors.
   
   
6. The RAM was the right type, pins aligned and all. But the second chip didn't quite fit in snuggly. I really felt like I had to jam it, and still, the bevel in the corner of the chip was not perfectly flush with the groove on the outline of the RAM socket. It was really close though.

***

Normally these sorts of small potential mishaps don't matter to much in my experience. I accidentally touch an exposed pin? No problem, clean it with isopropyl and carry on. But here something clearly went wrong (a first for me), so I'm really second guessing everything.

I don't have any thermal paste, but I ordered some quality stuff (two different kinds, a thick one and a CPU paste) just in case I now need it.

These new machines aren't easy to work with, everything has to be taken out just to get to the RAM and CPU, so I'm trying to diagnose as much as I can before going back in.

Based on these 6 potential issues, and the diagnostic lights, which seem the most likely? What should I prioritize exploring in the interest of time?

 

[4sallypat]

I would reverse the work and restore the original RAM and hard drive.

Then I'd do a 10 second press of the power button.

 

[hobowankenobi]

Doubt the thermal paste is it. Overheating? Sure. Powering on? Doubtful.

Doubt the RAM is it. Even if the RAM thing was a problem, I would expect power to the fan. I would expect a tone or a KP. Powering up well past what if going on, and then failing POST.

Doubt the drive is it. Should attempt to boot even with nothing installed.

Even a CPU issue....I would guess would not prevent initial power-up steps, like the fan.

PSU or a connection seems like the most likely cause. I don't know if there is an easy way to test iMac PSUs.

 

[orvn]

Thanks for the replies, I'll troubleshoot issue-by-issue and go from there. Will prioritize PSU.

 

[turbomacs]

It's probably your CPU. The tension bracket thing is exceptionally tight. Remove the heatsink assembly. Repaste your stuff then turn that tension bracket over and bend it a little to loosen it up. When putting the screws back in, do them in a cross pattern 3 turns at a time until they're all tight.