These randomly goes out. Sometimes unable to click, sometimes click but no action. People have said tilt iMac upside down and fix it each time. Didn't work for me so had to do a real fix.
In most cases, the actual mechanical button works fine (check with a multimeter). There is a spring loaded washer between the button cap and the real button underneath. And the real button is mounted in a round metal plate like a potato chip. I'd guess either the washer lose its spring tension over time or the metal plate bent from hard pressing (that corner is a natural carrying position pressing against the power switch) which causes the spring washer to dislodge from its position.
Fix require removing the power button which unfortunately require complete logic board removal to pull the CPU fan and left speaker.
Make alignment marks before removing the metal button housing so it goes back in the same position.
Metal plate button housing is glued on. #4 X-Acto blade is a good way to cut the glue without distorting the the metal plate too much. Don't cut too deep as the button itself is in the center. Try not to bend the button circular metal frame too much. Will need to have good fit to glue back on later without tension to loosen the glue. Ideally, #4 X-Acto can cut around 360 degrees to minimize plate distortion but unable to due to button inset into the iMac chassis.
Scrap and pick off as much glue as possible with X-Acto + tweezer. Acetone will clean the rest off.
If didn't make alignment marks earlier (I didn't) Tape the metal button housing on to center it. Mark alignment marks for reassembly. You want the 2 dots on the white button membrane to be vertical
Need to bend the washer like a potato chip so it offers some spring tension. Also need to bend the button metal plate like a potato chip so it fits nicely in the chassis to avoid unnecessary stress. This is not so easy as the chassis has 2D curvature. Here is the curvature that finally provided good results
I used 7mm Tesa tape (super strong double sided tape but it wasn't as strong as the original) Cut 5 pieces in this pattern to stick on the chassis. Don't peel and expose sticky surface on the button side yet. Use adhesion promoter on the surface before laying down Tesa tape if have any.
Check alignment mark. Use the tape along the button cable length to secure it against the chassis. This eliminates 1 axis of movement.
Level the iMac, insert the dummy button (one with power sign on it), spring washer, and test fit the button housing to ensure all the alignment marks are good and get a good click. Apply adhesion promoter on the button plate surface that will be facing the Tesa tape.
Finally, lift up the button metal plate (but leave the cable tape in place for alignment. metal plate can just rest against the iMac frame and not get in the way (or get someone to help you) to peel the Tesa tape. Make sure the spring washer is centered and get 1 try at gluing on the button plate. If didn't glue on well, its possible to remove the button plate, remove the tesa tape (less sticky than original and be peeled off in 1 piece by tweezers) and retry.
Apply 200C hot air to the metal button plate to promote adhesion. Trim any excess Tesa tape beyond the button plate. Apply hot glue around the button plate. Its hard to get glue gun tip down low on the button plate. Use hot air to help melt and shape the glue to help secure the metal plate and flatter to not block speaker when re-installing.
After all done, works like new
In most cases, the actual mechanical button works fine (check with a multimeter). There is a spring loaded washer between the button cap and the real button underneath. And the real button is mounted in a round metal plate like a potato chip. I'd guess either the washer lose its spring tension over time or the metal plate bent from hard pressing (that corner is a natural carrying position pressing against the power switch) which causes the spring washer to dislodge from its position.
Fix require removing the power button which unfortunately require complete logic board removal to pull the CPU fan and left speaker.
Make alignment marks before removing the metal button housing so it goes back in the same position.
Metal plate button housing is glued on. #4 X-Acto blade is a good way to cut the glue without distorting the the metal plate too much. Don't cut too deep as the button itself is in the center. Try not to bend the button circular metal frame too much. Will need to have good fit to glue back on later without tension to loosen the glue. Ideally, #4 X-Acto can cut around 360 degrees to minimize plate distortion but unable to due to button inset into the iMac chassis.
Scrap and pick off as much glue as possible with X-Acto + tweezer. Acetone will clean the rest off.
If didn't make alignment marks earlier (I didn't) Tape the metal button housing on to center it. Mark alignment marks for reassembly. You want the 2 dots on the white button membrane to be vertical
Need to bend the washer like a potato chip so it offers some spring tension. Also need to bend the button metal plate like a potato chip so it fits nicely in the chassis to avoid unnecessary stress. This is not so easy as the chassis has 2D curvature. Here is the curvature that finally provided good results
I used 7mm Tesa tape (super strong double sided tape but it wasn't as strong as the original) Cut 5 pieces in this pattern to stick on the chassis. Don't peel and expose sticky surface on the button side yet. Use adhesion promoter on the surface before laying down Tesa tape if have any.
Check alignment mark. Use the tape along the button cable length to secure it against the chassis. This eliminates 1 axis of movement.
Level the iMac, insert the dummy button (one with power sign on it), spring washer, and test fit the button housing to ensure all the alignment marks are good and get a good click. Apply adhesion promoter on the button plate surface that will be facing the Tesa tape.
Finally, lift up the button metal plate (but leave the cable tape in place for alignment. metal plate can just rest against the iMac frame and not get in the way (or get someone to help you) to peel the Tesa tape. Make sure the spring washer is centered and get 1 try at gluing on the button plate. If didn't glue on well, its possible to remove the button plate, remove the tesa tape (less sticky than original and be peeled off in 1 piece by tweezers) and retry.
Apply 200C hot air to the metal button plate to promote adhesion. Trim any excess Tesa tape beyond the button plate. Apply hot glue around the button plate. Its hard to get glue gun tip down low on the button plate. Use hot air to help melt and shape the glue to help secure the metal plate and flatter to not block speaker when re-installing.
After all done, works like new
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