[stomachmonkey]

The 21 and 27 PSU's are different.

I actually have one of each opened up right now.

I'd take pics for you but not home for a few more days.

If you can wait I can help you out.

And yes, there is always 12v at the power button.

Hitting it opens the circuit to the PSU which is why the slo blo is the first stop.

I would not worry so much about exact voltages that you are getting as long as you are getting readings the PSU is OK.

I generally recommend not opening those iMacs. They can be fiddly, especially the 21's.

Most common reasons they stop working is someone opened it to do a HD swap and broke a lead.

Really dead nuts easy to do on those things.

No power to HDD is usually someone borked the ribbon cable.