Posting this after a VERY frustrating week on a customer's dead 2011 iMac 27"....... Hope it helps someone..... Symptoms: iMac 'dead'...... no power, no fans, no chimes, nothing. Try with new mains lead - nothing. Open it up: Mains power tested & present at the PSU but no DC volt output & no diag' LED number 1 ( no standby trickle)........ bad PSU?....... Tested start button was functional with multimeter..... Tried shorting the two start button pins directly on the MLB socket........... Nothing. Try machine in minimum configuration (i.e. disconnect the various peripherals in case a short is preventing the MLB starting up ) - NO difference. Disconnect output multi-pin connector from PSU and test for DC outputs directly on the PSU output pins......... all pins at 0V........... bad PSU?......... Changed power supply: - NO difference. According to Apple diagnostic trouble shooting - now change logic board - not convinced this is an entirely logical step but had one here so did so - NO difference..............

Essentially as follows:- (for the 23") ( 20" and 30" are similar in principle):- 1) plenty of workspace / soft towel or similar on bench 2) Remove the two large-ish round finisher screws on either end of the hinge (hex key) 3) Remove the 4 small fixing screws under the hinge ( Torx) and ease the foot/hinge assembly off the screen. Springs etc all stay in place and the foot is easy to put back on afterwards. 4) Carefully ease off the two plastic side cheeks ( thin craft blades or similar ) starting at top or bottom edge. CARE - they are held by thick super-grip pads & will break if forced. NOTE the small cable to the power button on RHS. Warming with hair drier can help. 5) take the 4 screws out of the long metal plates now exposed on each side and lift the plates through 90' to disengage the hooks. NOTE how they went on and put to one side. 6) Locate the two small metal tags now exposed at the lower edge on each side - pull outwards about 5mm in order to disengage the catch assembly along the lower edge....