This might be a bit specific, but I am trying to test a trashed A1208 Intel iMac (Late 2006 17" Model). The power supply is dead (Fuse was blown, bypassed with a wire and bang poof a resistor and ic blew), but everything else (logic board, DC/DC Board, LCD) looks good.

I will be connecting a PC power supply (for testing) for the 12V and Ground, but I do not know what voltage is present on the Power Good signal.

enter image description here

Pictured is the Power Supply (Top) and the DC/DC board (Bottom). The Power Good cable is the gold/brown cable, which can be measured at the switcher board rightmost pin (With Ground as the two next to it).

Can anyone help me out? Most models around the same time have the same type of power supply (Mains to High Current 12V, with 5 Pins, 2x 12V, 2x Gnd, PG)

  • The Service Manual does not have info towards this. Just the usual "just replace" bs. – cde Jun 6 '14 at 5:12
  • stay away from it!! since you newer replace a blown fuse with a wire. but since there was a IC in the circuit you can look it up and you are going to have to replace it anyway. – Ruskes Jun 6 '14 at 8:39
  • @Buscar웃 Bypassing a fuse is a common troubleshooting step if you understand the risk, which I did. Electrical Engineering is a hobby. The supply was as good as dead anyway. At this point, I just dont want to spend 50 dollars on a new supply just to find out something else is wrong. Knowing what a good supply does to the Power Good signal (Pull high or low) allows you to replace the stock power supply with something else. – cde Jun 6 '14 at 22:58
  • Why exactly the downvote? – cde Mar 19 '15 at 3:21
  • I tried the same with a 2006 Intel iMac. Attached it to a 12V 25A PSU - "plus" on gray, "minus" on black. Left the brown one open. The iMac takes 0.7 ... 1.8A - but doesnt boot. But if I got you right, it should? - Or does the logic board need any other info to trigger the boot sequence? Wolf – Spotlight Apr 13 '15 at 17:30

Solved this on my own. For reference this is the information needed. The Power Supply is a 12V 15.5A Power Supply, with an Inverted Power Good Signal.
Logic Low (Or Floating) = Good, Logic High = Bad (I don't know what voltage level), tells the computer to shut itself down.

This forum post from 2013 had a similar issue (weak power supply).

Left to Right, when plugged into the DC/DC board (Part 820-1934).
enter image description here

  1. 12V (Grey)
  2. 12V (Grey)
  3. Gnd (Black)
  4. Gnd (Black)
  5. PG [Power Good] (Gold or Brown, Varies)

The 6x2 connector on the DC/DC board goes out to the Logic Board and SATA Harddrive. All outputs look to be on when the connector is unplugged.
enter image description here

  1. PG (Power Good, Direct from Power Supply Connector)
  2. GND (To LB and HD)
  3. 12V (To LB and HD) Switchable with P_ON
  4. GND (To LB and HD)
  5. 5V (To LB and HD) Switchable with P_ON
  6. 3.3V Switchable with P_ON
  7. ?P_ON/Power_ON?
  8. 12V Always On
  9. 12V Always On
  10. 5V Always On
  11. GND
  12. 3.3V Always On

Pin 7 on the 6x2 connector that I labeled Power_On, Connects to transistor pair most likely controlling the IRF7424 P-Channel 10A mosfet. Logic Low (Or Floating) turns ON the DC/DC converter's Lower Outputs, Logic High (3.3V) disables it. The Motherboard pulls this High when the computer is OFF. The Harddrive Power is connected to the Lower Outputs.

The setup is fairly common. Some newer iMacs skip the DC/DC board and have that all built into the logic board, but still take 12V input.

I connected an old ATX power supply, with a 12V 7.5A rail, half the current of than the original. It's also out of spec (10V instead of 12V ±5%), so the hard drive wouldn't spin up. I left the Power_Good FLOATING. But the computer booted. Now that I know the majority of the iMac is working, I can order a replacement power supply without worrying about wasting my money.

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