iMac 27'' Blown Power Supply Repair

[mitchw]

I recently had a second look on a blown iMac 27'' 2011 PSU (they used the same PSU in 2009 to 2011 year 27'' models).
It is a LiteON PA-2311-02A 310W power supply (Apple P/N: 614-0446).

I tried to repair it about a year ago, but due to lack of time and the sheer amount of blown components, I just replaced it with a new one.

However, I still wanted to repair it. The fuse was blown, so I replaced it. Used a 75W dim bulb and it glowed brightly when plugged in. Next, I checked all the components, and there was lot of shorted things.

I replaced bridge rectifier (shorted), PFC diode(s), PFC MOSFET, two main switching MOSFETS, PFC driver IC and perhaps something more, but I think that was it. However, when I plugged in with a variac at 80V, the fuse blew again, however, no further damage was done. But even when I checked all the components again, I was not able to find any shorts.

When powered via a 40-600V DC bench PSU (limited at 60mA), it starts to draw excessive current when the input voltage exceeds 110V DC. As the bench PSU is current limited, the iMac PSU "pumps". After a minute or so, the PFC driver IC gets warm, but nothing else. I tried the old PFC IC and the situation is the same.

Now, I am wondering, does perhaps anyone has a schematics of this model?
What checks should I do further?

[guruji]

Re: iMac 27'' Blown Power Supply Repair

repairing this power supply is tricky first you need to get 392v at the main cap that will tell you you pfc working remember the pfc works in tandem with l6599.please check you mosfets and bridge diode for leaks.also many time the small sot23 transistor go bad 2n4401 and 2n4402 p-ch and n-ch.schematics for this you don't get so you need to do a cold check with lots of components thats how i get them made but they are repairable no problem.Best of luck!!!

[Quaddro]

Re: iMac 27'' Blown Power Supply Repair

Shorted primary, my favorite.
But sometimes give me headaches too..

fuse blow or anything like that always mean shorted components.

Did you tried with bulb after you've change the components?

ic like 5vsb generator and offline switcher sometimes be the culprit of this problem.

Try to pull off the ic, one and another.
Then try to turn it on with light bulb connected.

So you can locate the real shorted component.

or, you can just simply connect it and let the bulb on.
It will warm the components.
The hottest one usually is the problem.

[satish s]

Re: iMac 27'' Blown Power Supply Repair

hi..my i mac 27inch power supply board (lite on pa-2311-02a) at primary side auxiliary voltage is not constant ..variation from 8v to 17v continuously i am unable to fined the problem in circuit board can any one help me with secahmtic circuit diagram Thanks advance

[sam_sam_sam]

Re: iMac 27'' Blown Power Supply Repair

hi..my i mac 27inch power supply board (lite on pa-2311-02a) at primary side auxiliary voltage is not constant ..variation from 8v to 17v continuously i am unable to fined the problem in circuit board can any one help me with secahmtic circuit diagram Thanks advance

Please post some pictures of the board in question this will help us to be able to help you with this board

[PeteS in CA]

Re: iMac 27'' Blown Power Supply Repair

hi..my i mac 27inch power supply board (lite on pa-2311-02a) at primary side auxiliary voltage is not constant ..variation from 8v to 17v continuously i am unable to fined the problem in circuit board can any one help me with secahmtic circuit diagram Thanks advance

Sounds like the source of that primary auxiliary voltage is starting up and then shutting down. I'm not familiar with the specific power supply, but if it has a separate auxiliary regulator for the 5V standby and internal housekeeping voltages, the problem may be there. I'd check to see if the 5VSB is shorted to Return, if the small value electrolytic capacitor filtering the auxiliary voltage is bad, or if the controller IC is not getting feedback voltage.

On the other hand, if the primary and secondary housekeeping voltages come from the main transformer, then the problem is probably in the main PWM circuit. I'd check the small value electrolytic capacitor filtering the PWM Vcc and the high value resistor that charges it during start-up.