testing imac a1207 power supply

[frenchscottie]

I have a imac a1207 power supply with the 5 wires output
2 greys = 12v
2 Blacks = ground
1 brown = PFW 3v - 6.5v to turn on
I think.

So do I need to put a 3v supply to brown and a black so that I can check the 12v legs?

I'm new to imacs not repaired many.
This one had a blown power supply and I got a replacement but still not powering on, so I want to check the replacement power supply.
I suspect the logic motherboard is damaged too but want to make sure.

 

[Markverhyden]

You check the trickle voltage against ground. No voltage, bad PS. If you get voltage then jumper trickle to ground, this simulates the power button if you have the PS all of the way out. Otherwise use the power button and see if you get voltages in other areas.

 

[frenchscottie]

So with just the mains connected I shouldn't get voltages?

Is the brown the trickle voltage, and is it supplied from the cmos battery - or whatever it's called on an iMac?

 

[Markverhyden]

Can't say specifically about that power supply. But that is the way it is with other power supplies that I have seen. All the mains are dead unless the trickle is shorted by the power switch board. How else would they power down HD's, etc?

I do remember doing the same type thing with G4/5 power supplies.

 

[frenchscottie]

You check the trickle voltage against ground. No voltage, bad PS. If you get voltage then jumper trickle to ground, this simulates the power button if you have the PS all of the way out. Otherwise use the power button and see if you get voltages in other areas.

Sorry I think i'm not quite getting it.
You say "check the trickle voltage against ground. No voltage, bad PS."
But the trickle voltage comes from the battery so if no voltage, surely that's nothing to do with the power supply.
I think I'm missing something.

 

[altrenda]

Here is some help from Apple.
http://support.apple.com/kb/HT2173


That power supply has a history of capacitor problems. If the one you bought was a pull and not a refurbished one, chances are it is bad as well.

 

[Markverhyden]

Sorry I think i'm not quite getting it.
You say "check the trickle voltage against ground. No voltage, bad PS."
But the trickle voltage comes from the battery so if no voltage, surely that's nothing to do with the power supply.
I think I'm missing something.

As I said I've not seen this machine. But all of the other power supplies I have looked at get the trickle voltage from the power supply. Otherwise you would not be able to test it without a logic board. So I'm pretty sure it is the same for the one you are looking at.

The cmos battery is for preserving certain settings, including running the RTC, when the machine is not powered up. Apples also have PRAM, parameter random-access memory, which stores many of these settings and the battery also supports that device.

 

[frenchscottie]

Well that's weird I just reconnected the power supply and it kicked into life.

 

[trevm999]

What I used to do

One lead on the grounded screw. The other testing the contacts on the output port. One should give voltage. Maybe you had a loose connector, or the PS is still bad but issue is not always consistent.

 

[frenchscottie]

I've checked it for loose connections, dry joints etc, all seem ok had it on and off several times, moved it about i.e twisted it etc but all seems ok.
Funny thing was, when it did kick in, it started up straight away as soon as I plugged it in, without pressing the power button but every other time,s been ok.

Oh we'll I'm off out to get ******.

 

[trevm999]

Probably just needed an SMC reset. How long would you guess it was without power before you got it? Used to always have people who brought in their iMac that wouldn't power up only to have me plug it in and it turning on fine.

 

[frenchscottie]

Probably just needed an SMC reset. How long would you guess it was without power before you got it? Used to always have people who brought in their iMac that wouldn't power up only to have me plug it in and it turning on fine.

The power supply was dead for about 8 days as they had waited till I got back from my holidays.