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PSU voltage: 12 V stable - 5 V NOT stable

Hi ....

i hope that someone here can help me.

I habe a problem with my G4 that won't turn on.

I tried everything i found on the web including change of 3.6 LiIon Battery and all the reset stuff.

I ended up with disassembling the G4 and took a look at my PSU.

I measured some of the voltages ...

AC / DC Board gives me more than 400 V of a stable DC voltage

DC / DC Board gives me a stable 12 V voltage between one black (GND) cable and 4 of the yellow cables of the mainboard-plug.

But between the black cable and the blue cable i get voltages between 0.8 and 3 and just sometimes up to 5 V .... it isn't stable at all .

I suppose there should be a stable 5 V voltage between the black and the blue cable??

I tried changing some of the capacitorc on the DC/DC board (2*10uf and 1*1uf) because i heard that they often are responsible for PSU problems but it didn't change anything.

Should i try to change the other capacitors too ?

Or could there be any other reason for the unstable voltage ?

Hope someone of you can help me

Thanks in advance

Oliver

Answer this question I have this problem too

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Obi B, not sure where you got the 400V. May be you can post a couple of images and show us where you are measuring your voltages. Take a look at the attached image and see if you get those voltages.

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Hii oldturkey03,

thanks for response, sorry for late answering.

since 3 days iMac is up und running. Don't know what exactly solved my problem.

Changed 3 capacitors on DC/DC board i described in my first post.

Resoldered soldering joints from onboard connector on DC/DC board

Used contact spray on mainboard-cable-plug (blue cable)

checked if cables were well fixed in the plug.

Reasembled, now it's running!

@measuring voltage:

PSU in my case (G4 15 inch) consists of two parts.

1st part: The AC/DC board that converts ~230V AC to ~418 V DC and is connected to the DC / DC board with a two-core-cable.

2nd part: The DC / DC board that is supplied by the AC/DC board and has a 14pin connector that connects to the mainboard/DVD&HDD by cable.

My plug:

7 black (bk) on one side

4 yellow (y), 1 blue (b), 1 yellow, 1 red (r) on the other side

X = plug connector lock

y(2) bk (1)

y(3)

y(4)

X y(5)

b(6)

y

r

Again sorry for late answering, thanks

Oliver

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Obi B will be eternally grateful.
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