Imac 21.5" Mid 2011 power supply schematic

Bruno09

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Aug 24, 2013
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Hi all,

I'm trying to fix an iMac 21.5" mid-20011 power supply unit.

Therefore I am looking for a schematic, made some search on the Internet, to no avail.

Could some one help please ?
Thank you in advance.

BTW, the AC input is shorted, I see a 0 Ohm resistance.
It is supposed to be in the 1 MOhm range.
 

FrancoisC

macrumors 6502a
Jan 27, 2009
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Montreal, Qc
If the A/C in is shorted, there is a good chance that one of the diode in the diode bridge is blown. If it’s just that, you don’t need a schematic to fix it I think.
 
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Bruno09

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Hi,

thank you for that.

I have no idea how this unit works, and there are many diodes on this board.

I will try to check them.
 
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FrancoisC

macrumors 6502a
Jan 27, 2009
524
265
Montreal, Qc
Here is a picture of a “standard” diode bridge, the first stage in an a/c to d/c conversion. When you’re lucky, the physical layout ressemble this diagram (4 diodes in a squarish layout). I presume you know hot to test them?
 

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Bruno09

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I have very basic knowledge about electronics.

Yes I know what a diode bridge is and how it works.
On this board, in the AC area, I can see 4 small identically sized diodes, not in a squarish layout though, marked D31, D32, ZD36, ZD39 (does "Z" stand for zener ?)
When using my multimeter in diode mode, the 4 diodes give me a ~700 (0.7 V ?) value.

Then I have 3 big diodes D1, D2, D3.
They all give a zero reading on the multimeter.

Then in another area (still in the AC zone) I have 2 other small diodes, D33 and D39, both give : 560.
 
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b06tmm

macrumors regular
Jul 23, 2009
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South Louisiana
Bridge rectifiers are typically a single component these days. I would just get a new power supply they are available on eBay and Amazon.

Power Supply, 205W 661-5299
 
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Bruno09

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Thank you.

I have already replaced the power supply and the Mac runs fine.

I am however trying to fix the old one.

I actually have located the rectifier which looks exactly like this :

Capture d’écran 2017-07-29 à 16.35.07.png

Wherever I check, all seems to be shorted, there is 0 Ohm everywhere (in the AC area, which is marked "Primary" on the board).
 
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MacTech68

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Mar 16, 2008
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Australia, Perth
IMHO, if the bridge is shorted, there's more damage there than meets the eye - I'd also find it odd that the bridge is shorted without the fuse being blown.

If you've not repaired Switch Mode Power Supplies before, it's not something I'd be attempting without a lot of assistance. These beasts can give you a hefty jolt when working, and even when unplugged for quite a while with certain kinds of faults. :eek:

In my (limited) experience of late model iMac Power Supplies specificlly, a bad, small, electrolytic capacitor in the pulse width modifier is often the culprit for a seemingly dead power supply.

Any earlier symptoms (before complete failure) may help the diagnosis. Also, it seems there may be different manufacturers used so a pic may help.

Just my 2c
 
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Bruno09

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Aug 24, 2013
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Thank you for your help.

The fuse (6.3A/250V) is blown.
So I am checking the input resistance after the fuse.
On the working power supply that resistance is about 1 MOhm (I no longer remember exactly, but it was in the 1 MOhm range).

There is not visible physical damage anywhere, all looks good.
I have unsoldered one pin of two big capacitors to check that they are fine, and they are.
(yes I have discharged them before checking, just in case)

I was planning to repair this power supply only if it is something obvious and easy to do.
I am not skilled enough to investigate more, so I will simply give up.

Thank for your help anyway !
 
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MacTech68

macrumors 68020
Mar 16, 2008
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Australia, Perth
There are still people selling their faulty PSUs on eBay - so you may make some $$ there.

Usually, I would expect the main switching transistor(s) to have shorted. That usually takes out the bridge, fuse, and possibly the pulse width modifier IC and some associated passive components. By the time you source them (for a single repair) it's often not economical.

People who fix them usually bulk buy the parts for common faults and as a result can repair them relatively easily.

I'm intrigued enough to know if the faulty one is an ACBel or Delta branded.
 
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