imac G5 psu component level question/challenge.

[finndigg]

I was hoping to help my friend repair her iMac G5 17" 1.6g Rev A computer by changing the caps in the Power Supply and Logic Board. But when removing the caps on the power Supply I noticed a broken zener diode and feel it may be useless to continue until I identify and change that component first. The diode in question is labeled ZD4 on the circuit board itself. (I have a picture but I'm not sure how to post it)

And while I was able to find an Apple service manual for the model online it is of little help in that it doesn't address component level repairs.

Would anyone be able to help me accurately identify this component so I can replace it?

[etnietering]

Re: imac G5 psu component level question/challenge.

I'm sure somebody here (maybe me, maybe not) would be able to help you. To upload a picture:

Below the text window for posting, under 'additional options' there is a button that says 'Manage Attachments' Click this, and the window that comes up will let you upload a picture. To insert the picture into your post, right click the link to the picture you just uploaded and select 'copy link' or 'copy link location' Then click the icon of a mountain in front of a yellow sky, and paste the link you just copied into the box. It will automatically tag it for you. I find it useful to click the 'preview post' button to make sure I did it right before submitting my post.

[finndigg]

Re: imac G5 psu component level question/challenge.

Ok. Here's a few pics, though I doubt they will be very helpful. The first (zd4) is one I took with the camera on my laptop. The second is the best picture I could find on the internet - and I made some marks pointing to the component. I used a magnifying glass to look closely at the component beneath a good light but I couldn't read anything besides the ZD4 printed on the board itself. The model is iMac G5 17" 1.6g Rev A.

[finndigg]

Re: imac G5 psu component level question/challenge.

Oh forgot to mention, the caps are removed in the first picture.

[Toasty]

Re: imac G5 psu component level question/challenge.

3.3v, 1w

[finndigg]

Re: imac G5 psu component level question/challenge.

Thanks Toasty!

A few follow up Questions:

I heard somewhere that one should get something called a "temperature
compensated" diode for the purpose served here. Is there anything to
this?

Further, As with capacitors some brands are favored while others
are advised to be avoided. So with diodes, are any particular brands recommended over others? - any brands to especially avoid?

And finally any recommended places to purchase? Thanks.

[Toasty]

Re: imac G5 psu component level question/challenge.

>>I heard somewhere that one should get something called a "temperature
compensated" diode for the purpose served here.<<
Please state source of information/reference.

I heard somewhere that Elvis is still alive....

1N4728A - Fairchild, ON, TI - any reputable maker.

Wherever you get the caps* for this is fine - DigiKey, Mouser, Newark

*You'll need 13 without the mains, or 15 with.

Toast

[finndigg]

Re: imac G5 psu component level question/challenge.

This may be a silly question but I don't know much about diodes. When I looked up 1N4728A at Mouser I noticed that the zener current varies by component. Some are listed as 76 mA while others as .2A and yet others as 276 mA. Which of these would be preferable, if any?

[etnietering]

Re: imac G5 psu component level question/challenge.

Well power = voltage * current

so if power = 1W, and voltage = 3.3v, then you can calculate the current:

1W / 3.3v = 303mA, so I'd go with the 276mA one.

[Toasty]

Re: imac G5 psu component level question/challenge.

None - see below* for a correction. I had to grab one and test it and examine the surrounding components.

The series resistor is a 1mm chip @ 240 ohms.

The current your looking for is the zener holding current. At there and above, it holds the voltage constant.

* I'm going to recommend going to a 1/2 watt zener (500mw). The holding current necessary for my original recommendation is too high for the 1/10watt chip resistor. Instead go to this one which holds at 10ma:

Mouser #: 583-1N5226B-B // Manuf. #: 1N5226B-B

http://www.mouser.com/ProductDetail/...FPb2G3UpTEc%3d

At that current, the resistor is handling 24mW; 1/4 of its power rating.

Toast

[ratdude747]

Re: imac G5 psu component level question/challenge.

congrats on catching a broken diode. most people would not notice it and go straight for caps.

buy a few diodes... in case it blows up and your up the creek or you mess the first one up. besides, its cheaper in most cases.

dont forget to check your polarity on both diodes and caps. always take pics before you go in case the silkscreen is wrong/confusing/non-existent. you seem to have already so another thumbs up on that.

keep up the good work! feel free to ask questions- look for the member junk parts and read his signature... the mark twain quote is the truth so when in doubt, ask.

[finndigg]

Re: imac G5 psu component level question/challenge.

Thanks for testing, Toasty.

[finndigg]

Re: imac G5 psu component level question/challenge.

I put in the diode and new caps on the power supply last night. But I would like to test it before hooking the mother board to it. I have a pin read-out diagram for the unit that I found. I did a search on this forum and there was some talk of using a jumper (jumper the plug)? but I couldn't find anything specific on what was meant by this. I just want to do the right thing before I plug it in - any help kicking me in the right direction would be appreciated.

[finndigg]

Re: imac G5 psu component level question/challenge.

Well as best I could tell, what was meant by "jump" was 15 to ground so I used a snipped cap leg to bridge it - held my breath and plugged her in: no smoke and flames. My test results were normal except pin 4,6,20 and 21 read as 5v rather than 5.1v and pin 22 looked closer to 26v than 24. Are these variations normal/to be expected?

[Player2]

Re: imac G5 psu component level question/challenge.

Well as best I could tell, what was meant by "jump" was 15 to ground so I used a snipped cap leg to bridge it - held my breath and plugged her in: no smoke and flames. My test results were normal except pin 4,6,20 and 21 read as 5v rather than 5.1v and pin 22 looked closer to 26v than 24. Are these variations normal/to be expected?

That sounds reasonable to me. 5v/5.1v is pretty negligible. The 24v should be for the backlight inverter, and with a load it should read closer to 24v. Congrats on getting it working!