iMac G5 20" Issues dispite replacing caps

[duffling]

Hi all,

I have recently replaced all the caps in a iMac G5 20" (first edition) and the bad caps in the power supply.(Delta PSU)

Idle heat is 48 degrees C and about 78 under load.

The machine had 6 visually bad caps but I replaced them all just to be safe.

After the caps were replaced the machine still shows issues (lines , shut downs etc) , it however does pass the AHT test without issue.

The caps were replaced and no damaged noted to the cap pads. Removal was done with a high quality solder station + desoldering station setup

I have noted flexing the case slightly causes the issues to appear. Pressing the caps or wiggling them to determine bad connects while the machine is running does nothing.

Would it be safe to assume the GPU has become detached (flex during desoldering) or prior to the removal of the caps?

Any tips ideas greatly accepted.

[weirdlookinguy]

Re: iMac G5 20" Issues dispite replacing caps

Is it an nvidia GPU by any chance?

[duffling]

Re: iMac G5 20" Issues dispite replacing caps

Yes it certainly is. It uses the NVIDIA FX5200. Its located under the CPU heatsink with the large G5 on it.

[weirdlookinguy]

Re: iMac G5 20" Issues dispite replacing caps

I don't know if that particular GPU was affected, but older nvidia GPU's and a lot of their chipsets are notorious for doing this.

[duffling]

Re: iMac G5 20" Issues dispite replacing caps

Well technically this processor was not a major culprit however board flex when desoldering plays a part. It's easy to push and pull trying to get the caps's out and break the BGA which I suspect has happened in this case.

It is a cap replacement done previously which worked and now is showing the same signs again however caps all are good.

It's either that or dry solder joints...

[Th3_uN1Qu3]

Re: iMac G5 20" Issues dispite replacing caps

I don't know if that particular GPU was affected, but older nvidia GPU's and a lot of their chipsets are notorious for doing this.

The FX5200 is old enough not to be affected. But if the solder did crack, you can always try baking the board... 7 minutes at 210C always does it for me. Do remove all electrolytic caps beforehand tho or you'll have popcorn.

[Toasty]

Re: iMac G5 20" Issues dispite replacing caps

That's a lot of work, and not just the new ones either...

Pull the heatsink and try a reflow on the chip à la Xbox 360 style.

[Th3_uN1Qu3]

Re: iMac G5 20" Issues dispite replacing caps

That's a lot of work, and not just the new ones either...

That's why i only like to do this to laptop mobos... All solid caps, nothing to blow up.

[Toasty]

Re: iMac G5 20" Issues dispite replacing caps

Solids have limits too. Reflow info limits temps and maximum time to somewhere around 3 minutes.

Going by the UCC guide, a 2 minute (max) ramp up to 180°C then a max of 40-50 seconds over 217°C with an over 230°C for no more than 20-30 seconds with a peak of 205-260°C max.



210°C is not enough to melt the majority of lead free solders. While saving the mounting of the on board components, you are just stressing those components without have a true effect on the faulty BGA.

Caution is all I'm advising. Preheating is essential, but using a (foil) shield over the other components and a direct heat application via a reflow station to the problem chip is, in my experience, safer.

Toast

[weirdlookinguy]

Re: iMac G5 20" Issues dispite replacing caps

Did I say older nvidia stuff was affected? I meant newer... my bad

[torgeson5kqs]

Re: iMac G5 20" Issues dispite replacing caps

I've tried the the reflow with a heat gun, foil and keeping within the time limits. However, same issue and the graphics vary from lines to plaid, etc. I would rather remove the GPU (nvidia 5200) all together and re-ball it. However, even after it's heated to proper temperature I still can not remove the chip. Has anyone actually gotten this chip off the board to do a proper re-ball?

[Toasty]

Re: iMac G5 20" Issues dispite replacing caps

Lines and plaid could be the panel or connections, not the board. Try hooking the panel to a different system.

Unless you have the tools and templates to truly reball the chip, I would not attempt to remove it. You are dealing with something that there is a very specific set of instructions for and does require the correct equipment. Perhaps a thousand tiny lead balls that need to be *precisely* placed and then pre-cooked to set in place. Then remounted using a specific temperature curve so that damage does not occur to other components and your efforts are not in vain.

Toast

[torgeson5kqs]

Re: iMac G5 20" Issues dispite replacing caps

I know its the nvidia chip because even with adding a second monitor I still get the issue and after heating the chip (post #11) the graphics look slightly different. I have 3 boards with all the same issue as this. They are basically worthless at this point unless the GPU can be replaced or re-balled. Seems like a waste to just toss them or sell as junk.

[Toasty]

Re: iMac G5 20" Issues dispite replacing caps

Keep an eye on eBay or Craigslist for busted screens and the like. Sometimes a good midplane can be picked up for cheap. Make sure you get it with a non-DOA guarantee. Or at least enough other parts value to make it worth it if it too, turns out to be bad graphics.

Biggest culprit I've seen so far is the heatsink cooling fins above the bottom fan are plugged with dirt, hair, and dust. These have it hard enough trying to stay cool.

Toast