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Power Mac G5 - Three LED Flashes with Good RAM


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Preamble: This investigative thread was first posted by myself on MacRumors on the 25th of November, 2013. Its aim was to provide information to the many Power Mac G5 owners in the PowerPC Macs forum experiencing no-boot or intermittent boot issues. To some here, it'll sound fairly basic, perhaps a little crudely worded at times, and may have some overlap with information already well known. It's a direct copy from MacRumors with only minimal alterations necessary to transfer it between the boards, but has not been altered in any form specifically for the 68kMLA.

 

Several years ago, I owned a Power Macintosh G5, a 1.8GHz Single Processor. The machine was a first generation, codenamed "Omega" and manufactured some time between June 2003 and June 2004.

 

When I acquired it, the machine would power on without a chime, and the LED on the front would flash 3 times, pause, then flash 3 times again, with the sequence repeating until the machine was powered off. The machine would never display any video, and over time the fans would slowly ramp up to maximum speed. The flashes of the LED indicate a specific error code, designed to help isolate a fault in the system.

 

 

A power-on self test in the computer’s ROM automatically runs whenever the computer is started up after being fully shut down (the test does not run if the computer is only restarted). If the test detects a problem, the status LED located above the power button on the front of the computer will flash in the following ways*:

 

• 1 Flash: No RAM is installed or detected.

• 2 Flashes: Incompatible RAM types are installed.

• 3 Flashes: No RAM banks passed memory testing.

• 4 Flashes: No good boot images are detected in the boot ROM (and/or there is a bad

sys config block).

• 5 Flashes: The processor is not usable.

 

* Note: The status LED lights up when the power button is depressed at startup. Do not count this light as one of the diagnostic flashes.

 

 

Now, the first step is to attempt reseating the memory, replacing with known good memory, and ensuring that the memory is in the correct slots. Power Mac G5s don't respond well to unmatched RAM or memory installed in the incorrect slots.

 

However, sometimes the issue goes beyond that. In the case of the system I experimented with, a reseat of the memory could occasionally allow the machine to start, but it would lock up during startup, requiring a hard power off. New, known good replacement memory would do the same.

 

 

 

A Theory

 

 

One issue I found commonly discussed on the various Mac forums relates to the solder joints below the memory slots. The theory is simple; reseating the memory flexes the logic board enough to create a momentary connection of the broken solder joints, resulting in a near successful startup attempt.

 

This sounded promising, since it was consistent with the behaviour my machine was exhibiting.

 

A common test for the issue was to slide out the fan assembly and run a hair dryer back and forth over the memory slots from the front side of the board. I've highlighted the area for reference:

 

 

step3_full_1_7.jpg

 

 

The theory is based on the fact that as metals expand when heated and shrink when cooled, the heat from the hair dryer would slightly expand the solder joints, temporarily rebonding the connections and allowing the machine to boot once again, or at least until the joints cooled and the connection was once again broken.

 

After 5 minutes under the hair dryer, my Power Mac G5 booted up, albeit briefly. The system locked up at the "Starting Mac OS X" screen, and the fans began to ramp up once again.

 

The longer I heated the machine, the longer it would last. With 15 minutes of heat, I made it far enough to format a new SATA drive and prepare it for installation before the G5 locked up once again.

 

Here's where it becomes interesting.

 

 

Removing the Logic Board from the system, I painstakingly reflowed each solder joint below the RAM slots with a soldering iron to rebond any broken connections, then refitted the board to the machine.

 

After powering it on, I saw the same 3 flashes, followed by the telltale sound of the fans ramping up. The G5 was still dead.

 

 

 

Tracing The Fault

 

 

Using the handle of a screwdriver, I probed around inside the enclosure, applying pressure at various points on the board to see if it would affect the machine's behaviour at all.

 

After applying moderate pressure to the area highlighted below, being careful to not damage the small surface mounted components in the process, the machine started up.

 

 

ram_slots_2.jpg

 

 

Removing the Logic Board for inspection once again, I found that the components are directly behind a large chip on the underside of the Logic Board.

 

 

logicboard_7.jpg

 

This is the integrated circuit that handles the transport of data between the memory, processors and system bus, and it looks almost identical to the render of the G5 System Controller from the Power Mac G5 Introductory Keynote. Skip ahead to 8:25 if you don't wish to watch the entire thing.

 

http://www.youtube.com/watch?v=ve3O6VtBmdc

 

Looking at the Block Diagram for the Power Mac G5 from the Service Source manual, it appears Apple refers to this custom IC as the U3 Memory Controller / Bus Bridge, and of course, it directly interfaces with the memory slots.

 

Bingo.

 

 

 

What's Happening?

 

 

I studied a document released by IBM in 2004, titled "Development of BGA Solution for the IBM PowerPC 970 Module in Apple's Power Mac G5". This document is freely available from IBM's website and is worth a read if you're interested in the minute technical details of modern circuit board assembly.

 

https://www-01.ibm.com/chips/techlib/techlib.nsf/techdocs/59BEA03A4FA9E41787256FD600710800/$file/BGA%20Solution%20for%20G5.pdf

 

The U3 Memory Controller / Bus Bridge is affixed to the board using a Ball Grid Array, or "BGA" for short. This chip packaging technique places the electrical connections on the underside of the chip, allowing for a compact chip package that is easier to place on more densely packed circuit boards.

 

One of BGA's disadvantages is that the electrical connections can be subject to stress and fatigue over time, either through flexing or heat expansion and contraction.

 

Given enough time, those electrical connections can crack, and the process can occur much sooner if the solder used is of a poor quality, such as the nVidia GeForce 8600M GT problem that plagued 2007/2008 model MacBook Pros, or with inadequate cooling, much like the "Red Ring of Death" issue that affected the Microsoft Xbox 360.

 

The Power Mac G5 does not have these factors playing in its favour. The machine runs more on the warm side, evidenced by the use of large heatsinks not only for the G5 processors themselves, but also the extremely large heat pipe arrangement on the back of the board to cool the U3 Memory Controller / Bus Bridge.

 

Adding to the problem is the fact that the Power Mac G5 uses a very large Logic Board that is subject to flexing with thermal expansion, and the removal and insertion of memory into the slots also flexes the board.

 

As these machines have aged, some of the electrical connections below the U3 Memory Controller / Bus Bridge have fractured, resulting in an intermittent boot or no-boot situation.

 

Since the Memory Controller can not interface with the memory slots, it is unable to detect any memory slots that can be used by the system, and so it throws the 3 flashes of the LED on the front:

 

 

• 3 Flashes: No RAM banks passed memory testing.

 

 

 

The Repair (Attempt)

 

 

So, if you could repair those electrical connections, the Power Mac G5 would run reliably again. Most homebrew BGA repairs involve either the use of a heat gun, to heat only the affected chip, or an oven, to evenly heat the entire board and reflow all the components, along with the affected chip.

 

BBISHOPPCM's World on YouTube has a video titled "Repair PowerMac G5 Motherboard Failure (NON-Watercool Only)". This is perhaps the most well known documented example of an oven reflow of a Power Mac G5 Logic Board.

 

http://www.youtube.com/watch?v=4521HSi0r4E

 

 

This is where I need to stress that reflowing the chip is a difficult process, compounded by the large size of the Power Mac G5 board. While it can repair a Power Mac G5 Logic Board, it isn't guaranteed to work, and if not performed correctly, it can irreparably damage the Logic Board. Unless you have experience with PCB work, I would recommend speaking to someone that is more confident with BGA repair to get this task done.

 

 

Due to the large size of the G5 Logic Board, it is very easy to warp the board when heated. It is absolutely critical that the board is supported during the heating process. If you watched the video from BBISHOPPCM's World linked above, you would see that he used coffee mugs evenly placed to support the entire board, not just the affected corner.

 

You will also need to strip the board of as much as possible, as there are some wires and components that could melt or explode in the process, such as the PRAM battery. As a result, this can be an involved repair at times. It's worth visually inspecting the board and removing as much as you possibly can first.

 

 

I opted for the heat gun method, and I can't say it went well, although it doesn't mean this method isn't possible to perform correctly. If I had to try this again, I would try the full board in oven method.

 

Using a heat gun, heating the affected chip and the memory slots, I have managed to warp one Logic Board using excessive heat. After this occurred, the board appeared bent and would no longer sit flat. The heat pipe would no longer affix to the board, and the board would no longer fit the case correctly, let alone the reliability and functional issues that come with a warped circuit board.

 

If I had to use a heat gun again, I would heat only the affected chip, and would try to heat it for no more than 30 seconds to one minute.

 

 

 

In Closing

 

 

While I would like to write a definitive guide on how to repair the G5 Logic Board, I haven't had success with the one occasion that I attempted this repair, and the failure to repair this board allowed me to perform the dissection necessary to write this article. However, I do believe that I have identified the problem spot on the board, and if I could source another G5 to repair, I would likely be successful the second time around.

 

The aim of this write-up is to add to the already large amount of information out there about repairing the Power Mac G5 and to assist those who are seeking this information to repair their own Power Macs.

 

As many Power Mac G5s have since been scrapped and harvested for their enclosures, there are fewer available to enthusiasts and collectors. Prospective owners that wish to attempt to revive a dead system may find this article useful. If even one machine is repaired using this information, then it's been well worth my time in writing it.

 

Best of luck.

 

 

Regards,

~ Mic.

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I've had a couple of G5 towers that when the power button was pressed, the white light would come on, and the fans would start, but no startup chime and no boot. No flashing of the white light - it would come on and stay on.

 

Based on this old posting at MacRumors, I tried the hair dryer trick, and the machine would then boot and run normally. I didn't even need to take the side of the case off the machine. I could blow the hair dryer through the front of the machine for 15-30 seconds and then all would be well.

 

This was only necessary if the machine was shut down and stayed off for a period of time. Quick restarts such as for system updates, etc. were not a problem.

 

I don't have these machines any more, but I can vouch there there is some kind of thermal issue there.

 

Having tried the heat gun trick on some white Macbooks for video problems with small success, I can see how this "repair" on a G5 MB would be problematic.

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I have question reguards to this. I too have a dp 2.3 g5 powermac. In looking at the picture, for the underside there doesnt look to be a way to maybe mount some type of chipset cooling fan? Kind of like when some pc motherboards had a little fan on the chipset for cooling purposes. Also I remember reading somewhere here about by now due to age of g5s that its best to re-do the thermal paste. Would it also help to do on the underside of the mb? I dont think its an issue since I'd imagine the heat pipe area would have adequate thermal tape or something?

Im just asking this as I dont currently have any issues yet on my g5. But if there is some precautionary steps or something to help avoid these inevitable problems I'm all for it. The only other thing i have done so far is make sure to compressed air the unit regularly, and made sure to put in 1/2 the max ram it can take now, and finish it out in the future. In other words making sure to not touch the mem slots any more than i have to.

 

Thanks in advance.

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Been having fun setting up a Dual 2.3 recently - it demands RAM of exactly the same timing (eg. DDR400 3-2-2), otherwise it will not boot. I've lots of sticks of DDR RAM, but only could get 3GB up and running as I didn't have any more matched pairs.

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So whats the plan with this machine?

 

The machine I used to document for this thread was parted out a couple of years ago, but it took me a while to get around to writing up the findings. The case eventually went to a case modder who then turned it into a Core i7 thing.

 

 

Been having fun setting up a Dual 2.3 recently - it demands RAM of exactly the same timing (eg. DDR400 3-2-2), otherwise it will not boot. I've lots of sticks of DDR RAM, but only could get 3GB up and running as I didn't have any more matched pairs.

 

Of course, before anyone starts removing their Logic Board for a BGA repair, make sure you have the correct memory in it first! Incorrect memory or memory in mismatched pairs will cause exactly the same symptoms and is so much easier to resolve.

 

 

I have question reguards to this. I too have a dp 2.3 g5 powermac. In looking at the picture, for the underside there doesnt look to be a way to maybe mount some type of chipset cooling fan? Kind of like when some pc motherboards had a little fan on the chipset for cooling purposes. Also I remember reading somewhere here about by now due to age of g5s that its best to re-do the thermal paste. Would it also help to do on the underside of the mb? I dont think its an issue since I'd imagine the heat pipe area would have adequate thermal tape or something?

 

Im just asking this as I dont currently have any issues yet on my g5. But if there is some precautionary steps or something to help avoid these inevitable problems I'm all for it. The only other thing i have done so far is make sure to compressed air the unit regularly, and made sure to put in 1/2 the max ram it can take now, and finish it out in the future. In other words making sure to not touch the mem slots any more than i have to.

 

Thanks in advance.

 

I haven't attempted to mount a fan on the reverse side of the board before. It's possible something like a small MacBook blower would work and chances are it would fit behind there, but I can't confirm it. Considering the size of the heatsink, a MacBook blower may not even move the required volume of air needed to cool it down significantly anyway.

 

From what I understand, there is a cut-out in the metal that separates the Logic Board area from the Optical Drives / Hard Drives. The fan in the Optical and Hard Drive area can draw air upwards and over the chipset heatsink fins. I haven't smoke tested the case to verify the airflow or seen any Apple documentation yet to confirm this either, but it's what I've heard.

 

I think the U3 Memory Controller / Bus Bridge (aka. the Chipset) uses thermal paste. I recall this being the case from when I stripped my machine down at least. It's entirely possible that the Thermal Paste will have dried out by now, so if you can replace it, it's probably not a bad idea.

 

The challenge is that the chip sits behind that heat pipe and heat sink arrangement, and it's secured to the Logic Board with plastic pins that are rather easy to break if you're not extremely gentle. If they break, you'll have a really hard time securing the heatsink to the Logic Board, which is even worse for thermal transfer than having dried thermal paste. Not to mention extracting the entire Logic Board is a challenge in itself, requiring removal of the processors and a number of other components. So unless you can handle working with delicate electronics, I'd avoid messing around with it for now.

 

At least it sounds like you've taken care of it, making sure to avoid flexing the board near the memory slots and cleaning the dust out of it from time to time. Hopefully it won't require any major repairs for some time yet.

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