Power mac G5 and motherboard replacement

[konfusion]

Hello!

I got a nice PMG5 dual 2.7 ГГц, early 2005 (agp, pci-x), but, unfortunately, this machine currently suffers from issues.

Now I'm doing my best to bring it up and running:

• In most cases it does not want to start (black screen, no chime).
• Sometimes it starts, boots into mac OS and somewhat works, but detects only one CPU out of two.
• If I apply some mechanical pressure to CPU+LCS assembly it always starts, but with one cpu detected.
• As liquid cooling tend to leak and destroy those machines, LCS was the first thing for me to disassemble and check. Fortunately, it is in perfect condition and shows no single sign of leakage anywhere.
• I tried to swap both CPUs and saw no changes -- G5 starts with both CPUs in upper slot. So, I can consider both CPUs to be fine.
• Also, I did a visual check of both CPU sockets for bent or damaged pins, no visible problems there.
• A also tried to replace PSU with a guaranteed working one with no results.
• It seems to me that most likely the motherboard is dying, maybe bad contact in CPU sockets, or maybe bad contact in north bridge BGA solder joints.
• So, I tried to replace the motherboard with a motherboard from 2.0 GHz G5, and it didn't start (just blows coolers at full RPM).
• Next, I got a motherboard with p\n 630-6910 from another 2.7 GHz G5 (original p\n of motherboard in my mac is 630-6911). No success too.

Now I'm stuck with no ideas what to do, as there's no information about G5 hardware repairs (schematics, tricks, etc) anywhere.

I hope than someone could help me here.

So, questions are:

1) Are there any limitations on motherboard+CPU combinations for G5s?
2) If they are, can they be bypassed? Maybe some jumpers on board, or ROM re-flash?
3) Maybe someone have ideas to try?

Thank you!

 

[mustagcoupe]

You will need the board for a 2.7 with probably the same part number, the other boards from slower g5s will not interchange, i tried without sucess. The 2.7 G5 might not be the best choice. Imo it was a step to far even when they were new and they were not particularly reliable even when new. I bought a damaged 2.7, converted it to air cooling and got it to run long enough to test and while its very fast it runs uncontrollably hot and loud, i doubt the liquid cooling is much better especially with the age. If you want the 2.7ghz because it was the fastest ghz powermac i would expect to have to do a lot of maintenance and repairs to it to keep it running. I would track down a few more of them for spare parts and combine them into one working machine and some spare parts to keep it running. If you just want a fast g5 the later dual core 2.3 should come very close to matching the dual processor 2.7 while running cooler, quieter and more reliably. If your processor cards are truly both working and undamaged you could probably sell them for enough to buy another more reliable g5 as undamaged 2.7 processor cards are not that common.

 

[konfusion]

Thanks! This was expected - I guess that U3 chips (north bridge) in 2.7 motherboards should be specially binned ones that could run at impressive 1.35 GHz.

Unfortunately, 2.7 g5 are getting very rare to find, even spares for them. And if they are found, they often cost inadequate prices. So, I'd like to tinker with 2 motherboards I already have, as I have nothing to loose -- none of them is working properly anyway.

The thing that I can't understand is why 630-6910 motherboard refuses to work, as it definitely should have the same "binned" U3? Also, unlike 2.0 motherboards -- after power on it doesn't ramp up coolers immediately, but after ~20 seconds of black screen.
Seems that I should desolder and dump ROMs on both 2.7 motherboards and compare them to begin with.

UPD: I'd like to restore and keep this machine in my collection just because it was top of the line back then, such a crazy attempt to squeeze most possible performance from g5 technology.

 

[mustagcoupe]

Apple did a lot of strange things with the G5 and given how hard they were pushing the 2.7s they might have programmed the 2 motherboard revisions differently to get a different result, possibly also the processors if they had a different revision of the 2.7 processors. Do you know if the replacement board is even fully working. It may have another defect preventing it from booting. I do not know anything about dumping the roms on a g5, i dont know if that has ever been done or where the rom chips are located. If you think cracked solder balls on the u3 could be the cause try pushing on the board around the u3 and other components when you boot the computer and see if it changes the behavior. If you find one you could try desoldering it, replacing the solder balls and soldering it back on, its not a task for the faint of heart though, there are also services you could pay to do it for you but i don't know how expensive that would be.

Side note G5s are really really strange, it seems like apple did the development in a hurry or somewhat incompetently. For example the sensor for the clear plastic airflow door, it seems that you can unplug that and it thinks the door sensor is in place and working for some reason. On some computers removing that door seems to cause all the fans to spin up, on some its only the drive bay fan. Removing that door also sends up a completely useless cpu intake fan speed error in diagnostics even if its one of the g5s that only spins up the drive bay fan making troubleshooting a bad door sensor extremely annoying if you dont know that. If you run the thermal calibration and it passes you can unplug some of the fans and boot back into the apple diagnostics cd and run the diagnostics and it will pass like the fans are plugged in and running at the correct speed. And the fan for the u3 northbridge is just asinine. A small squirrel cage fan sucking air through a poorly sealed duct wastes a ton of cooling potential/efficiency.

 

[konfusion]

The second board should be working, but no guarantees that it didn't just die during years of storage. It looks clean, has no dust, dirt, or any scratches and damage on it, that's all I can say for now.

I tried to push motherboard near U3, but didn't notice any changes. Usually (from my PC experience) bga chips with bad connection should be pushed towards the PCB to make contact. In case of G5 this is tricky thing, as U3 is on the back side of the board.

If I won't succeed in making the second board work with flashing, re-soldering U3 and replacing CPU connectors on original one sounds like a valid option.

Yep, I agree that G5s are weird in many ways. This exact 2.7 ramps up only drive fans if airflow door is removed, and CPU fans RPM is reduced comparing to normal operation (to blow up CPUs for sure if door was removed 😁).
Also, CPU exhaust fans operate separately for both CPUs depending on load, which makes zero sense for a liquid cooled system.