Save my Mac Pro 3,1 - PSU failure etc

[FlicTheBunny]

Hi,

Long story short I bought a Mac Pro 3,1 a while back for very little money and it worked then it didn’t. I chalked it up to bad luck and scrapped it.

I bought another one yesterday, with 2 3.2 Quad-cores and 32GB RAM BUT it’s also dead.

No fans, no power LED, and no chime. Powers up for 1secinds, GPU fan spins, then it powers off. All the RAM diagnostic LEDs on any installed riser card are solid red.

The diagnostic LEDs suggest it’s not getting good power on startup, LED 7 and 9 illuminate but not 8. LED 8 is the POWER GOOD indicator.

I don’t feel like giving up a second time. I know right now a lot of people don’t rate these machines as worth the effort, but I feel like giving this thing a fair swing as it’s a rarer CTO config, and I feel like the PSU is the possible cause.

Here’s what I’ve tried, none resulted in any change:

• Swapping the PSU from the other 3,1
• Removing both RAM risers
• Removing all but 4 DIMMs and installing 2 on each riser in the slots closest to the logic board
• Removing the GPU
• Using a Apple GeForce 120 GPU
• Changed the NVRAM battery
• Disconnected the mains power from the system for an hour then retrie
• Messing with the SYS_RST button on the board

My first goal is to try to establish the pinout of the PSU. It uses 4 large plugs (behind the optical drive bay).

The top three I know are 12V and ground only thanks to this photo

In this article about tapping the PSU for GPU power:

Mac Pro Pixlas Mod - The House of Moth

Modify your Mac Pro power supply to relieve the board and feed powerhungry GPU's

thehouseofmoth.com

Obviously in order to replace the PSU with a ATX unit I will need to suss out what goes where in the lower connector in terms of, I expect, 3.3V and 5V.

 

[FlicTheBunny]

Okay I found this diagram on a forum that seems to agree with the previous one in terms of 12V and Ground locations sbd seens to give the rest. More importantly, it also gives a location for the PWR_ON Pin that might lead me to be able to hot-wire the PSUs I have to read the voltages.

According to the author of the thread, W/O is a “wire out” and C/B is a “circuit board” connection.

I’ll have to see what that actually means in terms of the wiring loom. I suspect (unconfirmed) that the W/O goes to a longer cable that runs to the RAM riser area. It’d make sense as it’s mostly 3.3V.

Looking at the split of these cables, I think the logic board runs off almost entirely down-stepped 12V. J1 and J2 are likely equivalent to 8-pin EPS on a normal board and provide power for the chunky CPUs. J3 is probably 12V to the board/PCIe and also to the drive bays (which are powered via headers on the board).

J4 seems to carry the low-voltage stuff including 5V for the drives and 3.3V for whatever needs that.

 

[FlicTheBunny]

I split the PSU open and found something interesting. All the cables break up into 3 distinct groups. One small cluster attaches to the upper board alongside the high-voltage side. One group of fairly thing wires attaches to a riser board on a plug. The remainder (all 12V and ground as far as I can see) all link to the main lower PCB directly.

This would indicate to me that the three areas correspond to the three present voltages. It makes sense the large low-voltage section would control the 12V as it makes up the bulk of the power capacity. The two smaller sections would thus be 3.3V and 5V I think.

 

[FlicTheBunny]

So, to update this thread, I obtained another Mac Pro 1,1 - 3,1 power supply and the same thing happens.

I pulled the CPUs and they were absolutely awash with thermal grease. Took some cleaning up!

I also noticed the side door has a huge coffee spill stain down it.

I disconnected all the fans in case one of them was shorted. No change.

At this stage I an pretty sure the board is dead.

So another board (that's guaranteed tested working) is on its way. Let's see if this improves the situation.

Fear not. If I get it running I intend to try and get the PSU pinout anyway! I am pretty sure someone else would like to know what it is, at least.