Power Mac G4 MDD FW800 power issues

[mac27]

I'm troubleshooting a FW800 MDD (Dual 1.25 GHz) with some weird behavior and am in need of some help on this one.

Description of the problem: The machine turns on / powers up; fans spin; HDDs spin up but then slowly spin down to a very low RPM (abnormal behavior). There is no boot chime and no video. The power LED glows briefly when first depressed, but does not turn back on and stay on like it's supposed to. There are no warning chimes.

Things I've tried so far:

- My first thought was RAM issues - have tried many different configurations, at least 3 different known-good sticks, and also no RAM at all. No change in behavior. Also no warning chimes when no RAM is installed, which is odd.
- Resetting PRAM (No response to this - same behavior).
- Removing all non-essential PCI cards, AirPort card, HDDs.
- Replacing speaker with known-good unit.
- Re-seating graphics card.
- Re-seating main power connector on the logic board.
- Re-seating CPU daughterboard and heat sink. (Inspected CPUs; they look normal).
- Application of new thermal paste.
- Installation (and removal) of PRAM battery.

Your first thought might be ... dead/failing PSU! That's what I thought also, so I've already sent it off to the MDD PSU repair service (ACX Electronics, Austin, TX). They replaced several of the usual-suspect caps and reported the PSU should be working normally. I also measured the voltages and they are normal. So I am wondering if this is actually something wrong with the motherboard or the CPU daughterboard. I do not have replacements for either on hand.

I've found some similar threads around, such as this one, but they're either not quite a match (e.g., slightly different symptoms), for a different model like the AGP, or they fixed theirs by doing things I've already done.

So ... I am at a loss on this one.. Any suggestions would be really appreciated.

 

[chriscarruthers]

Seems like you’ve tried almost everything except… PMU reset?

 

[herd]

Did it work before, or are you trying to diagnose a new machine? Could it be booting, but without graphics? Is there HDD activity? Does the USB power up (...does a USB mouse light up)?

 

[mac27]

Seems like you’ve tried almost everything except… PMU reset?

Just tried that as well (twice, about 5-10 minutes apart with power-on in between), no dice unfortunately.

Interestingly, there is a section in the service source manual that describes this exact set of symptoms:



I've done the PMU reset, re-seated the cards, and I also reseated the processor daughterboard as well just now. *sigh* I have a feeling I'm gonna be looking at a new daughterboard, at least.

Did it work before, or are you trying to diagnose a new machine? Could it be booting, but without graphics? Is there HDD activity? Does the USB power up (...does a USB mouse light up)?

This machine was working normally until this problem randomly appeared one day. I hadn't done anything to it whatsoever. One day it was working fine, the next I went to power it up and this was happening.

I think if it were booting I'd get a chime at least, the fact that there isn't even a warning "beep" with no RAM installed is concerning to me.

I haven't tried connecting peripherals directly to the tower (hasn't been much point yet), but there is no power to USB peripherals over ADC and no display. The ADC display's power LED does flash and fade out when you tap the power "button" while the tower is on, but that's it.

 

[mac27]

Close-up of the pins on the daughtercard (top) and mobo (bottom) - they look normal to me:

 

[MBongo]

Pins might look normal to the naked eye — but no harm in carefully cleaning them anyway… as oxidation, corrosion, etc. can and does take place in these now 23 year old machines.

Another oddity is that CPU daughtercards warp… and applying slight, yet brief downward pressure (by hand) to the heatsink upon a cold boot, can reveal this possibility… if it does boot as it should with such pressure.

ALSO… test with no RAM in the J20 slot. Have seen a few of these lately, where the J20 slot seems to have expanded over time… and possibly in relation to the close proximity to the GPU.

Just a few things off the top of my head. But if you want to test with another motherboard I have quite a few available. (Yours for the cost of shipping alone perhaps.) DM me.

 

[mac27]

Thanks for these suggestions. I've tried many configurations of RAM, and just tried again with no stick in J20 to make sure. No change (and still no warning beeps with no RAM installed at all, which is odd).

I also tried various pressures on the heatsink manually during boot, no change with that either.

For cleaning the daughterboard connector and slot, would some contact cleaner on a toothbrush be the best way to go about this? I've not manually cleaned one of these processor daughterboard connections before and will need to be exceedingly careful.

 

[MBongo]

Yes, I have carefully used a soft bristle toothbrush with both DeoxIT - D100L and then also 91% isopropyl alcohol. (One or the other and sometimes both.)

 

[mac27]

Thanks. So to update my list from above, I've now (in addition) also tried:

- Resetting PMU (3 or 4 times now in total)
- Re-seating CPU daughterboard and cleaning connections on both sides with Deoxit
- Applying pressure to heat sink in various fashions during power-on

@MBongo has very generously offered a suitable motherboard so replacing that will be the next step ... will update later.