G4 MDD will only boot with ADC connected

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G4 MDD will only boot with ADC connected

There have been many similar threads to this one, but I have not found any which report a solution.

My problem.

G4 MDD Dual 1.42 that I have owned since new. Initially ran with 23" Cinema Screen HD, then with Samsung Syncmaster via DVI for several years, and then headless as a server.

Several years ago, I experienced the inability to boot problem (Power button lights, fans whir, and then stops). I gave up and stored the machine.

Recently, my Powermac G5 had display problems, and so I pulled out the MDD to see what was happening. It booted first time connected to the ADC Cinema display (which was the screen used by the G5).

Out of interest, I then tried booting with the DVI display, and the original problem occurred - no boot.

To summarise, the MDD works fine (stable, no software issues or kernal panics for long periods with 10.5.8) with the ADC display or ADC + DVI displays. It will not boot without the ADC display (only DVI connected, or headless)

I have done all of the usual - Pram reset, battery change, reseat memory, power lead out etc.

I have reached the same conclusion that Japamac reached in another thread - there is some loopback from the ADC which is compensating for a weak suppy in the MDD. However at the video card, there are no changes in 25V, 5V, or 3.3V lines when disconnecting the ADC, and all voltages are in spec. All screens and returns are commoned with or without the ADC. (Video card is original ATI 9000 XT Btw)

I have seen that this may be a front board problem, but the machine boots fine from front MDD button or Display button when the ADC is connected.

Any input would be appreciated

G4 MDD Dual 1.42-OTHER, Mac OS X (10.5.8)

Posted on Sep 7, 2012 12:36 AM

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Answer 1

japamac

Level 7

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Sep 7, 2012 1:00 AM in response to StevenA75

Not only is there a power loop with the ADC, there is also a USB loop.

As to where the failure is without the ADC monitor connected, can't say.

Should be able to run normally with a non-ADC equipped graphics card if the issue is confined to the ADC circuit.

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Answer 2

StevenA75 Author

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Sep 7, 2012 5:08 AM in response to japamac

Thanks Japamac. When you say powerloop, do you refer to the 25v or to the 5v,3.3v?

As far as I can see, the 25v is not a loop, only a feed as the return is commoned to the chassis, so the only impact from having the ADC connected would be a drawdown on the 25v rail, which does not appear to be the case.

Do you know if the 5v and 3.3v are monitored by the ADC screen, or if the ADC Screen is actually returning some power on these lines? If the ADC display is just monitoring these lines, that only leaves the USB - all other lines are common to DVI - I will try removing this in the monitor and see what the impact is.

Whatever the problem, it is very immediate - it must be caught by the POST hardware checks as without the ADC, the MDD dies as soon as you press the power button (Btw - MDD passes AHT fine).

Additionally, it will not boot without the graphics card, although I am not sure if this is to be expected.

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Answer 3

japamac

Level 7

24,400 points

Sep 7, 2012 8:11 AM in response to StevenA75

As far as I can see, the 25v is not a loop, only a feed as the return is commoned to the chassis, so the only impact from having the ADC connected would be a drawdown on the 25v rail, which does not appear to be the case.

It shouldn't be a loop, but due to some malfunction, it acts as one; no ADC monitor, no boot.

That's my take, anyhow......

As to where the fault is..... logic board, PMU, graphics card (electrical), graphics card (firmware), don't know.

A PMU reset would give some info, successful or not.

Installation of a non-ADC card would also shed some light on the issue.

Do you know if the 5v and 3.3v are monitored by the ADC screen, or if the ADC Screen is actually returning some power on these lines?

I haven't really investigated more than to know that the USB voltage (+5v) is two way, just like the I/O.

3.3v would likely be a monitoring signal (?).....

Whatever the problem, it is very immediate - it must be caught by the POST hardware checks as without the ADC, the MDD dies as soon as you press the power button (Btw - MDD passes AHT fine).

That would indicate to me a fault in the logic board, likely localized in the AGP slot.

Rather than POST, it seems the DC circuit is not complete, which it should be even without an ADC capable card installed.

The AHT is limited in the scope of both logic board and graphics testing. I/O functions and related controllers as well as graphics VRAM are about all that are tested.

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Answer 4

StevenA75 Author

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Sep 7, 2012 10:15 AM in response to japamac

Finally got this diagnosed thanks to your suggestions.

It is nothing to do with the USB.

The power on sequence (pressing the MDD button, Touching the screen), pulls line 13 of the ADC connector low.

The Power on Circuitry then latches placing a positive voltage on line 3 of the ADC connector (0.5v measured at the GFX board) - probably 3.3v where it is critical

My guess is that this latch is in two places, the ADC Screen and in the Front Panel. On my machine, and I guess many others from all of the posts, the machine is not latching, but when the ADC screen is connected, this pulls the line high and the boot can continue.

Remove the ADC connector with the MMD running, and this line drops to 0 and the Mac shuts down immediately!

So time to pull the front panel board, but good news as this means that everything else is OK and this is anice little server (albeit a bit noisy)!

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