Need Help: Rev A iMac won't power up

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doug-doug the mighty's picture
Joined: Apr 14 2004
Posts: 1355

When I went to bed last night, my Rev A iMac was sleeping nicely, running AfterDark (and 9.2.2).

This morning, I went to wake it up and check mail and such only to find it not powered on. I tried the power button and got nothing. I checked my power strip and other devices are okay as is the connection to the iMac (ergo, it is getting juice).

Not being too familiar with the signs and symptoms of analog board death, I am wondering if it has finally happened to me. For those knowing better, how can I verify? And upon a physical examination of the board, is there any visual queue I could spot to let me know this is indeed the case?

If there is an alternate diagnosis, any suggestions are welcome. I plan to crack the machine open sometime tomorrow morning, and pull the hard drive - the only irreplacable data is my email, which is not time sensitive for my accessing it (but problematic as this is my only IDE Mac.

TIA

--DDTM

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--DDTM ('Fritter Critter' since Apr 26 2004 - 18:16)

'If it ain't broke, take it apart anyways. If you can't take it apart, break it so that you can fix it.'

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doug-doug the mighty's picture
Joined: Apr 14 2004
Posts: 1355
UPDATE

I unplugged the iMac and left it for a few hours this morning. When I returned, I plugged it back in and tried to power up. It did come to life, but was making some 'arcing' sounds from the left side of the machine (as you face it). The area near the computer has some dust and I do wonder if I have some dust bunnies that may be interfering with the machine. I will open it up and inspect that area to see if that is the case.

It bears noting that the arcing sounds coincide with flickering on the screen.

If the problem is not debris causing a short, then I will back up my drive to my 8500 or my clamshell and make one of those my primary.

Any input on suspected sources of the problem are welcome.

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--DDTM ('Fritter Critter' since Apr 26 2004 - 18:16)

'If it ain't broke, take it apart anyways. If you can't take it apart, break it so that you can fix it.'

Eudimorphodon's picture
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Joined: Dec 21 2003
Posts: 1203
"Arcing Sounds"

The major failure point in those machines is the flyback transformer for the monitor. (Which is on the "analog board".) The arcing sounds are probably coming from it.

If you have a Old-Macintosh (The wide 15 two-row pin connector) to Multisync VGA adapter lying around you can probably continue using your machine by removing the plastic cover from the bottom, disconnecting the cable to the internal screen, and screwing the VGA connector in its place. (You'll need a "decent" 15-17" VGA monitor. A really old or low spec one won't support the unusual 100Hz 640x480 video mode the Mac will spit out at first. Once you've booted up you can use the Monitors control panel to select a more "normal" video mode, including resolutions higher then the built-in monitor supports.)

Supposedly disconnecting the internal monitor from the logic board prevents it from powering up and shorting the power supply, allowing the computer to run on the external monitor. Your mileage may vary.

--Peace

eeun's picture
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Joined: Dec 19 2003
Posts: 1891
A link

Feeling brave?

Hopefully that'll offer some assistance during your exploration tomorrow. Depending on what you find in there, some 5-min epoxy might get you up and running again - at least for the short term.

You might also want to check the solder points where the flyback connects to the AB.

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doug-doug the mighty's picture
Joined: Apr 14 2004
Posts: 1355
Well...

I finally cracked it open and filleted it all the way down to the chasis. The boards are all clean inthe upper part of the case with a few minor dust patches (thin grey film). There is no discoloration on the boards from any kind of carbon build-up or obvious component failure.

This means that the arcing is highly localized to the flyback transformer and most likely internal. The only thing I can do now is to reconnect the wires and power it up to visibly watch for the flash (if there is one - darkened room if need be), unless anyone has a better idea.

I did look closely at the flyback transformer for signs of a crack or other failure, but it looked good. I do need a lot more light to do a good inspection and will use a directed light source to examine it again tonight.

So lets recap the analog board: Asides from being the placental-aparatus for the CRT, and driving the fan, what else feeds off of this? I did not see anything clearly being a problem except the fan (which I would need to drive from a different switchable source on the main feed somewhere...).

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--DDTM ('Fritter Critter' since Apr 26 2004 - 18:16)

'If it ain't broke, take it apart anyways. If you can't take it apart, break it so that you can fix it.'

doug-doug the mighty's picture
Joined: Apr 14 2004
Posts: 1355
UPDATE

Finally got the extension cord from my parents garage and hooked it up.

I had taken the iMac apart to examine for damage or anything visibly obvious. I put it all back together and plugged it up. I did miss one wire when I plugged it all back up - a two conductor brown link from a spot near the AC cord. When I plugged it in, the GFI in the wall blew right away. I do not think anything bad happened, but I have missed things before. I reconnected the linkage, reset the GFI and plugged it back up.

As it stands right now, the iMac will not turn on, even after resetting the CUDA. I am unsure of where the failure is. I do know that the arcing in the CRT was causing the iMac to turn off unexpectedly. I hope that the failure to power up is related as opposed to something being touched by my black thumb (meaning a new problem introduced when I cracked it open).

So let me ask a side question here: What alternatives to the internal PSU do I have? My intention with this question is to determine my alternatives to feed power to the mobo. If I were to proceed with my plans to rebuild my iMac with an LCD vice the CRT, the analog board is no longer needed. The implication of this is that much of the circuitry on the PSU board is then useless. Somewhere, I think I have the pins for the PSU connector to the mobo, but the question bears asking.

What is the smallest PSU circuitry I can put in my iMac case to feed just the mobo?

__________________

--DDTM ('Fritter Critter' since Apr 26 2004 - 18:16)

'If it ain't broke, take it apart anyways. If you can't take it apart, break it so that you can fix it.'

doug-doug the mighty's picture
Joined: Apr 14 2004
Posts: 1355
UPDATE - found my service manual :D

I found a soft copy of my service manual for this variant of iMac. Now I can follow the circuit path to figure out why the power board is non-responsive and work on plans to bypass it...

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--DDTM ('Fritter Critter' since Apr 26 2004 - 18:16)

'If it ain't broke, take it apart anyways. If you can't take it apart, break it so that you can fix it.'