iMac dead screen 2

[glod]

Have several "dead screen" 400MHz slot loaders, family number M5521, too good to trash. Have removed both top and bottom housings. All chime when switched on (a good sign?), green power switch LED comes on, CRT powers up (static crackles), hard drive starts and continues to spin, but screens stay black. External VGA monitor also displays black screen.Trickle voltages all check out OK for main boards. 4 amp PAV fuse OK. Can be put to sleep, but still power up in black screen mode. Everything seems fine apart from the screen displays.
Is this a hardware or software "stuck in sleep mode" problem? OSX firmware conflict? I've seen lots of similar problems listed, but no work through technical thread.
Can anyone help out please? Thanks

 

[jimoblak]

Seeing that you can pull them apart... can you try putting a hard drive from a bad mac into a good mac? This would determine whether you are dealing with bad hardware or software.

- - picklefish - -
Why is everyone in this forum responding to me as picklefish?

 

[glod]

I swopped over a known good hard drive with one from the "dead screen" iMac. The dead screen Mac is still dead! What does this prove? If the voltages all check out OK on the main PCBs, could one of the boards still be faulty? I guess I have only checked the trickle power supplies, or are these checks conclusive? I am hoping it's not the flyback or CRTs at fault. I don't think so.
If 3 similar iMacs all have similar problems from a similar user location, is there a chance that an OS9.2 to OSX upgrade has been done wrongly? (Firmware upgrade not done). If so, is this software problem/conflict reversible? Some sites I have visited talk of permanent power supply corruption on the motherboard!
Should I press the CUDA switch or reset the PRAM in desperation?
Thanks for any help

 

[jimoblak]

The hard drive swap proves that this is not a software error in the main operating system. You may be right to suspect CUDA/PRAM. I would rely on tech articles from apple.com...

http://docs.info.apple.com/article.html?artnum=95065

(I think this is the wrong model but this was the first article to come up in a search for 'CUDA' on apple tech support)

A faulty OSX install (without firmware upgrade) could not have done this. You proved this with the hard drive swap.

- - picklefish - -
Why is everyone in this forum responding to me as picklefish?

 

[glod]

I have followed the instructions on the Apple and other sites for resetting the PRAM, after checking the battery, and resetting the CUDA switch, now known as the power management switch. Still no screen, despite everything sounding like it's trying to come on. The EHT seems to power up (static crackles), but despite this the screen (and external monitor) stay black. Where do I go next????
I can't just throw away (potentially) good iMacs.
Has anyone else had this problem?

 

[JeffTullin]

FWIW, I thought I'd killed my iMac upgrading to OSX10.3 because of this update PRAM software.
Searching the Mac website as detailed in the docs gave two different places to look, and the updater that came up top of the search was the wrong one.
It tyook me two days of trying to reset the PRAM and load the correct updater (which only works under OS9 - WHY?) before I got a working system.
During that period I had : dead screen, green screen only, fuzzy screen, and one displayed with a huge skewed display.

I thought Macs were easy until that..

 

[glod]

Success!!!
I was playing around with one of the dead screen iMacs (slot loading, indigo 500MHz), and had already tried the power management reset before, with no success. Today I connected an external monitor, and did a start up as before. The monitor came on with a happy mac icon, and I watched in disbelief as it started to boot up into OS 9.2 with no problems !!
It wasn't until a few minutes later that I realised the "dead screen" iMac had also come on!! Its screen was away from me so I couldn't see it.
Now no problems at all. I've put it to sleep, restarted etc etc. and it's fine. Anyone got a reason for that?
I had left it a for few days after doing the PRAM reset , battery replacement and power management reset - to be honest I'd given up on it. Is this a software "stuck in sleep mode" problem which I've seen posted on several sites?
I've now got two other "dead screen" iMacs to resurrect.
I'll let you know what happens.