iMac Chimes- Screen White- 4 LED lights on logic board are ON. Stalls at that point.

This is a discussion on iMac Chimes- Screen White- 4 LED lights on logic board are ON. Stalls at that point. within the iMac forums, part of the Mac Forum category; I have a mid-2007 24 inch iMac Model A1225, that suddenly stopped working. It chimes normally on power-up, and the screen turns white with nothing ...

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    Question iMac Chimes- Screen White- 4 LED lights on logic board are ON. Stalls at that point.

    I have a mid-2007 24 inch iMac Model A1225, that suddenly stopped working.
    It chimes normally on power-up, and the screen turns white with nothing displayed.
    It stalls at that point and will not go any further.
    I use a USB wired keyboard but holding down Option key, C or F does not have any effect.
    I have an external USB HD with a complete bootable OS on it but it doesn't see that.
    I previously removed a working 750 GB internal hard drive with its OS and installed a 2000 GB upgrade in 2012. So I removed it and put the original 750 GB drive back in. No difference.
    Tried unplugging the HD, but still no difference.

    The four green LEDS on the logic board are glowing. Actually when I power on, the first three glow and then after it chimes the fourth LED glows. An Australian site said the first 3 indicate that the power supply is good. The fourth is an over temperature indicator but it glows even when the computer is cold. The hard drive appears to have a temperature sensor which had fallen off. So I put silicone heat sink white paste on it and glued the mounting clamp onto the surface of the hard drive. But that didn't make any difference either.

    Got a belt off the screen ribbon cable! Ouch!

    I put the old 2000 GB HD in another computer and it is clicking and will not mount but I don't think that is the problem. Any suggestions?
    Thanks

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    HALF FIXED:

    Apparently this is a fairly common problem called the White Screen of Death (WSOD) and I found it in other areas of this forum. It is caused by some kind of corruption of the NCRAM (PRAM on Power Mac processors). There is a CR2032 disc battery on the logic board which supports this memory when the computer is turned off and this can cause similar problems. My battery was OK reading over 3 volts even under load. So I rest the NVRAM by holding down Command-Option-P-R all at once until it chimes twice. This appears to have fixed the problem because now it comes up with a grey folder with a question mark on it. That means it cannot find a hard drive etc with the operating system on it. So now I have to fix that problem. To reset NVRAM see

    About NVRAM and PRAM

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    I was able to install Mac OS from a CD no problem after resetting NVRAM. I may upgrade to Mavericks. NVRAM stands for Non-Volatile Random Access Memory which stays intact when the power is turned off. The older PowerMac computers had PRAM or Programmable Read Only Memory. As far as I recall the 1980's Commadore 64 used EPROM Erasable Programmable Read Only Memory to store its whole 32 kilobyte operating system which booted up almost instantly.

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    Commadore 64 was 64 Kilobytes.... Wasn't it??
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