Replaced screen & drives - no more chime!

Hello,

I hope someone can help me with this: My daughter's iMac 2007 7,1 24" recently broke down half way with apparently a graphics card problem (single user mode works, graphical mode does not). So I went out and bought a used one. When I came to pick it up, the seller turned it on, and it started normally until the folder/question mark icon, which was OK because the seller had wiped the hard drive. The LCD had cloudy spots, and the seller stated that the DVD drive wasn't working. So I bought it, took it home, opened it, replaced the LCD with the one from the other machine, replaced the DVD and installed an SSD instead of the hard drive. I turned it on - no chime, black screen, of course no boot. Darn!

Here's some information:

• When turned on, the fans behave normally (they shortly kick in at higher speed, then run at low RPM)
• If I take the RAM out, I get the beeps for 'no RAM'
• The PRAM battery is at the full 3V
• The mouse has USB power
• The Caps Lock key on the keyboard does NOT light up when pressed
• I tried zapping the PRAM with the key combination - did not help, but I am not sure if the keyboard is being recognized because of the CapsLock thing
• I tried 'resetting' the SMC (afaik a simple disconnect of power)
• Diagnostic LEDs 1,2,3 turn on normally, 4 remains off

I figure that the first step must be a chime/POST success. As I said, the machine came up ok before, and I doubt that I have shot the main board by replacing the screen, HDD & DVD. The RAM is good. The hardware I installed worked on the old machine. For testing, I unplugged the LCD data cable on the old machine, and it chimed, LEDs 1,2,3 were on, LED4 off. Just what I expected.

So here are my questions:

1. Could a faulty/misinstalled cable (HDD etc, temperature sensors) prevent the machine from passing POST?

2. Would you recommend to try and restore the firmware via a restore CD? Will this even be possible if there is no chime? I am a little reluctant because the CD may get stuck in the drive and not come out again. Besides, how could the firmware have been damaged by the replacements I did?

3. What can I do to narrow down the cause for this? Unplug as much as possible?

Any help will be greatly appreciated.

Greetings from Germany,

Christoph