black screen, no chime, both fans start briefly,

I have this problem on a 2007 20" iMac after attempting to replace the hard drive (original problem was a bad drive). Immediately upon closing the case and turning it on I got thisL black screen, no chime, both fans start briefly, dvd ticks a couple of times, then it repeats itself indefinitely.

Tried all the key combinations and holding power button down, etc.

Gave up and put it away for 3 mths (have a new mb pro :))

This is clearly some mistake I made in taking it apart (following iFixit instructions here iMac Intel 20" EMC 2133 and 2210 Hard Drive Replacement)

Trying to fix it again.

First I removed the new HDD to see if that was it.

I have a Time Machine backup attached by FW, and even with no HDD I should be able to boot past this point, right? I mean I should at least get past a black screen?

Anyway I removed the RAM on advice above - and +something+ happened… With no RAM in I get the no-RAM beep and it seems to go a lot further: fans stay on, lots more DVD noise seems to be trying harder, but in the end I still don’t get anywhere.

Adding the RAM - either chip in either slot - puts me back to the original problem… Pulled out the RAM and put it back several times no luck. Also no luck with any of the keyboard options while the RAM is out.

Any ideas?

Also, what does the fact that it seems to get further with no RAM mean? Does it just mean that the real problem occurs after the RAM check? Or is the problem related to the RAM?

Note that the RMA was fine before I replaced the HDD