Display very dim and going black post-logic board replacement

This is my first attempt at offering a question and I am really sorry if it has already been asked elsewhere, but so far I am finding similar problems, but not quite the same.

I have a 2007 Core 2 Duo Macbook which recently required a logic board replacement. Not a major surprise due to its age, but prior to the logic board going it was absolutely fine, including the screen.

I have a friend who works with Macs and he had come to the conclusion the prior problem was that the machine required a new logic board, which I worked through and replaced myself. In my bid to check and see if the repair had been a success I quickly clipped back in the keyboard and fired it up. I got the startup sound and a grey screen momentarily and then it went black. If I clicked some buttons, the desktop was very faintly visible momentarily before disappearing to black again. This is the thing, the desktop is there, but not really - it's like the brightness is at a really low level. Furthermore, it works when connected to an external monitor, so I assume the logic board repair has worked (yay!), but now there is this problem.

I double checked the display connection, but that is definitely in the logic board. Is it likely to be the inverter cable? Have I perhaps caught the wires while taking it apart and putting it back together again? Is there anything I should look out for to ascertain if it is that? Or has the lamp coincidentally/not coincidentally blown?

I tried resetting the PRAM as I saw mentioned elsewhere, but I don't think I am doing it right - the Macbook just keeps playing the startup sound and then getting stuck. It was a bit odd but I mention it in case this is a logic board related symptom.

Anyway, apologies for the long winded post but any help or advice on what to look out for is gratefully received.