Had another thought, If the spare monitor does not work then maybe your processor had packed up, this happened last year too me I purchased a new processor, installed with fan and nothing would happen but HD was humming.
This could be a number of things.. Check CPU first. Also be sure factory thermal garbage pad hasnt melted and come in contact with the CPU. They like to short out.. If this isnt the case. Z-3 could be shorted or leaky. Bus diodes could be shorted. Or power supply area of board is shorting. I fix a ton of these.. Be ready for fun! If you need help guive us a buzz. We can walk you through some of this. Ken www.ikenfixit.com