Marshall 20 y'old guitar amp help

[E_maN]

I got myself a guitar combo amplifier, a Marshall Lead 12, i think its called, and it got some problems, for the any sound to come out of the speakers, you need to plug something in the lineout/headphones jack, just a plug, not a speaker or nothing, and then it will come sound out of it, do you guys think that there might be a misconnection on the back? cuz i "think" it should be the other way.

thats the major problem, and then we have the fuzz noice, when i dont play anything, a humming noice, very loud. got any clues what it might be?

thanks!

[i_got_a_mohawk]

Take it to a tech, probably could do with a once over