I purchased a Marshall 250 dfx with a couple broken connections. Soldered most.
I am missing one component on the board and was wondering if anyone here has an idea
Because track orientation. If OP flipped that board so we could see traces you would observe they run left to right (as the picture is currently orientated), making tracks/circuit routes that run up and down the board difficult to run. Thus, jumpers are used to "hop" over the tracks on the PCB. Kind of like bridges over rivers.
In effect, it's like a poor man's double sided PCB. though instead of tracks on the component side, they use wires. If you go look at a computer motherboard you'll notice the same. All tracks on one side go east > west, tracks on the other side go north > south.
Because there would be overlap with the other printed stuff. Imagine if you drew four lines on one side of a piece of paper and you had to draw another one going perpendicular to them. You can't just draw over the four lines, cause then they'd all be connected and theey aren't supposed to be.
When wires are like that on a board they're called 'jumper wires' or 'jumpers', cause they jump over the other traces on the board. Hence why they're labled J1, J2, etc.