Hi askwho.
"PA in" means power amp input.
C1 ... C15 refers to a 15 pin connector; some of those pins carry power, as Polo16 told you, others carry signal, audio, etc.
Now that this is clear, *do not build this amplifier*.
It's poorly designed, very unreliable, and burns so much that Marshall had to mount it on a small plug-in board (hence the need for the 15 pin connectors) so the musician can repair itself .... by buying and plugging in a new board, that is.
Search within this Forum for "Chipamps", they sell amp kits (board and all parts, except power transformer) which will help you build one.
In fact, that Marshall *is* a chip amp, but stressed way beyond what it can really deliver.
You don't want that.
There are also many other kit suppliers; "Vellemans" comes to mind, you'll find others.
Good luck with your build and feel free to ask for advice.