Corrosion, plus a capacitor not rated for the RF power that was flowing through it. If you look the capacitor body has lost all the wax conformal coat normal for those age ceramics, and has been well baked.
Looks like SRBP, so the traces will be very fragile, so a big tip on the soldering iron, and gentle handling and lots of solderwick and a dental pick to lift each leg of the transistors are needed, otherwise the trace will come with. The heatsink is the ground plane, with all the mounting screws there providing the connection to it, and is common in older RF designs, as it is all stripline based. the component leads bent like that are standard, but the advantage is that you can do a lot of repairs with large SMD parts in many cases, especially with coupling capacitors, supply capacitors and such, just watch power ratings.
Replace the resistors ( not sure on the component to the right of the cap, might be a ferrite bead or a very low value inductor) and capacitor, but be sure the replacement ones are rated for the RF power they will handle.