Reddish residue around solder joints on circuit board.

[wikwox]

While removing one of the pots on my Fender Pro 185 I noticed that many of the solder joints had a reddish deposit on them, very soft and easily brushed away, is this the rosin in the solder?

[J M Fahey]

Yes.

[wikwox]

Does it come out of the joint with age? I've never seen concentrations as numerous as this.

[Enzo]

It comes out of the center of the rosin core solder when the part is soldered in.  In the manufacturing process, the boards are usually washed after soldering, which removes the stuff.  Most technicians have a can of flux remover on the bench to clean up after their repairs, but not all. SO if the parts were hand soldered at some point, the brown stuff would have been there ever since.