To open up a 319 remove the four screws around the grill, the grill
then will come off, just pull up, be careful the capsule is now exposed, there is a screw at the bottom that should be removed, and then the four screws on the plate for the switches. Now before you try to remove the internal works you have to unscrew the 2 screws on the little board attached to the switch plate. be very careful, the wires are easy to break. Now slide the circuit board back in the housing and carefully side the whole assembly up and out.
I like to replace all the caps in the electrolytics in the amp with
better quality caps, also the ceramic cap used between the capsule and the fet with film or silver mica caps.
the final thing that one might do is rebias the fet.
Marty
Oh, and while the circuit is the same as a 219, the layout on the board is very different.