You should have two nulls, each 180 degrees opposite each other, one being at the 0 point on the knob. To do what Clark says assumes the mica cap on top is set correctly. If the filter was not aligned correctly, you will have to visually assure the phasing varible is set at 1/2 capacitance and then do an alignment. Once set correctly you adjust that mica for the null to be 180 degrees out as Clark described.
Some sets had a mark on the shaft so you knew where the cap set point was. If you didn't see one and your null is wrong you will have to open the box to see.
How do you get into that can? Is it possible to do with the cabinet on?
I tried adjusting C36 without opening the can with zero success. The nulls are about 90 degrees apart. I would love to get in there to see what's going on with the mica caps and to put an alignment mark on the shaft of the phasing variaible cap but it doesn't look easy.