Sir Carl. . . .

After initially consulting the schematic for that set and seeing that it is using a variable inductance tuning mode of the IF transformers and then additonionally seeing that the pictorial of the chassis components seems to be suggesting that those units are smaller 3/4 inch by 3/4 inch cased units that seem to be clipped into a larger mounting plates.

Then, those might be the "Automatic" type of IF transformers, which have a proclivity for problems in two modes.

The fabrication mode for those internal C21 and C22 resonating capacitors is not the use of sealed silver micas units, but instead a completely open type of construction The associative terminal lugs have additional mini rectangular tabs of brass or tinned brass resting against the plastic infrastructure Two sets are right against the plastic while the opposite sets are placed so that there is a slight spacing from the other set. That then leaves initial spacing for a square sheet of mica with silvered, sprayed on electrodes sets having been placed on both sides of the mica. The silver pad dimensons being just a wee bit larger than the metal electrodes. With the mica insert placed between the opposing sets of two contacts another plastic sheet on top then permits the insertion of a central hollow brass rivet. Its compression then permits the flat brass contact strips to be pressed into the silvered contact areas of the mica sheet.

With time, and mainly, due to its its open construction, contaminating acidic and caustic air elements carried by the hygroscopic characteristics of air humidity, will be reacting with those exposed contact areas. The silver coating, blackening into a silver oxide surfacing and typically laterally migrating. Erratic contact area action can then develop between the compressive contact media.

Its correction is then dependent upon the disassembly of the unit and mechanically modifying, to permit the insertion of fixed silver mica units in place of the gutted initial capacitor construction. Or, the replacement caps may be installed at the bottom of the external terminal lugs of the transformer.

The very sharp distinct popping noise, drop in received performance, is indiative of an erratic contact action between contact and sprayed on silver mica contact (black oxidized) area.

Sometimes you can have the volume at max and OFF station and then take a soft "tapper" , akin to a rubber cushioned screwdriver handle, and then make a sharp tap to the bottom base of the IF transformer and it will make a one shot repetition of that familiar sharp crack.

(Probably then having made a "healing" of that erratic contact area . .but only for a short time)

Sometimes, it takes an additional tap at a 90 degree tap fom the first, to make it show, but that does clue in as to a connectivity problem with in the IF transformer.

Finally, go to the top search feature of this site and check off forum box and then and insert "silver mica disease "and be prepared for an onset of about two 'tousand and 'lebenteen different slants to this situation.