[Mondial]

Mike, at his point you probably need to do a full alignment. The 48-482 uses an oscillating quadrature detector for FM (FM1000 tube), and if it is not adjusted exactly right the FM will sound terrible. The IF amps are common to both AM and FM, so aside from the alignment, the tubes and associated circuitry is probably ok since your AM and SW reception is good.

Try going through the Philco alignment procedure using the loading resistor, but I had better luck using a sweep generator for best symmetrical bandpass of the IF. Also it is important to set the free running freq of the quad detector ( primary of transformer Z300)at exactly 9.1 MHz with no signal, then with FM modulated signal adjust secondary of transformer for best linearity of the demodulation curve, centered on the IF bandpass.

Alignment made a tremendous difference in the FM performance when I did it on my 48-482.