If you have
no reception below 800kHz both day and night, inspect the tuning cap, it may have a slightly bent plate that is shorting out either the antenna coil or the oscillator coil as the plates are meshed. Inspect it closely as you run the tuning up and down. You may be able to hear a scratchy sound as you tune below 800kHz.
Can you patch the speaker? Elmers glue and tea-bag paper works well.
Of course, recap it and replace the #47 bulb! Why even ask?
jr
PS: I suspect that you are looking for info on the Zenith J420T, (J402T does not exist):
http://www.radiomuseum.org/r/zenith_j420t_ch4j60t.html
Is that it?