You will find a schematic for your S-7404-6 under "Rider's by Original Index" here on Nostalgia Air. Air Chief radios were made by Firestone Tire and Rubber Co.
The next step would be to have all the tubes tested and replace any that test weak. I've found that it pays to check the tubes first. From your description, it may be the rectifier tube that converts AC to DC voltage.
Another possibility would be the filter capacitors in the power supply. If you hear a hum in the background with the radio on, the filter capacitors will need to be replaced.
I hope this helps you get your Air Chief back to its prime performance. If not, please post a reply; there are many others here on Nostalgia Air who may have a solution to the situations you encounter. Nostalgia Air is a very helpful forum.
Good Luck,
Mark
: I purchased an airchief radio with a repair bill in it dated 1941....stock# S-7404-6.....I plugged it in and it worked great for about an hour,,,,and then lost the sound,,,,,turned up all the way you can hear it just faintly,,,,I live in Michigan near Detroit and would like to repair it and find out what is wrong with it,, can you help....sincerly,,,Al Butler