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« on: December 04, 2022, 1359 UTC »
The first question I can answer confidently for you. Yes, you can transmit outside the ham bands with the DX-40. Unlike modern transceivers with digital control that can prevent you from doing so, the DX-40 is a simple radio from a different era, at a cost efficient price point. A crystal for just below the 40m band (or above the 80m band, say) should work just fine for you.
Audio will be a different story. What you have there is a screen modulated transmitter designed primarily for CW. AM was achieved with a high impedance microphone like a D-104. Wiring the headphone output of an MP3 player to the mic jack may modulate the radio to some degree, but it may not sound great. It certainly won't hurt to experiment but watch out for ground loops on the audio, etc and you may find you'll need to pursue some kind of impedance matching audio transformer, or a different way to modulate the radio entirely.
Also, when working on equipment of this vintage be extremely mindful of high voltages that are present, and that can remain present even when the radio is OFF. Take precautions to bleed power supply caps before working on the radio.
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