Many thanks to SWLing Post contributor, Jesse (W9JES) who writes:
I’ve been busy enhancing my Radio Shack DX-390. I added an IF-Out jack for my SDR, changed out the light for a LED, added a latch circuit for the light switch, added static protection, and disabled auto-mute. My blog with full instructions is at www.w9jes.com
Thank you, Jesse! Here are links to the various modifications Jesse has documented: