[MrFixr55]

Hi Bryan, 

That looks like an early 1940s mod done by a Pro, using an adaptor sold by RCA.

This input has a sensitivity of approximately 0.5V to 3V s. If your "modern" turntable has a magnetic cartridge, a preamp is needed.

You state that the shortwave signals that you successfully receive are loud.  If so, then the IF alignment is good.  If you are getting a lot of loud buzzing noise instead of voice / music on the broadcast band, then definitely lengthen the antenna, and if possible, raise the height.  Also, consider opening the link and connecting a made ground to terminal 2 instead of the AC Outlet's ground.  You may get less noise.  If your shed has a water supply via a buried pipe, use that. Otherwise, go to Homeowner #&!! (Home Depot) and get a ground rod or even a piece of conduit (the ground rod is better) and hammer it into. the soil.  Sandy or loamy soil is best, clay is not as good.

You may be tempted to "diddle" with the IF, RF or oscillator adjustments.  Avoid doing so.  If you really must, then mark the current screw positions, and do not turn more than 1/4 turn in each direction.  If no improvement, return to the original position.

Hope this all helps!