Video on the Strange Tuning of the Radio Shack DX-390 Receiver

I'm more of a single conversion guy myself, but in working with the DX-390 I came to appreciate the benefits (especially regarding image rejection) of the double conversion technique. 

While working on the DX-390, I discovered that the BFO control on the front panel DOES NOT change the BFO frequency.  It was fun to try to figure out why the designers did it this way.  It does make sense once you consider the limitation imposed by that PLL main tuning oscillator that only moves in 1 kHz steps. I hope the video explains things.  

Here is the drawing I used in the video: 

And here is a drawing that shows how a single conversion superhet with a fixed or switchable  (usually crystal-controlled) BFO works: 

Earlier this month  I did a blog post on my repair of a broken DX-390: 

https://soldersmoke.blogspot.com/2020/08/fixing-up-radio-shack-dx-390-aka.html