Here you go:
DX-390 mute modification
Lay radio face down on a soft surface. Remove all batteries and
then remove all screws holding back of radio (don't forget the one
in the battery compartment) and remove back. Remove screws holding
circuit board in place. Lift circuit board (carefully) from the
left side while flexing left side of case to clear volume control
shaft.
Once left side is clear, move board left to clear antenna and power
jacks. once board
is clear, turn it over toward you (be careful, wires are short).
In the upper left of the board , locate the volume control pot.
Just to the right of this is a metal can. Just above the can is a
multi-pin connector marked CNT 1. Locate the third wire from the
left on this connector. This wire is marked 10. Cut this wire. Mute
mod is now done. Tip: cut the wire with enough left so mod can be
undone if desired.
Note: Sometimes this radio has a processor lockup happen when you
do this procedure.
Leaving the batteries out for a few hours before opening it up may
prevent this. If you do experience this problem, remove batteries
and let sit overnight. The radio should work normally after this.

-----Original Message----- 
From: Butch Bussen 
Sent: Wednesday, November 23, 2011 4:58 AM 
To: [log in to unmask] 
Subject: Re: RadioShackDX390-DX392,SangeanATS818-818CS Receiver/Recorder. 

If this is the radio I'm thinking of, it is a good radio, but one thing I 
hated was the way it acted when you used the tuning knob.  When the pll 
switched, it shut off the audio and made a pulsing sound in the speaker 
and it was difficult to tune in a station using the dial.  I took mine 
apart and cliped a wire, probably still have the mod here somewhere on 
this computer, and that helped a lot.
73
Butch
WA0VJR
Node 3148
Wallace, ks.