(This is the model with the cassette recorder and has a totally different main board than the ATS818 “no cassette” model)
Write down your memory settings, because the mod will clear all the memories. Begin by placing the receiver on a flat, WELL LIT surface, face down on a Magazine or folded towel to avoid scratching the front of the unit.
Remove all plugs from the left side (ext. pwr, headphones, ext ant) and switch both lock switches in the “lock” position.  Remove all batteries and  let the receiver sit without ext pwr or battery power for 1 hour to ensure that the memories are absolutely cleared.   Use a #1 phillips screwdriver (magnetized is best) to Remove the 5 screws that hold the rear panel, including the one in the lower left corner of the battery compartment. Gently pull the lower half of the rear cover loose, then bend the lower half away and yank the top half loose (it tends to hang up on the handle, but will break loose with a little effort).  Be sure to remove all 5 screws from the rear cover to avoid breakage of the front posts during reassembly.  Position the unit so that the speaker is at your lower left, and the cassette capstan main flywheel is on your lower right.  At this point, use a ruler to zero in on the work area of the main board.  The work area is approx 6 centimeters from the top of the green main board and 2.3 centimeters from the left edge (this is of course assuming you have not moved the unit from the previously instructed position).  Another good location mark is to measure about
1.7 centimeters above the upper left corner of the uppermost RF Ground shielding plate on the left.  At this area you will see 2  tiny surface mount transistors.
They are both rectangular in shape, and have 3 leads soldered to the main board.  You will note that the one on the left is mounted in a horizontal position, while the right one is mounted in a vertical position.  This is a very distinguishable arrangement and the transistors are only about 3mm apart.  Looking at the LEFT transistor (horizontally mounted) you will see a single circuit trace coming out from under the transistor and up, right between 2 solder joints. All you need to do  is cut the trace where it runs between the 2 joints and you are done.  Use a sharp Xacto blade or similar to gently go over the trace until it is cut.  DO NOT press Excessively hard and crack the main board!  Simply  make the same gentle, Repetitive slice until the trace is cut, then reassemble the unit.  Plug an external DC pwr source (6VDC, negative tip) into the unit, unlock the lock switches),
Turn main pwr on, then use a tiny screwdriver or toothpick to press the “reset” Button in the battery compartment just next to the 9K/10K tuning switch.

  U R all set!! Set your clock and memories and enjoy the lack of audio muting