Sunday, January 17, 2010

Mods for the Radio Shack DX-390 / Sangean ATS 818 Part 1 Disable Mute

Let's start out with a collection of radio modifications for the Sangean ATS 818 / Radio Shack DX-390 world-band receiver. I've owned my DX-390 since 1993 and it is a great radio that I would place in the "middle class" range of other portable receivers on the market. It has some nice features such as single-sideband AM (SSB) reception and strong FM sensitivity as well as a great AM sensitivity with the large ferrite rod. This radio has a surprisingly good sound coming from the 4 inch speaker. It has tone and RF controls to enhance your listening experience as well. Of course there are some drawbacks (no radio is perfect) including a highly sensitive shortwave circuit which can easily pick up static and nearby noise sources. There is also limited filter functionality for signal and audio bandwidth and station memory capacity commonly found on more expensive radios. It also mutes the audio while tuning and scanning for DX. The backlight only stays on for a short time and is not adjustable. I find myself pressing the light button a lot when tuning in a dark room.

The good thing about this radio is most of the drawbacks can be fixed to make it a much better radio for portable or home use.

As I mentioned earlier, the radio mutes the speaker when tuning. This is extremely distracting when trying to find active stations, chasing DX, or fine tuning SSB signals. I will show you how to disable PLL muting. It takes about 15 minutes to do this mod.

Luckily, Sangean and Radio Shack incorporate the muting signal on IC403, pin 47. A single wire travels from the controller board to the receiver board circuitry. We can cut one wire to eliminate it.


Minimum tools required: Phillips screwdriver, wire cutter, electrical tape/heat shrink tubing

Time required: 15 minutes


  • Disconnect all power from the receiver including the D and AA batteries for 30 minutes before opening the case. This is necessary to discharge the memory backup capacitor and avoid PLL lockup.
  • Remove the volume knob by pulling it straight out.
  • Turn the receiver over and remove 5 black screws. One of the screws is in the battery compartment.
  • Separate the two halves of the receiver and lay the radio back down on its face.
  • Locate and remove 5 silver screws from the circuit board.
  • Gently flip over the circuit board while making sure not to bump the AM antenna wires. You will have to slide it sideways to clear the headphone and ext antenna jack.
  • You will notice two connectors near the bottom of board nearest to the battery compartment.
  • Locate the connector CNT1. This is a multi-wire connector that has 12 pins. The pin numbers are printed on the board.
  • Cut the wire going to pin 10. Make sure to leave about 2 or 3 inches of wire to the connector in case you want to reverse the mod.

    • Apply electrical tape or heat-shrink tubing to prevent the cut wires from shorting.
    •  Place the circuit board back into place and secure it with the silver screws.
    • Put the back of the receiver on and secure with the black screws.
    •  Put the batteries back in and turn on the receiver to test it.
    • If you are having trouble hearing stations on shortwave or AM after the mod, remove the batteries and let the radio sit without power for a few hours. This is is a result of the PLL chip lockup and the reset is needed. The radio utilizes a memory capacitor that must be discharged.

    Your receiver now has the capabilities of SW radios that cost much more. Enjoy DX hunting! The next part of this series will cover the backlight timer. I will show you how to extend the 15 second limitation of the backlight and add an LED.

    Jesse, W9JES


    1. Hi Jesse, many thanks for posting the mute mod, it worked a treat for me but could you also post the backlight mod please, cheers Jim

    2. My latest DX-390 find from eBay ( $36 Priority Mail)

      Refuses to staty OFF!

      Comes back on after about ten seconds to the last station selected.

      Only stays off after removing the D-Cells

      and/or AC adaptor...

      Is this a diode problem

      Or an IC Chip issue..