Sunday, January 17, 2010

A collection of radio modifications for the Sangean ATS 818 aka Radio Shack DX-390 part 1

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 put in the "middle class" range compared to other portable receivers. It has some nice features such as single-sideband AM (SSB) reception and strong FM reception as well as a decent AM band. This radio has a surprisingly good sound coming from the 4 inch speaker. It has a tone control to shape the audio as well. Drawbacks include an over-sensitive shortwave circuit which easily picks up static and nearby noise as well as limited filter functionality. 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 the bedroom.

The good thing about this radio is most of the drawbacks can be fixed to make it a much better radio overall. Here are some enhancements for this radio.

As I mentioned earlier, the radio mutes the speaker when tuning. This is extremely distracting when trying to find DX stations or tune SSB. There are two ways to fix this problem. The first is to use headphones and turn on the volume while tuning/scanning. You will be able to faintly hear the audio, but will get a blast when you stop tuning. Instead of taking the risk of becoming deaf, try this easy modification to disable to muting. It takes about 10 minutes to do this mod after waiting 30 minutes to discharge the capacitors.

DISABLE AUTO MUTE

Minimum tools required: Phillips screwdriver, wire cutter, electrical tape.

Time required: 10 to 40 minutes

Side effects: Changing bands or turning the receiver on/off could cause a loud noise if speaker is turned up.


Procedure

  • 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.
  • 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.
  • Seperate the two halfs 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.
  • You now have two options - protect the wires from shorting or add a switch.


    Option 1:  Reassemble the receiver and enjoy DX hunting

    • Apply electrical tape or heatshrink 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 12 hours. This is caused by a processor lockup and the reset is needed.

    Option 2: Add a switch to enable/disable the mute function externally
    • If you cut the wire towards the middle, you know have enough slack solder the wires to a SPST switch.
    • The switch can be added right between the tuning and volume knobs. I recommend using a switch available at Radio Shack, model number 275-406. This is a pack of two switches for under $3.
    • After the hole is drilled, solder the wires to the switch and attach it with some hot glue, screws, or rivets.
    •  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 12 hours. This is caused by a processor lockup and the reset is needed.
    Your receiver now has the capabilities of higher end equipment. 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.


    73,

    Jesse, W9JES

    7 comments:

    1. Did you ever post your DX-390 backlight timer mod anywhere? Choppergirl a.t air-war.org

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    2. 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

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      1. Hi Jim. I am in the process of sharing how I performed the backlight mod in the next couple weeks. Thanks!

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    3. Jesse I got a Radio Shack Dx-390 radio from Ebay not to long ago. What I though was a problem with the radio turned out to be the auto speaker mute. I did find in the radio that someone has been doing some unknown modifications. An extra jumper wire and a diode, both on the foil side of the main pc board. the jumper is in the audio section. The diode is coming off one of the pins of the external antenna jack to ground. The black band on the end of the diode is facing ground. Now the jumper could be there to bridge a micro crack in one of the pc traces, I just don't know. But there is absolutely no reference to that diode been there in the service manual (I d/l both the owners and service manuals from Mods.dk). Both the jumper and the diode show signs of after market soldering, (a good soldering job). I have made up a blow up picture of that section of the printed circuit board, the foil side from the service manual, and used windows paint to add the jumper and diode. You can see the picture at https://www.dropbox.com/s/dn61idph4d102ak/blow%20up%20of%20cb.jpg My email address is kc7byp@msn.com

      Steve

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      1. Hi Steve. Sorry it took so long for me to see your comment. I've been away from my blog for quite some time and am trying to find time to share some ideas with everyone again. Could you please upload the picture again as the link is no longer valid. Thanks! W9JES

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    4. mod podswietlenia, podłącz pod guzik podświetlenia zwykły włącznik on/off i juz masz na stałe lub czasowo podświetlenie

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      1. Here is his translation.. "Backlight mode , connect the button backlight normal on / off switch , and already have a permanent or temporary lighting"

        You could absolutely add a switch to control the lamp. Bypass the current circuit and and connect +6VDC to one leg of the lamp and the other through a SPST switch to ground. I'd prefer to not drill holes in my radio and perform my mods internally so they can be reversed if needed. Thanks for sharing!

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