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Astatic AST-878DM Desk Mic Electret Element Upgrade

Shadetree Mechanic

Shadetree Mechanic

Delaware Base Station 808
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Oct 23, 2017
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I bought one of these a while back because it is one of the few desk microphones still available. Unfortunately I found that the audio has no bass. Great for DX, but not what I was looking for. The solution was to put an electret element from an old telephone handset in it. The electret elements that you can buy from everywhere have too much bass, the element from a telephone is perfect for voice communications. Go figure.

I have had great success upgrading the President and Anytone hand microphones too, check it out:
Shadetree Mechanic

Thread 'President Electret Microphone Upgrade'

The stock electret microphone that comes with the President radios now days is hard to beat. Changing to a dynamic mic will usually give a lower tone that sounds good, but it lacks the punch of the stock mic and consequently is not as loud. A power mic will bring up the audio level, making the dynamic element loud enough. But the punch is still gone.

There is a seller on ebay that sells a multiple element retro fit kit for the D104. Just put all the elements in parallel, giving more gain. And I just happened to have a hand full of the small electret elements that came from telephone...
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The new element is powered from the PTT switch where it switches the 9v battery power to the amplifier board. You can see the red wire attached to the bottom row of double pins at the bottom right of the board. The two pins on the bottom left were to switch the audio wire on and off, but this causes a scratchy sound when the PTT is pushed. The solution is to jump the pins together with a blob of solider. The audio gets switched inside the radio when the PTT is pushed, so having the mic switch it is unnecessary. Radios with VOX are like this from the factory.

(Note the big diode bodged onto the board)

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Power is going to the element through a 1K resistor. (If you dont use a resistor, you will blow the tiny diode on the back of the board) The two electrolytic caps are to isolate the amplifier board from the element power. They are wired together negative to negative, but a non polarized cap would do too. I just didn't have one handy. Two 4.7 microfarad caps in series for a total of about 2.3 mfd will pass plenty of bass, and the bass response can be tailored by reducing the value at the expense of gain.

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I didn't get any pictures of the element, but it goes in super easy. Basically you take the old dynamic one out, then put the electret under it. The electret pushes down into the foam, and it all goes back together like normal. This also keeps you from losing the dynamic one incase you ever want to go back. Also, the amplifier board has a lot of gain with the electret driving. I suggest starting with the mic level on the radio all the way down. Then slowly turn it up while talking into the mic. Stop when normal radio output is reached. The best way to listen to your transmit audio is with another radio that has no antenna and headphones. I have a monitor radio in my station set up for this. I do a quick check every time I power up just to make sure nothing got bumped etc.
 
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