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D-104 loading...

Discussion in 'Amplitude Modulation' started by W1BR, Jan 27, 2021.

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  1. K4YNZ

    K4YNZ Ham Member QRZ Page

    So did you try the crystal element? My Dad had a low gravel voice. He often used a D-104 stock head. He tried some HEIL products, including the EQ, with little luck.
    D-104 made me sound like a YL.
    You can retard the lows and boos the highs. But if your voice is all bass not m
     
  2. W1BR

    W1BR Ham Member QRZ Page

    All my D104 crystal elements had turned to mush.... even a few NOS from local estates that were stored in damp cellars and ruined. I was lucky to find the Kobitone.
     
  3. K5UJ

    K5UJ Ham Member QRZ Page

    Mike, thanks for posting those photos of the Astatic element interior. Those are keepers for future use in other Astatic topics.

    I hope Frank "Not giving any crap or taking it is a pretty good lifestyle" GFZ is doing okay!

    I was in a QSO once with a friend who had three D104 heads he could quickly swap for audio comparisons. A was the Astatic xtal element, B was the Astatic ceramic element and C was a Kobitone element. A sounded best by far, B was a close second and C was so bad we told him to throw it away. So, if you can't find a genuine Astatic xtal element that's good, you might consider some other crystal element microphone as a temporary substitution until you can find a good D104. Turner made crystal element mics. There were also imports from JA land that were okay. Another good one is the Shure 737A. Shure xtal element mics are fine but the key thing in all of this is an element with a wide diaphragm. Xtal mics made for lavalier use for example don't sound as good because they are small.

    If the rig being used is a Ranger, you can ditch that solid state preamp in the mic. base because an old tube rig like a Ranger doesn't need it. Just bypass the active component stage in the base and run the mic head right to the rig. You can adjust the rig's audio response by changing the coupling caps and bypass caps in the audio stages. Normally I make the input grid resistor 10 meg for a xtal mic because that provides the widest response but you can take that down to 3 megs to attenuate the lows. The grid resistor in the ranger is in a bizarre location as I recall, inserted inside the sleeve of the cable from the mic jack to the first stage. You can make all the coupling caps .05 mfd instead of .1, and reduce the value of the bypass caps to add more high frequency to the audio. The audio gain pot should be advanced just enough to fully modulate the rig without hum. Staying a few inches away and tweaking the audio pot to achieve this should keep hum down and prevent prox. effect.
     
  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    Set them in front of a big speaker playing loud music, To reform the elements.
     
  5. KE0ZU

    KE0ZU Ham Member QRZ Page

    Nice pics and Zed page, Mike!

    I too have done some REPAIR on these elements, although a bit on the small side they are pretty easy to work on. I've been amazed just how beat-up some of these can get and still be repairable.

    [​IMG]
     
    AC0OB likes this.
  6. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    Where did you find that chart, or did you develop it experimentally yourself? It is highly consistent with what I have observed with mine, although I believe the 500k load would kill the lows at a greater rate than what that curve shows. Attached is the curve published by Astatic in their D-104 data sheet. Also attached is the response curve built into my homebrew pre-amp. I use a 20 megohm load on mine.

    The combination mic and pre-amp should give me ~ 18 dB of presence rise, followed by selectable low-pass filters with either brick-wall cutoff at 3400~, or a more gradual roll-off that begins at around 5000~. No attempt is made to limit low frequency response beyond that of the microphone, and the pre-amp is flat down to 30~ or lower. I never get reports of bassy audio nor tinny or shrill. The near subsonic rumble of distant aircraft sometimes shows up clearly on my monitor scope long before it is audible in the shack, so the stock unamplified D-104 is indeed capable of picking up low audio frequencies.

    There is a fundamental difference between "bassy" (to much low or too much mud frequency) audio, and muffled audio (excessive high frequency cutoff). There is likewise a difference between tinny (excessive low frequency cutoff) and "bright" audio with a high amount of upper midrange and high frequency boost.

    Many stock amateur AM rigs and most SSB rigs, particularly the "vintage" ones, suffer from excessive cutoff at both ends, along with considerable distortion, giving the signal the infamous "space-shuttle" /"EAS alert" sound.
     

    Attached Files:

    KA0HCP likes this.
  7. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Don, great descriptions of what we 'hear' vs. cause. b.
     
  8. K0OKS

    K0OKS Ham Member QRZ Page

    I am curious how this chart was created. Are these loads connected at the element itself? Or how are they set up? I assume you need to buffer it following the load?
     
  9. KE0ZU

    KE0ZU Ham Member QRZ Page

    The response curves shown in the above post I clipped from some piece of Astatic literature somewhere, but I've no idea where or when

    However, this is the response of one of the elements I repaired some years ago with a 10Meg load, the input resistance of an HP-3400 Volt meter, which is my typical process. Also note the vertical scale is different, by a factor of 2, than the classic Astatic scales, however the response is quite similar.

    [​IMG]

    [​IMG]

    [​IMG]
     
    Last edited: Jan 29, 2021
  10. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    It's the resistive load the mic looks into. With a tube type pre-amp, it would usually be the resistor from grid to ground at the 1st audio stage. With some FET pre-amps, the gate itself supplies a resistive load of several megohms. Some transmitters, for example the Heathkit Apache, has a separate 500k resistor in the line from the mic input terminal to ground to choke the signal before it even reaches the grid of the tube.
     
    K0OKS likes this.

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