D-104 loading...

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

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  1. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    I have one I need to repair. The element generates a signal, but the quality is atrocious after some CBer punched a bunch of holes in the diaphragm, like a salt-shaker. I have another element with a dead crystal. I am contemplating salvaging the good diaphragm from the dead element and using it to replace the damaged one.

    If you were able to take yours apart without destroying it, how did you detach the diaphragm from the little rod that's linked to the crystal, and the rim of the diaphragm from the plastic case? What kind of glue did you use to re-assemble it?
  2. KE0ZU

    KE0ZU Ham Member QRZ Page

    I just used heat from a soldering Iron or a second or two, then a solder sucker to remove what I suspect is Bees wax or something similar, with the perimeter secured to the plastic shell using rubber cement. When I re-assembled them I used small amounts of Bees wax. On the crystal I used tiny dabs of epoxy to hold the bridle.

    Since Rochelle salt crystals are susceptible to moisture I'd certainly think they would have absolutely been sealed at the factory, and although I've never verified same, I've always assumed they were, and I've never noticed any holes, except in one instance similar to what you described.
  3. N1BCG

    N1BCG Ham Member QRZ Page

    What transducer was used to emit the audio for the mic to pick up?
    K4KYV likes this.
  4. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    Actually, there is one hole pierced in the diaphragm at the factory, probably to equalise air pressure on both sides. Years ago when I was still new at the game, I purchased a new element from the factory to replace the one in my mic that had gone bad, probably from excessive heat and humidity in my un-air-conditioned upstairs bedroom shack at the time. When the new one arrived, I noticed what looked like a hole accidentally punched in the cardboard container it came in. When I opened the box and took out the element, I noticed a hole in the diaphragm. Right away I assumed it had been damaged in transit somehow, that a sharp object had managed to pierce the box all the way through to the diaphragm. Also, I figured the little round disc of what looked like fibreglass insulation was intended as protective material during shipping. So, I carefully plugged up the hole with a tiny drop of epoxy and removed and discarded the fibreglass padding. The mic seemed to work OK, but later on after having disassembled a few other mics, I noticed they all had the hole in the diaphragm, and that the fibreglass padding was still in place. Mine had been undamaged all along, despite the hole knocked in the cardboard container. The hole in the diaphragm was supposed to be there.

    A classic example of Murphy's Law. If the cardboard container had not been accidentally pierced, I wouldn't have questioned the hole in the diaphragm, but the hole in both container and diaphragm made my hypothesis too convincing. Astatic should have included a note in the package explaining that the fibreglass padding was supposed to be there, likely in order to deaden resonances in the tinfoil diaphragm. In any case, I carefully pierced a new hole in the diaphragm using a tiny drill bit, and fabricated a new fibreglass pad from a piece of wall insulation. However, I couldn't tell any great difference in the sound of the mic before and after I re-modified it back to original.
  5. KE0ZU

    KE0ZU Ham Member QRZ Page

    Look at post #29 on page 3, or follow THIS LINK.
    Last edited: Jan 30, 2021
  6. KE0ZU

    KE0ZU Ham Member QRZ Page

    After reading your post above, I thought "that can't be" so went down and opened up 5 different elements I have in a drawer. I'd never looked at any of these before and to my surprise I found you are correct, every one has a hole in the diaphragm, although they are VERY small. I then opened up my station and test bench mics, both of which I'd repaired long ago, and they also had them. Obviously I never noticed them before.

    I've repaired 3 different elements and all used what seemed to be Bees wax to attach the bridle to the diaphragm, but looking at these tonite I also found they apparently used at least 2 other compounds for that purpose.

    And a third surprise was that on one of those 5 the entire front screen surround, and the "ring" were one cast piece, with only the back cover actually being held on with screws. The front screen was pressed in from the rear.
    Last edited: Jan 30, 2021
  7. K5UJ

    K5UJ Ham Member QRZ Page

    I was at the Indianapolis hamfest 2 or 3 years ago walking around and came upon a ham with a table selling this and that, some of which were 5 Astatic D104 elements for around $5 each. I examined them closely and noticed that they all had holes in the diaphragms so I took a pass, thinking they'd been wrecked by someone and were no good. A week or two later I was mentioning this to Don and found out the hole is supposed to be there. I had no idea who this seller was, but a friend of mine who was there at the time knew who the ham was and gave me his call sign. Naturally, he was one of these guys with zero contact information anywhere, other than a mailing address on QRZ. As a last resort I mailed him a note to try to buy the Astatic elements but he never responded. I cannot for the life of me understand why some hams refuse to put an email address out on QRZ. You can create some throw away burner address using gmail or something and check it once or twice a week, just to give folks a way to reach you in some minimal method. No spam to your regular contact addr., or phone number this way. You won't be putting your entire life on line. What gives....guys on the lam? Paranoia? I don't get it. Don't like Google? Use something else.
  8. WB2GCR

    WB2GCR Ham Member QRZ Page

    The material that connects the diaphragm to the stake is "sealing wax".
    Used on envelopes at one time...

    Melts at a reasonably low temperature. To remove the diaphragm, one can with a great deal
    of care lift up and cut/slide a razor blade under the aluminum edge. After it is all loose, heat
    the wax, not the aluminum point and lift off the aluminum.

    When you go to replace it, do it from underneath, the wax will them climb up the post,
    not run down... I think beeswax melts too low and is too soft. Fwiw.
  9. WB2GCR

    WB2GCR Ham Member QRZ Page

    This is my shot of the guts of a D-104 element.
    To this day I have no clue how the Rochelle Salts element is assembled!
    Note the two thin, thin aluminum (?) or tin ribbons coming out from the side?
    And then there is the wrap of foil... wazzat? And the red stuff...

    Attached Files:

  10. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    I think I may have solved the mystery of the holes punched in the D-104 diaphragm. Evidently this was supposed to improve bass response. The idea is that the case of the crystal element forms a sealed air chamber behind the diaphragm. If the chamber were completely air tight, the air trapped in the chamber would limit motion of the diaphragm, so Astatic put a small hole in the diaphragm to allow the passage of some air in and out of the chamber as the diaphragm vibrates. Since bass frequencies tend to vibrate at greater amplitude than than frequencies in the 3-5 kc/s range, the pressure of the dead air trapped in the chamber tends to limit the low-frequency response.

    Modification of the crystal element by piercing additional holes was supposed to further improve the bass response. The modification below was described by a ham, not a CBer. I would still be very reluctant to pierce additional holes in a good element, since these are rapidly becoming unobtanium, but if one has extras on hand, maybe one that still has output but performs marginally, it might be an interesting experiment to try this modification to see if it really works. I'm not sure about removing and discarding the fibreglass pad either. YMMV

    I suspect that as the story of this modification began to circulate in the CB community, that the mic sounds better after you "punch holes" in the diaphragm, with no further description of the procedure, some CBers interpreted this to mean simply stabbing the diaphragm multiple times with a sharp object.

    An alternative, suggested by Timtron who apparently had heard tell of this modification, to punching holes in the diaphragm might be to carefully drill a ventilation hole in the back of the plastic case of the element, being careful not to let any debris from the drilling operation fall inside the case. If the hole didn't make the hoped improvement, the modification could easily be reversed by stopping up the hole with epoxy.

    Holes in diaphragm.jpg


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