I thought I'd share this little bit of information, since it worked out so well.
Its probably a good guess to say there's a fair number of amateur radio operators who have, or have had, an Astatic D104 mic, aka the "chicken choker" or "lollipop". I have no idea what an original D104 should sound like as I've never encountered one that was in pristine working condition. Even the D104s that were thought to be "good" had their flaws.
I bought a used D104 online, basically for its classic looks. It was cheap enough. And its element was as raspy and gritty sounding, as you might imagine.
I tried that condenser element sold online. Wasn't all that thrilled with it.
I tried the Heil 5.1 "vintage upgrade kit" and I thought it was "ok", even wrote a review on eHam about it.
Then by chance, while doing some research about old Shure 520xx series mics, I came across an element that I thought would be worth trying in the D104 - namely the Shure R96 element, which prices out at $27 with free shipping from several online sources.
Long story short - I used the foam from the Heil kit, having to open up the hole to accept the larger element. The R96 is a large diaphragm dynamic element. When doing so, I made the opening large enough for the round rear section of the element, and simply cut a small slit on either side of the opening so the wiring tabs and wire would fit into those yet maintain a good seal. I could have used some dense neoprene foam I found on Amazon - about $4 for a sheet 3/4 inches thick and four inches square. I used the Heil impedance matching transformer low impedance side to the element, high impedance side to the IC-7300. The mic already got wired with an 8-pin connector to fit my Icom radios, so no changes there. It also had a blocking capacitor to keep Icoms "DC on Pin-1" off the mic.
First test - tone set flat on the IC-7300. Way too much treble. Had to back it down to -5 and boost the bass to +5 just to be even remotely satisfied. Gain worked ok at 75 percent, with 2 set on the compression.
To correct the excessive high end, I put a .2uf (mind the decimal please) tantalum capacitor across the elements leads (in the base of the D104).
Second test - tone set flat and perfect response. Very even response. For DX I can add a little treble. For extended rag-chew, add a little bass.
I ended up running it thru a small preamp I've got here, and dropping the compression to 1 on the IC-7300. The radio's gain is now set to 25 percent.
Anyway, I just thought I'd share what I found to be a really good fix for those old classic mics.
The same R96 element will fit into the gasket found on the Shure 520xx series (Green Bullet) mics, if you need one of those fixed up too. You can fit the transformer right into the back of the bullet shaped shell.