D-104

[kopcicle]

Parallel technology,
About a year ago I was faced with diaphragm replacement in a number of of regulators. This is what happens when a system is designed around obsolete parts. A flat diaphragm didn't have the range of movement. A conical diaphragm had even less movement. Making a copy of the original was the only recourse. I wish I had photos on this network. Suffice to say that the result looked very much like the Astatic mic diaphragm, just a bit larger and thicker and made from full soft copper rather than aluminum alloy.

Positive and negative dies were made from 4130 steel in a manual machining operation on a lathe. In the interest of time only this was done manually rather than with the aid of CNC. By the time the code was written and the materials delivered to a shop both die were approximated and perfected the old fashioned way.

Process is simple. Place the thin sheet between the dies located in the lathe and run the tailstock into the headstock . The heat work hardens the metal and only final trimming remains.

I and several others are thinking that along with the glue separating between the original salt/ceramic and the backing that damage to the diaphragm is the most common sin found in the mc-321 cartridge.
We are collecting data for replication of the diaphragm in hopes that at least a few can be saved. Initially I'm hoping that high temperature plastics will suffice for dies and any other materials will come later if necessary. I've already found various alloys, heat treat, of various thickness to experiment with. I also have a fair bit of experience with aluminum heat treat.

A fools errand some may say but the thought persists. A fine niche for a nervous twitch my grandfather might say.

I'll report back with the results from the first die test. Whenever that happens ...

 

[Robb]

Humidity and time have taken their toll on these elements.

However, condenser mics have a diaphragm that is restricted as well. Be a neat trick if you can charge the diaphragm and turn it into a condenser element.
Sensitive at an arms length away, wide dynamic range, low distortion, and flat response. Better than the OEM xtal element - by far.
Like I said, it would be a neat trick - indeed . . .

Using a 9v battery and a xformer to jump the voltage up to ~50v could be done inside the D-104. Might require more than one battery to supply enough current.
Some thoughts . . .

 

[kopcicle]

http://www.extremenxt.com/blog/?page_id=77

 

[nomadradio]

The bottom two pics above appear to be the ceramic-element "MC321" cartridge. The top pic looks exactly like the MC320 crystal element.

73

 

[Jay Mojave]

Hello All: While shoveling out the Advanced State of the Art work complex, em the Garage, I come across a few marked "Bad" D-104 mic heads. So whats the fix for these D-104 mic heads? There all pretty and chrome looking, how to save them? Someone should sell a kit or something? That's sounds close?

Jay in the Great Mojave Desert

 

[oldtymer]

Hello All: While shoveling out the Advanced State of the Art work complex, em the Garage, I come across a few marked "Bad" D-104 mic heads. So whats the fix for these D-104 mic heads? There all pretty and chrome looking, how to save them? Someone should sell a kit or something? That's sounds close?

Jay in the Great Mojave Desert

Heil sells a kit & W2eny on Ebay has one also

 

[Robb]

Kopcicle; are you going to go thru the chemical process to use Rochelle Salts to rebuild the 320 element?
That will be a trick. I see that he salts are available thru photographic supply sites.

SO; how do you go about making the salts work in the element?
Easy bake oven?
That might work.
Done it before?

 

[Redbeard U812]

SO; how do you go about making the salts work in the element?
Easy bake oven?

Sorry, but I just had too!

 

[Robb]

Sorry, but I just had too!

I think what Kopcicle has in mind is rebuilding the mic element by taking it apart and either remelting the Rochelle salts or using fresh salts to form a crystal structure that has adhesion. Rochelle salt, don't know if you are aware of, has the piezoelectric effect. So, by either heating the salts present in the element or replacing them with new, uncontaminated salts, will make the element a working unit. If he can pull it off.

Either using a heat gun, hot air station, or a HID lamp can get there - if done carefully. A small electric oven might also work. The melting point of Rochelle salts is 75C; so carefully heating them will possible work. Let's see what he does and how he does it . . .

 

[unit_399]

Hello All: While shoveling out the Advanced State of the Art work complex, em the Garage, I come across a few marked "Bad" D-104 mic heads. So whats the fix for these D-104 mic heads? There all pretty and chrome looking, how to save them? Someone should sell a kit or something? That's sounds close?

Jay in the Great Mojave Desert

I used to repair D104s using a replacement crystal element from RBMICRO. Clicked on their site, and found that they are sold out and are not producing any more elements. Damn!!

http://www.rbmicro.com/index.php

- 399

 

[nomadradio]

What we found is that the rochelle-salt element is usually just fine. Yes, I know the conventional wisdom is that they deteriorate from years of humidity. The failure we usually find is much more mundane and entirely mechanical in nature, not chemical.

Two weak points in the crystal-element "MC320" cartridge cause it to become weak and tinny sounding.

Using the mike as a 'straight' D104 without amplification makes this deterioration apparent at once.

The amplified version may fool you into thinking the cartridge is okay. The amplifier in the base of the mike has a buttload of gain, and will make up for a weak cartridge, at least for a while. Eventually that "hollow" sound takes over and the condition of the cartridge becomes too poor to ignore.

First, there is a daub of wax at the center of the aluminum diaphragm. It bonds the upside-down "Y" part that is glued to opposite corners of the rectangular crystal element inside. This piece, or "yoke" is made of fairly rigid brass.

The bond to this wax can come loose. Reheating it can fix this problem.

Much more common is failure of the glue that attaches the crystal element to the element's plastic "cup" housing.

The crystal element rests on two tiny blocks of hard rubber. The glue holds these each to one diagonally-opposite corner of the element. Each block is also glued to the inside of the cup. The Y-shaped yoke that links the other two diagonal corners of the element to the diaphragm is arranged to put a maximum twisting motion onto the element.

The tube-radio guys covet this mike to use without the preamp. Lets them run power without signal-rectification and 'squeal' problems from the preamp circuit.

The D-104 cartridge was the original DX speech processor. The published frequency-response curve shows a peak around 2 kHz, meant to maximize punch for AM operators of the 1930s.

I'd love to find the alloy used for that aluminum diaphragm, and make a die to form new ones. No way to disassemble a MC320 without destroying the old one.

Pretty sure a bit of the right glue, a new diaphragm and wax would rehab the MC320 cartridge back to original performance.

Maybe.

73

 

[nomadradio]

So here is the 1937 sales brochure for Astatic's crystal mikes.

Note the company's location is Youngstown Ohio. They didn't move to Conneaut until 1943.

This was genuine hi-tech 82 years ago.

Hopefully I didn't shrink these too small to make out the text.

73

 

[Redbeard U812]

Just had a friend show me one he had converted. Placed a speaker in the head of the D104. Claims it out performs the two that are original, one of which used to be mine. Wish I had never given that one up. Sold it with the Siltronics 1011D, which is in the bone yard now. Shame on me.

 

[543_Dallas]

The tube radio guys use the non amplified version because the impedance of the element matches the rigs. Even a good element will sound bad when connected to a low impedance. The amp board does a decent job of matching that high impedance element to a modern rig. Unfortunately some people crank it up and talk into the mic at arms length.

 

[timelake]

To kopcicle and to nomadradio

Wanting to restore my D104 faithfully I dissected my MC320 and while searching for possible repair notes online, I was brought to this thread - Very glad it did.

I had not expected to find focused AND engaging comments on MC320 construction as found here so I must first thank you Kopcicle for posting your comments as well as for the excellent close up photos, and I also thank Nomadradio and Robb for your followup comments.

Although there’s has been no recent update since your last posts, I hope to evoke one with my comments, below.

As affectionately depicted in Norman Rockwell paintings, families in 1930 America would gather around to warmly enjoy this radio experience. As a radio enthusiast since 12, I came to realize my love for radio was due to listening to that same warm fire-side chat AM audio.

Inkeeping with this radio experience, I wholeheartedly prefer to restore my D104 faithfully, by reconstructing its MC320. And by this blog, I now realize that perhaps I’m not alone in this pursuit.

Though long discontinued MC320 elements are nowhere to be found, alternatives do not appeal to me. However entries in this blog consider reconstruction, which I find inspiring.

I applaud the thoughtful comments herein - by you, by Nomadradio and by Robb - because by having examined MC320s carefully, the observations made are accurate and suggestions offered are sensible.

So by my comments here, I hope to inspire a re-marketing of these MC-320 in their original technology, with the same salt crystal vintage technology, to be sold perhaps in a kit format if not preassembled.

To borrow a proven business paradigm, ribbon/velocity mics have been around decades and they too and are still coveted, albeit in different audiophile circles than radio. They are, if not more than as challenging to restore yet their ribbon transformers and ribbon motor assemblies are replicated and marketed online, allowing ribbon-mic enthusiasts to restore their original ribbon mics.

I believe that salt crystal mic kits, with preformed diaphragms, will be as viable a marketable venture as those ribbon mic subassemblies, if made and offered responsibly with consumer-friendly policies.

So there, I've expressed my gratitude to you both for your inspiring comments, contributing what I could to foster overdue support for a growing international community of AM enthusiasts.

I've submitted the above in goodwill, that those in position to move forward with such a venture, do so. Thank you all for a great thread!