Opening up a carbon transmitter

[dsk]

Hi, I have this carbon transmitter who came with my 1935 telephone for moist areas. I t did still have this transmitter, but it is no good transmitter so I decided to have a look inside.

Not everything is easy to get in the photos, here the spring and rim of the cover and diaphragm was covered of an almost invisible layer of wax.  The pictures under should be the rest of the story.


dsk

[andy1702]

I've often wondered if dead carbon transmitters are repairable. This one seems to come apart quite easily so it liiks like the answer should be yes. Have you tried putting new carbon in there and seeing if it works?

[dsk]

No, I have no source of the right carbon granulate, but a carefully stirring with a small screwdriver made this one work better. I also have German war transmitter where the spring here is a threaded ring, but still the same, that one became usable from no sound at all with the same treatment.

dsk

[TelePlay]

No, I have no source of the right carbon granulate, but a carefully stirring with a small screwdriver made this one work better. I also have German war transmitter where the spring here is a threaded ring, but still the same, that one became usable from no sound at all with the same treatment.

dsk

Someone had a small vial of recovered granules on eBay for a bit over $100 US. The granules are more rare than the rarest phone. They don't make them anymore. This pdf file details how they were made. It's quite interesting how they start with a specific anthracite coal, grind it, eliminate any magnetic particles, then bake it at high temperature in an oxygen free furnace and sort of the just right size particles. Another WE production line "machine" scrapped long ago.

The Patent Office pdf file is attached below, just above the image.

If you want to build one in your basement, the attached image is a side view cross section of the image in the above pdf article.

===============

A shorter description or general overview of the carbon mic can be found at:

     https://mysite.du.edu/~jcalvert/tech/phones.htm#Trans

which if you scroll down to the transmitter section says:

"A carbon transmitter is shown in the diagram at the right. This schematically represents the classic transmitter of the 1920's and 1930's as used in a desk set. The principles are exactly the same in earlier and later examples. Carbon transmitters used in later handsets were more compact, and the carbon chamber was a different shape to facilitate use in different orientations.

The diaphragm is of thin aluminum, and presses on the front of the carbon button. A damping spring controls the motion of the diaphragm, and makes electrical contact. Insulation is provided by fiber washers and bushings, so that both the front and back of the carbon chamber are insulated from the frame and from each other. The carbon granules are from ground anthracite, passing a 60 per inch screen, but retained on an 80 per inch. They are carefully cleaned and heated in hydrogen to passivate the surfaces. The granules are held between carbon discs at front and back. This transmitter was sensitive between about 300 Hz and 3000 Hz, which determined the voice-signal bandwidth, and strongly peaked at about 1000 Hz. Later developments evened out the response considerably by 1927, with further smaller improvements continuing.

The carbon microphone is ideal for creating electrical noise, which appears as the "carbon hiss." The noise increases with the bias current, limiting the output of the transmitter. This noise is scarcely noticed in telephone applications, but is very disadvantageous in more critical ones, such as radio broadcasting and recording. In spite of this, carbon microphones were used until electronic amplification permitted the use of capacitor microphones and dynamic microphones, which are not plagued by noise. Crystal and electret microphones are used at the present time, but they are possible only because of electronic amplification by solid state devices. The sensitivity of the telephone carbon microphone was about -15 dB re 1 V/μbar, and its internal impedance was 30 to 100Ω.

The carbon microphone is subject to several diseases, mostly caused by damage to the carbon granules. They can "pack," greatly reducing the resistance, and the output, of the microphone. This can happen when too large a current is passed through the microphone, causing the granules to heat and fuse. If the circuit to a carbon microphone is opened while current is flowing, and the circuit contains an inductive element, the inductive surge can damage the granules. This doesn't happen, of course, when a microphone plug is simply pulled out of a jack, since there is no inductive kick."

So, save them if you get them just in case you need them.

[andy1702]

As far as I know carbon is carbon is carbon. It doesn't wear out. What happens (so I'm told) is the little granules get stuck together after many years, especially if the phone has been inactive, due to moisture in the atmosphere. Stirring them up will break up the clumps and that's why they start to work better.

The problem here in the UK is that most carbon transmitters seem to be almost impossible to get into without ruining the casing. Some so-called restorers put electronic microphones in the handsets, but to me that is not restoration. I always try to re-use the original parts where possible.

[TelePlay]

There is carbon and there is diamond. Of the carbon, there are many types of carbon from soot (fine powder) to big chunks such as electric motor brushes and electrodes.

1)  The carbon in transmitter capsules is unique in size, shape and quality.

2)  Transmitter particles are between 60 and 80 mesh in size (60 by 60 to 80 by 80 openings per inch in the grading screen).

3)  Then, the shape of the particle is round to oval rather than broken chunks which could catch on each other like puzzle peices and clump up.

Quality deals with the percentage of impurities in the carbon granules, the type of impurity, and the hardness of the carbon. Activated carbon (which is heated to drive off attached contaminants making it ready to absorb stuff) used in filters is full of holes and soft since it is made to have maximum area to capture contaminants in air or water. Metallic impurities, especially magnetic metallic impurities, are bad in that they will interfere with the function of the granules including corrosion and magnetic coupling or clumping. Organic impurities ( plant material) reduces or insulates electrical transmission.

So, 60 to 80 mesh hard, round and pure granules is what the patent office filing state is produced by the process and equipment and found in transmitters. The carbon can clump over the years due to moisture causing problems. The only solution is to dry out and break up the problematic carbon. How that's done is up to the person working on the transmitter.

C₁₂ is C₁₂ but the differences in carbon are as many as the uses of carbon. And, yes, diamonds burn in a fire just as fast as a chunk of coal.

[dsk]

As far as I know carbon is carbon is carbon. It doesn't wear out. What happens (so I'm told) is the little granules get stuck together after many years, especially if the phone has been inactive, due to moisture in the atmosphere. Stirring them up will break up the clumps and that's why they start to work better.

The problem here in the UK is that most carbon transmitters seem to be almost impossible to get into without ruining the casing. Some so-called restorers put electronic microphones in the handsets, but to me that is not restoration. I always try to re-use the original parts where possible.

Transmitter of this kind is rather hard to get, but I think it is an interesting step in the development of carbon transmitters.

Regarding electronic replacements, no it is not restoring, more modernization, I have done that twice to be able to use older phones without complaints from the other end of the line, that is when you not restore, but want to use an old telephone regularly, and have no other option.

dsk

[Haf]

I have done that twice to be able to use older phones without complaints from the other end of the line, that is when you not restore, but want to use an old telephone regularly, and have no other option.

dsk

dsk,

I know this problem too as I regulary use only old phones. And I disagree with modification. If you see elderly people walking slow- would you ask them to modify themselfes to keep up speed with the younger?
OK, relating to phones, of course they should fit at least the parameters when beeing new. But if anybody complaining about my voice on phone, well...just because they are not used to analogue technology. I grew up with it and I still remember when someone called from far away, it was clearly audible that he was doing so. And I think that is absolutely reasonable. And if you yourself use old phones, sure you have to do it like it was recommended. Speak into transmitter etc, there was a nice 1927 film called "Now you're talking" explaining the customer how to use the phone
https://www.youtube.com/watch?v=CgYH3w4Rzxk

Haf

[TelePlay]

Having some carbon granules extracted from ruined transmitters, I was able to get a grab a few photos of a few granules.

They are not all round but more than 50% seem to be round or oval.

The image on the right shows the center round spot being measured at about 0.22"

It takes about 11 averaged sized granules to cover half of the spot or 22 granules on the whole spot.

That puts each average granule at about 0.01"

The mesh size stated was 60 to 80 squares per inch.

Dividing 1 inch by 70 (the average mesh) yields 0.014" per mesh side or 0.014" by 0.014" and the wire has to be subtracted to the photographed granules showing about 0.01" each on average is close to the mesh size of 0.014" less the wire (which I do not know the thickness of but assume it to be 0.005" so all seems to generally match or correlate with the information given above.

I still think they tried to get a round or oval particle but just could not get every not so round particle out of the production run. As such, having or starting with a hard chunk of near pure, hard carbon, such as a motor brush, it may be possible for one to grind out replacement particles and if done so between two plates causing the grinding to be a rolling, who knows how well they would work. Only way to know is to grind some and test them in a transmitter.

[TelePlay]

With my equipment, this is the closest I could get in focus to show the particles.

I would like to thank our old friend static electricity on the plastic vial I was using for keeping the granules in the same place for each image capture. As soon as I dumped the few out, they went right to the plastic and stuck like metal filings on a magnet.

[dsk]

The grains I have seen are not particularly rounded, more like those pictured. I remember that I was told from a tech when I was child, the granulate was finer in the transmitters used in German and Swedish telephones with the impedance of about 200 ohms, bigger in the telephones with all the current going trough the transmitters. (Circuits equal to the WE302) and sometimes even bigger in LB telephones. The 2 last used impedance of about 50 ohms.  All the transmitters used in Norwegian made phones in from 1934-67 was Belgian made. (STK later ITT)  By my own opinion, the European transmitters was more noisy than the North american transmitters, but might be giving a louder signal. 

Regarding modifications, prefer to not modify, I have done so in a telephone with fixed transmitter with broken diaphragm, and no more coal.  I have a phone with empty transmitter and receiver housings her I did put in about 50 years newer components, (Still carbon transmitter) and in 2 fake/Indian brass candlesticks who already is a huge mix of old and new parts.  I have also tested out this in a field telephone handset with M-35/U capsule witch I could not get. 20 years later I go it and its back to original.

For you over there Audiosears looks like a possible supplier. Not actual for me.

dsk

[TelePlay]

The grains I have seen are not particularly rounded, more like those pictured.

These are 2 dimensional images so unless you look closely at the light reflecting of the center, roundness is hard to see. The granules with the white dot in the middle show a round granule. If not, then they are more flat given the light used to illuminate the particles. The granules in the images are from a WE F1 transmitter.

My thoughts on grinding new particles would be to crunch up a hard carbon source using two 1/4" steel plates with spacers hold them apart, first the thickness of the original large crunched particles and then reducing the spaces until it is 0.01" and in the size reduction process, grinding the steel plates to give the crunching a "rolling" effect on the particles.

The carbon would then have to be screened using first a 60 mesh screen to get rid of the too large particles and then with a 80 mesh screen to get rid of the too small particles and dust. And then a way to check them out, to compare them from original granules to the "newly" ground granules.

[19and41]

I remember in grade school, seeing a description of a carbon microphone that could be assembled at home.  It used the center carbon rod from a carbon zinc battery laid between two razor blades with the wire connections to the blades and the rod bridging the two.  It was said to be effective with a public address or guitar amplifier.

[TelePlay]

This currently reactivated topic shows carbon insertion in one type of transmitter.

     http://www.classicrotaryphones.com/forum/index.php?topic=6762.msg77778#msg77778

[dsk]

First time I opened up one, as a kid, I refilled it with iron filings, (didn't know better) kinda worked, but I had to use very little, and it was extremely position sensitive.

And, John this seems as a more practical way to open a capsule.

dsk