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Carbon microphone revitalization
I have a NOS T-17 carbon microphone that I use with my BC.-191.
The modulation level is rather low, but I cannot easily determine whether the problem is with the transmitter or with the microphone. Two questions: 1) I have been told that carbon microphones can be revitalized by leaving them in an oven for an hour or so at a temperature of about 180 F. Did anyone try to do that? It would be easier to put the entire microphone in the oven rather than just the capsule, but I am not sure on whether this could cause damages to the non-metallic parts 2) I would like to test the microphone by measuring the output voltage with an oscilloscope while talking. Does anyone have an idea on how much (peak) voltage I should see on the scope when powering the microphone with 12 V, through a resistor of known value? Thanks & 73 Tony I0JX |
Carbon microphone revitalization
Antonio Vernucci wrote:
1) I have been told that carbon microphones can be revitalized by leaving them in an oven for an hour or so at a temperature of about 180 F. Did anyone try to do that? It would be easier to put the entire microphone in the oven rather than just the capsule, but I am not sure on whether this could cause damages to the non-metallic parts I've never heard that, but back when telepehones uses carbon mikes, it was popular to fix them by removing the cartridge inside, and banging the cartridge on a table top. Geoff. -- Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM |
Carbon microphone revitalization
"Geoffrey S. Mendelson" wrote in message ... Antonio Vernucci wrote: 1) I have been told that carbon microphones can be revitalized by leaving them in an oven for an hour or so at a temperature of about 180 F. Did anyone try to do that? It would be easier to put the entire microphone in the oven rather than just the capsule, but I am not sure on whether this could cause damages to the non-metallic parts I've never heard that, but back when telepehones uses carbon mikes, it was popular to fix them by removing the cartridge inside, and banging the cartridge on a table top. Geoff. Additionally, that procedure is a legitimate and it is intended to break up and separate the clumped up carbon particles. A frequent procedure for outside pay phones back in the old days. You would smack the mic on the metal plate under the phone. Paul P. |
Carbon microphone revitalization
I've never heard that, but back when telepehones uses carbon mikes, it was popular to fix them by removing the cartridge inside, and banging the cartridge on a table top. Geoff. Oh, gosh! I didn't know the cartridge was supposed to be removed before banging. I thought that the fix was to just whack a carbon microphone on the nearest indestructible object. The mikes that came with the BC-191 and/or BC-375 often had metal housings and could be used as hammers in an emergency. Old Chief Lynn, W7LTQ |
Carbon microphone revitalization
Antonio Vernucci wrote:
1) I have been told that carbon microphones can be revitalized by leaving them in an oven for an hour or so at a temperature of about 180 F. Did anyone try to do that? It would be easier to put the entire microphone in the oven rather than just the capsule, but I am not sure on whether this could cause damages to the non-metallic parts I have done this before, although not to a T-17. The idea here is to bake all the moisture out of it, because the moisture causes the carbon granules to cake up and stick together. It took me a lot longer than an hour or so. 180'F isn't really very hot. 2) I would like to test the microphone by measuring the output voltage with an oscilloscope while talking. Does anyone have an idea on how much (peak) voltage I should see on the scope when powering the microphone with 12 V, through a resistor of known value? I do not. BUT, I would suggest first of all measuring the static resistance of the microphone and comparing that with the resistance of a known-good microphone. I might also try using a telephone transmitter element (in the US we have lots of Western Electric T-1 transmitters everywhere) and comparing that with the test microphone in measured sensitivity. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
Carbon microphone revitalization
I do not. BUT, I would suggest first of all measuring the static resistance
of the microphone and comparing that with the resistance of a known-good microphone. The static resistance is about 100 ohm, but I do not have another carbon microphone for comparison I might also try using a telephone transmitter element (in the US we have lots of Western Electric T-1 transmitters everywhere) and comparing that with the test microphone in measured sensitivity. --scott I tried to power the microphone with 12 V through a 1200 ohm resistor. Talking loud into the microphone and with the mouth very close to it, the scope (put across the microphone leads) shows a peak voltage of about 600mV (or 1200mV p-to-p). Perhaps it is good enough, but I am not sure on whether the bias current is too low, and I should then try again using a lower resistance. 73 Tony I0JX |
Carbon microphone revitalization
"Antonio Vernucci" wrote in message . .. I do not. BUT, I would suggest first of all measuring the static resistance of the microphone and comparing that with the resistance of a known-good microphone. The static resistance is about 100 ohm, but I do not have another carbon microphone for comparison I might also try using a telephone transmitter element (in the US we have lots of Western Electric T-1 transmitters everywhere) and comparing that with the test microphone in measured sensitivity. --scott I tried to power the microphone with 12 V through a 1200 ohm resistor. Talking loud into the microphone and with the mouth very close to it, the scope (put across the microphone leads) shows a peak voltage of about 600mV (or 1200mV p-to-p). Perhaps it is good enough, but I am not sure on whether the bias current is too low, and I should then try again using a lower resistance. 73 Tony I0JX Microphones vary but 100 Ohms is about right. Take the audio from the series resistor rather than the microphone. Set the voltage so that the drop across the mic terminals is about one to two volts. You can run more but the least is the best. Most devices, transmitters, etc., using carbon mics use an imput transformer with a primary of somewhere around 100 ohms and a secondary suitable for what you are feeding, usually a high impedance. The exciting voltage for the mic goes through the primary usually with a capacitor across the battery to insure a low impedance audio frequency path and minimum noise. See what you get and post again. -- -- Richard Knoppow Los Angeles WB6KBL |
Carbon microphone revitalization
Antonio Vernucci wrote:
I have a NOS T-17 carbon microphone that I use with my BC.-191. The modulation level is rather low, but I cannot easily determine whether the problem is with the transmitter or with the microphone. Two questions: 1) I have been told that carbon microphones can be revitalized by leaving them in an oven for an hour or so at a temperature of about 180 F. Did anyone try to do that? It would be easier to put the entire microphone in the oven rather than just the capsule, but I am not sure on whether this could cause damages to the non-metallic parts 2) I would like to test the microphone by measuring the output voltage with an oscilloscope while talking. Does anyone have an idea on how much (peak) voltage I should see on the scope when powering the microphone with 12 V, through a resistor of known value? Thanks & 73 Tony I0JX Tony, I just looked at a couple of single-button mics. The best one is an old military T-32 that used an early. large telephone-type element. Using a Simpson 260-8 VOM, I get a 400-500 ohm upward swing, with a whistle into the mic. Several "known good" T-17s get 200-400 ohm swings, less that the T-32. I find that I can get a very good idea of what's up by this simple test. I find also that its not so much the absolute static resistance but the amount of "swing" and hence, activity of the element. Of course, an element with several Kohms worth of static resistance would probably indicate a suspect element. Regarding T-17s, I've never been able to truly resurrect an element. I have found that the bypass capacitor, inside, can be leaky or shorted and can cause the problems. That, and some pretty corroded plugs and PTT switches. If you have a T-17D, they can't practically be fixed and require a new element be fitted. If you disassemble one of those, the carbon granules will spill out owing to the construction of that particular model. The straight T-17, the A, B, and C models can be more easily retrofitted. The H-33/PT handset element or one from one of those 1950's style telephone receptionist boom-mic/headsets work very well. de K3HVG -- Posted Via Newsfeeds.com Premium Usenet Newsgroup Service ---------------------------------------------------------- http://www.Newsfeeds.com |
Carbon microphone revitalization
Maybe we need to discuss what you really want to accomplish.
My experience with T-17 microphones 50 years ago is that they are just not very sensitive. I assumed this is deliberate - to make yourself heard over the noise in an airplane you have to yell into the mike. Now if you want to preserve an authentic T-17 then I guess something like baking the transmitter is about your only hope, and you can only hope to make the mike as good as it was when originally manufactured. I remember lots of guys in the old days, who were not interested in historic preservation, simply took the carbon element out of the T-17 and attached the wires to a Western Electric F-1 element and taped it in place. F-1 was the element used in the 302 telephones, forerunner of the later T-1 used in the 500 type sets. If you want to preserve the appearance of the T-17 but not the authenticity, then maybe you could get the original element out and replace it with either a T-1 or an electret mike behind the faceplate and nobody will know the difference (except you will sound a lot better on the radio). I was working with some military handsets a while back that use the equivalent of a T-1, and replaced the old transmitters with new T-1s. I also got an electret T-1 replacement from Mike Sandman - it was designed for use with a modular-corded handset, but with a little hacking I got it to fit into the older kind of handset. Has anyone worked out the circuit to use an electret like you get from Radio Shack - they have 2 and 3 terminal models - to replace a carbon element? Jim W6JVE |
Carbon microphone revitalization
Jim Haynes wrote:
Maybe we need to discuss what you really want to accomplish. My experience with T-17 microphones 50 years ago is that they are just not very sensitive. I assumed this is deliberate - to make yourself heard over the noise in an airplane you have to yell into the mike. Now if you want to preserve an authentic T-17 then I guess something like baking the transmitter is about your only hope, and you can only hope to make the mike as good as it was when originally manufactured. I remember lots of guys in the old days, who were not interested in historic preservation, simply took the carbon element out of the T-17 and attached the wires to a Western Electric F-1 element and taped it in place. F-1 was the element used in the 302 telephones, forerunner of the later T-1 used in the 500 type sets. If you want to preserve the appearance of the T-17 but not the authenticity, then maybe you could get the original element out and replace it with either a T-1 or an electret mike behind the faceplate and nobody will know the difference (except you will sound a lot better on the radio). I was working with some military handsets a while back that use the equivalent of a T-1, and replaced the old transmitters with new T-1s. I also got an electret T-1 replacement from Mike Sandman - it was designed for use with a modular-corded handset, but with a little hacking I got it to fit into the older kind of handset. Has anyone worked out the circuit to use an electret like you get from Radio Shack - they have 2 and 3 terminal models - to replace a carbon element? Jim W6JVE Jim, I'm surprised you didn't need an amplifier with that electret element? That's what the carbon-compatible mics use in 2-way and avionic installations to get the required output level and the correct low-Z. Also, the larger telephone elements won't fit inside the T-17 housing and would have to be mounted, up front, as you mentioned. I did a military mic article for the June 2007 issue of Electric Radio, if anyone's interested. DE K3HVG -- Posted Via Newsfeeds.com Premium Usenet Newsgroup Service ---------------------------------------------------------- http://www.Newsfeeds.com |
Carbon microphone revitalization
"K3HVG" wrote in message ... Jim Haynes wrote: Maybe we need to discuss what you really want to accomplish. My experience with T-17 microphones 50 years ago is that they are just not very sensitive. I assumed this is deliberate - to make yourself heard over the noise in an airplane you have to yell into the mike. Now if you want to preserve an authentic T-17 then I guess something like baking the transmitter is about your only hope, and you can only hope to make the mike as good as it was when originally manufactured. I remember lots of guys in the old days, who were not interested in historic preservation, simply took the carbon element out of the T-17 and attached the wires to a Western Electric F-1 element and taped it in place. F-1 was the element used in the 302 telephones, forerunner of the later T-1 used in the 500 type sets. If you want to preserve the appearance of the T-17 but not the authenticity, then maybe you could get the original element out and replace it with either a T-1 or an electret mike behind the faceplate and nobody will know the difference (except you will sound a lot better on the radio). I was working with some military handsets a while back that use the equivalent of a T-1, and replaced the old transmitters with new T-1s. I also got an electret T-1 replacement from Mike Sandman - it was designed for use with a modular-corded handset, but with a little hacking I got it to fit into the older kind of handset. Has anyone worked out the circuit to use an electret like you get from Radio Shack - they have 2 and 3 terminal models - to replace a carbon element? Jim W6JVE Jim, I'm surprised you didn't need an amplifier with that electret element? That's what the carbon-compatible mics use in 2-way and avionic installations to get the required output level and the correct low-Z. Also, the larger telephone elements won't fit inside the T-17 housing and would have to be mounted, up front, as you mentioned. I did a military mic article for the June 2007 issue of Electric Radio, if anyone's interested. DE K3HVG Since T-1 elements are fairly plentiful one can get the carbon granules out of one to repack the T-17. I did this a long time ago to repack a Western Electric 375 double-button broadcast mic. It worked fine but was, of course, tedious. Where the original carbon granules have been fused due to excessive current (too much voltage) simply drying them out will not fix them, they must be replaced. I don't remember how the T-17 element is constructed but most carbon mics are made so that replacing the carbon can be done. -- -- Richard Knoppow Los Angeles WB6KBL |
Carbon microphone revitalization
When I used to use them, we always repaired them by several sharp bangs
against against anything sturdy enough not to be damaged by the "hammer". W4ZCB |
Carbon microphone revitalization
"Harold E. Johnson" wrote in message news:_TZhl.555961$yE1.439407@attbi_s21... When I used to use them, we always repaired them by several sharp bangs against against anything sturdy enough not to be damaged by the "hammer". W4ZCB Depending on the design of the carbon cell there is a tendency for the carbon to settle and "pack". Shaking or rapping the microphone will distribute the carbon so that the mic works again. Note that carbon mics are also position sensitive. Some may not work well if used in some position other than they were designed for. One of the design objectives for the Western Electric T-1 cell, used in the 500 series telephones, was to make them as much as possible independant of position and immune to packing. Packing of a sort can also be produced by moisture causing teh granules sticking together due to moisture or because they have been fused by the application of to high a voltage. There are other causes of poor performance, such as oxidation of the contact surfaces. Fixing that requires opening up the mic and cleaning the surfaces, which may be difficult where one of them is the diaphragm. Carbon microphones were popular for communications equipment because they are amplifiers and, thus, can have very high output compared to a generator such as a dyanamic, magnetic, or crystal microphone. -- -- Richard Knoppow Los Angeles WB6KBL |
Carbon microphone revitalization
On Feb 2, 6:48*pm, "Richard Knoppow" wrote:
"K3HVG" wrote in message ... Jim Haynes wrote: Maybe we need to discuss what you really want to accomplish. My experience with T-17 microphones 50 years ago is that they are just not very sensitive. *I assumed this is deliberate - to make yourself heard over the noise in an airplane you have to yell into the mike. Now if you want to preserve an authentic T-17 then I guess something like baking the transmitter is about your only hope, and you can only hope to make the mike as good as it was when originally manufactured. I remember lots of guys in the old days, who were not interested in historic preservation, simply took the carbon element out of the T-17 and attached the wires to a Western Electric F-1 element and taped it in place. *F-1 was the element used in the 302 telephones, forerunner of the later T-1 used in the 500 type sets. If you want to preserve the appearance of the T-17 but not the authenticity, then maybe you could get the original element out and replace it with either a T-1 or an electret mike behind the faceplate and nobody will know the difference (except you will sound a lot better on the radio). I was working with some military handsets a while back that use the equivalent of a T-1, and replaced the old transmitters with new T-1s. I also got an electret T-1 replacement from Mike Sandman - it was designed for use with a modular-corded handset, but with a little hacking I got it to fit into the older kind of handset. Has anyone worked out the circuit to use an electret like you get from Radio Shack - they have 2 and 3 terminal models - to replace a carbon element? Jim W6JVE Jim, *I'm surprised you didn't need an amplifier with that electret element? *That's what the carbon-compatible mics use in 2-way and avionic installations to get the required output level and the correct low-Z. *Also, the larger telephone elements won't fit inside the T-17 housing and would have to be mounted, up front, as you mentioned. *I did a military mic article for the June 2007 issue of Electric Radio, if anyone's interested. *DE K3HVG * * Since T-1 elements are fairly plentiful one can get the carbon granules out of one to repack the T-17. I did this a long time ago to repack a Western Electric 375 double-button broadcast mic. It worked fine but was, of course, tedious. * * Where the original carbon granules have been fused due to excessive current (too much voltage) simply drying them out will not fix them, they must be replaced. I don't remember how the T-17 element is constructed but most carbon mics are made so that replacing the carbon can be done. -- -- Richard Knoppow Los Angeles WB6KBL - Hide quoted text - - Show quoted text - Yeah; doesn't 12 volts sound a bit high? While approx 100 ohms plus 1200 phms = 1300 ohms and therefore 12/13000 = about 9 mlliamps current, just wondering if a combination of packing, moisture and maybe a bit of carbon granule sparking at too high an energising voltage could be contributing to the problem? Energising voltage of around 3 volts was typical in many dry cell telephones sets; or even on sets where 48 volts is/was fed out from the telephone Central office . |
Carbon microphone revitalization
On 2009-02-08, terryS wrote:
On Feb 2, 6:48Â*pm, "Richard Knoppow" wrote: "K3HVG" wrote in message ... Jim, Â*I'm surprised you didn't need an amplifier with that electret element? Â*That's what the carbon-compatible mics use in 2-way and avionic installations to get the required The electret elements from Radio Shack include an amplifier, which is why you have to power them; but I don't know if it brings the output level up to that of a carbon mike. The electret T-1 replacement I got from Mike Sandman obviously has enough amplification built into it somewhere, since it directly replaces the T-1. But I didn't tear it up to see what is inside. |
Carbon microphone revitalization
Jim Haynes wrote:
On 2009-02-08, terryS wrote: On Feb 2, 6:48Â*pm, "Richard Knoppow" wrote: "K3HVG" wrote in message ... Jim, Â*I'm surprised you didn't need an amplifier with that electret element? Â*That's what the carbon-compatible mics use in 2-way and avionic installations to get the required The electret elements from Radio Shack include an amplifier, which is why you have to power them; but I don't know if it brings the output level up to that of a carbon mike. The electret T-1 replacement I got from Mike Sandman obviously has enough amplification built into it somewhere, since it directly replaces the T-1. But I didn't tear it up to see what is inside. All those electret elements have a FET-IC in them, which is a J-FET with controlled leakage so that they don't need a grid leak resistor inside them. Apply voltage across them through a resistor, pick the modulated signal off through a capacitor. With a little tinkering with series and shunt resistances you can make one fit right in place of a carbon element. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
Carbon microphone revitalization
Scott Dorsey wrote:
All those electret elements have a FET-IC in them, which is a J-FET with controlled leakage so that they don't need a grid leak resistor inside them. Apply voltage across them through a resistor, pick the modulated signal off through a capacitor. With a little tinkering with series and shunt resistances you can make one fit right in place of a carbon element. --scott Very interesting, gentlemen. I'm going to have to give this a try as I've, heretofore been using the element tken from H-33/PT handsets and those old single-headest PBX operator's units. Might either of you have the part numbers? I fear, also, that RatShak may well have deleted this item as they have hundreds of other components. de Jeep/K3HVG |
Carbon microphone revitalization
K3HVG wrote:
Scott Dorsey wrote: All those electret elements have a FET-IC in them, which is a J-FET with controlled leakage so that they don't need a grid leak resistor inside them. Apply voltage across them through a resistor, pick the modulated signal off through a capacitor. With a little tinkering with series and shunt resistances you can make one fit right in place of a carbon element. Very interesting, gentlemen. I'm going to have to give this a try as I've, heretofore been using the element tken from H-33/PT handsets and those old single-headest PBX operator's units. Might either of you have the part numbers? I fear, also, that RatShak may well have deleted this item as they have hundreds of other components. They are all pretty much the same other than polarity... you can grab any cellphone headset or computer headset from the dollar store and use that. If you want a nice electret element, try the Panasonic WM-61A, Digi-Key part number P9925-ND. It's pretty small so be careful soldering, but it is better made and lower noise than most of the cheapies and still only costs a couple bucks. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
Carbon microphone revitalization
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