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Carbon microphone revitalization
"Antonio Vernucci" wrote in message . .. 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 microphones are not generators, they need an external source of power. Typical voltage is around a volt or two, the less the better. The simplest way to test the mic is first to measure its resistance and see how much that varies when the mic is shaken or tapped. It will change some even on properly working mics. In the absense of a proper transformer check the audio quality using a battery and series resistance. Pick up the audio across the resistor. Carbon mics are prone to "packing" which is when the carbon granules stick together. Sometimes this is caused by moisture, sometimes by being subjected to too high a voltage which causes the granules to fuse. To get rid of moisture the element must be baked. If it is possible to remove the carbon granules and bake them separately that is best because a much higher temperature can be used. It might also work to put the element in a sealed box with a moisture absorber in it for an extended period. Nearly all carbon mics need to be stirred up pretty frequently. About the only ones that seem immune from this are the elements used in late WE telephones. They are designed so that they are not position sensitive and are well sealed so they don't much pack. Other types are position sensitive and won't work in some positions, for instance facing up or down. Its normal for most of these mics to need shaking pretty frequently. Carbon mics were very popular for many uses because they are amplifiers. This results in very high output. The vices are high noise and high distortion, at least in single-button types. The highest voltage that should be applied to a carbon mic is probably around six volts, but the lower the voltage that will result in adequate output the better since higher voltages tend to cause arcing and fusing. -- -- Richard Knoppow Los Angeles, CA, USA |
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