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705lowout [2007/05/20 19:44] (current)
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 +====== Model 705 Microphone Low Output Mod ======
 +**From the Ten-Tec Reflector December 22, 2000**
  
 +The reason you are having low output from your TT 705 desk microphone is due to a design flaw in its bias circuit.
 +
 +The Primo  EM-60  ( EM-100 in the 1201 kit ) electret cartridge used in the 705 requires 1.5 volts DC minimum to bias the JFET inside the electret well into the constant current region. Actually, 3 to 4 volts is much better.
 +
 +The existing TT design has way too much resistance (24K) in line with the electret such that it's bias voltage never rises much above .5 ~.6 volts DC; and the JFET never gets out of the constant resistance region, and consequently cannot develop adequate gain. 
 +
 +To fix your 705 mic, tack solder a 3300 ohm resistor across R11 ( which is 22K now ). 
 +
 +You will find that you will have plenty of output after this mod .
 +
 +If you are interested in enhancing the low end freq response as well, increase C2 to .47 uf or larger.
 +
 +Also you should verify that the 705 is being run on at least 7 volts by measuring its operating voltage at the M+ pad on the 705's printed circuit board. ​ Some older Corsairs have 10K behind the mic connector pin 4, which should be reduced to less than 68 ohms.
 +
 +
 +73,     - Phil    KF6ZZ

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