[TenTec] D-104 Microphone Notes

[Robert & Linda McGraw K4TAX]

A few general comments about using D-104 microphones on modern radio
equipment.  The original D-104 mike is a Hi Z mike designed to operate into
a mike input of some 100 K ohms or more, typical of vacuum tube input
stages.  Today's rigs typically do not provide an input Z in this high
range.  To that end and to drive lower impedance inputs, Astatic added a
pre-amp in the base of the G stand model T-UG8.  This pre-amp output level
is adjustable and provides adequate source impedance for today's solid state
mike inputs.  One must exercise caution as the pre-amp is very capable of
overdriving the input stage of most of today's radios.  To that end,
reducing the mike gain on the radio will not correct overdrive of the input
stage.  The correct approach is to reduce the output level of the pre-amp in
the base of the G stand. A control accessible from the bottom of the mike
stand will accomplish this task.  Be sure the 9V battery in the base of the
G stand is good too.

A more common and more difficult issue to identify is the condition of the
mike element itself.  Original D-104 mikes have a crystal element.  The
D-104C has a ceramic element.  Of particular note, the crystal element is
very prone to deterioration due to heat and moisture.  Usually storing the
mike in an environment where the temperature exceeds 100 degrees F will
cause permanent damage to the element.  Secondly, moisture developed from
the breath while "close talking" the mike will also cause damage.  In both
cases, this will cause the element to become brittle and generally "sound
bad".  As a note, the D-104C typically does not exhibit the failure
conditions from heat and moisture.  It however does not have the clean
communications sound of the original D-104.  The D-104C has a somewhat
different frequency response overall and the peak in the upper mid-range
does not exist as in the D-104.

In conclusion, a GOOD D-104 mike with the correct level into the radio will
sound quite good.  If you are getting poor audio reports, distorted audio
reports, most likely you have a bad mike or a pre-amp incorrectly adjusted
for your radio.

73
Bob, K4TAX