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The what and the why of Wireless Set No.19 Microphones.


In the past there has been some discussion and confusion about what microphones work with the 19 set. This is a
short note to clarify the possible configurations.

There are two possible types of microphones that were used with the 19 set: dynamic and differential carbon granule.
They are pictured in Wireless For The Warrior page WS 19 Control 15.

The dynamic is the familiar Microphone, Hand, No. 7 with the rubber noise exclusion cone which is part of the
Microphone and Receiver Headgear #1 It is a moving coil unit, like a small speaker, and generates a voltage as a
result of diaphragm motion.

The carbon granule is Microphone, Hand, Power No. 1A and is part of the Microphone and Receiver Headgear No.
2. The interesting thing about this microphone is that it appears to be a DIFFERENTIAL carbon granule unit. By this
I mean that it has 3 wires, a common and two signal wires. It appears that the carbon granule capsules are connected
in a push-pull configuration such that if pressure is applied to the diaphragm, the resistance of one side increases and
the other side decreases.

The control boxes are of three types:

Those that provide no power to the microphone and contain no transformer. These will ONLY work with the
dynamic microphone used in the Headgear #1. ALL the control boxes are of this type, with the exception of the
Junction Distribution Boxes #1 and #2.

Those without any snatch plugs, such as the #10. Since there is no microphone connection, the type is moot.

Those that can provide power to the microphone, the Junction Distribution Boxes #1 and #2. These are identical save for the provision of a call buzzer in the #1. These boxes will work with EITHER headset as follows:

If used with the dynamic microphone, the microphone output on pins 4 & 5 of the snatch plug is coupled to the set
through the microphone transformer. Except for the insertion of the transformer, this is the usual mode of operation.

If, however, the carbon microphone is used, the operation is as follows. The vehicle +12 VDC is connected to the
centre tap of the microphone transformer primary through a current limiting resistor and splits into two approximately
equal currents, each of which flows through a carbon granule capsule to ground. This is clever for several reasons:

A) Motion off centre of the diaphragm makes the resistance of one capsule go up and the other go down, producing
a differential mode signal. Noise on the +12 VDC supply line is common mode, and is therefore rejected. A decent
transformer has very good common mode rejection.

B) The DC current in the two halves of the transformer primary cancel each other so the transformer core does not
saturate. This is a cool design, like the output transformer of a push-pull amplifier. (Thanks Chuck)

C) The powered microphone has gain, so it can work without the IC amplifier.

NOTE: The Junction Distribution boxes have no ability to key either transmitter. They ONLY provide an internal communication facility for the AFV.

So, what does this all mean?
Here are the simple rules:


The dynamic mike will work with any control box.

The carbon mike(Microphone & Receiver Headgear #2)will ONLY work with Junction Distribution Boxes #1 & #2


By the way, you can test your dynamic microphone simply with a 'scope or the IC function on your 19 set.

For the scope, just look at the output of the capsule, or at the snatch plug, or at the terminal strip in the control box (this is the easiest). Speak into the mike and you should see voltage. The pinouts are in the Working Instructions or in
Wireless For The Warrior.

To use the set, just switch your headset to IC. Press the presstle switch and speak into the mike. If you hear yourself,
the mike is good. Period. You should also hear side tone on either the A or B sets when the switch is pressed, but then you are also turning on the transmitter!!

Alister Mitchell.


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