[ARC5] [Milsurplus] Carbon mic pops

[Richard Knoppow]

         This is a conventional carbon mic input. A low voltage power 
supply is fed through the primary of a transformer to the microphone, 
the output being taken from the secondary. The components marked in your 
drawing "microphone filter" comprise a power supply filter.
      I don't know the resistance of a T-17, mine is not in great shape 
but it seems to me it measures around 1Kohm.  I have a RS-38A in very 
good condition that measures about 1K so that's probably a reasonable 
value. Standard Western Electric 300 and 500 series telephones put about 
4.5V across the microphone element, this is probably not far off. 
Assuming my resistance values are about right the voltage from the 
supply will be divided in about half by the 1000 ohm series resistor. 
This is partly for filtering but is mainly to limit the current through 
the mic element.
     I think you are seeing a switching transient caused when the PTT 
switch puts a load on the supply. The voltage will drop suddenly causing 
the inductance of the audio transformer primary to produce a transient. 
Its probably a click. If you get a "rush" it may be the resistance of 
the microphone changing from the current going through it. I have a 
couple of field telephones and it seems to me they click when the 
butterfly switch is moved to "talk" from listen. If I am right and this 
is just the way the thing is, you may want to put a transient limiter 
across the output. Bell System used varistors across the earpiece of its 
Type 500 telephones to eliminate strong transients from switching or 
ringing currents. There may be other solutions to this.
     Remember a carbon microphone is a variable resistor so it draws 
current when connected.
On 11/29/2016 10:44 PM, hwhall at compuserve.com wrote:
> Attached is the schematic of the mic circuit & amp that I'm looking at.
> The mic internal wiring looks like just a set of PTT contacts in series
> with the carbon mic button + the small cap across the mic button.
>
> The primary of T-4 has no current thru it until one of the mics is
> keyed, then suddenly current flows thru primary & carbon mic, inducing a
> transient in T-4 that appears as a loud initial pop in the headphones. I
> don't see how they prevented this pop from happening every time a mic
> was keyed.
>
> By hitting Send I'm about to remind myself whether these forums allow
> images attached.  :-\
>
> Wayne
> WB4OGM
>
>
>
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL