[ARC5] [Milsurplus] Carbon mic pops

[Richard Knoppow]

    These caps have nothing to do with the "pop". That is a switching 
transient caused by turning current on and off through the inductance of 
the mic transformer primary. The purpose of the capacitors is only to 
filter the DC supply. The 1K resistor in series with the filter inductor 
is to limit current through the microphone. Carbon mics need relatively 
little current to operate but they are also amplifiers so the choice of 
current is often made to obtain a desired level on the line.  The higher 
the current the more likely it is that the carbon will be "burned" and 
performance degraded.  Standard Bell System practice was to put about 
4.5 volts across the mic. The F-1 element used in the old Type 300 
phones has a resistance of about  500 ohms so the current is on the 
order of 10ma.  Probably about the same here although the resistance of 
the T-17 is probably higher.

On 11/30/2016 1:31 PM, hwhall at compuserve.com wrote:
>>Are the two mike filter capacitors originally 50uF?
>
> Yes, that's what was on schematic & what was found installed. Restuffed
> them with 47uF new ones.
>
>>
> Since "new" capacitors are so much better then what was used originally,
> I wonder if you paralleled the caps with high value resistors, would
> this would reduce the pop?  Perhaps a resistor value that would cause
> 1/10 the microphone current to leak around the caps?
>>
>
> Never thought about that, Jim& it seems sort of counterintuitive, so I
> probably wouldn't have. But that wouldn't cause a current thru the T-4
> primary, so when the mic is keyed there's still be a sudden current in
> the windings & a transient out the secondary side (the pop).
>
> If there was constant current thru the primary, with the mic element
> getting switched in & out of that current, it might make a difference.
> But there's apparently no provision for that kind of switching. I'd
> thought once about whether the mic's contacts could be rewired as a
> short across the mic element, so that current always flowed, & the short
> opened only when you wanted to talk, but the T-17 contacts aren't put
> together that way.
>
> Wayne
> WB4OGM
>
> -----Original Message-----
> From: antqradio <antqradio at sbcglobal.net>
> To: milsurplus <milsurplus at mailman.qth.net>
> Sent: Wed, Nov 30, 2016 10:05 am
> Subject: Re: [Milsurplus] [ARC5] Carbon mic pops
>
> Wayne
> Are the two mike filter capacitors originally 50uF?
> Since "new" capacitors are so much better then what was used originally,
> I wonder if you paralleled the caps with high value resistors, would
> this would reduce the pop?  Perhaps a resistor value that would cause
> 1/10 the microphone current to leak around the caps?
> Jim
>
>
> ------------------------------------------------------------------------
> *From:* "hwhall at compuserve.com <mailto:hwhall at compuserve.com>"
> <hwhall at compuserve.com <mailto:hwhall at compuserve.com>>
> *To:* arc5 at mailman.qth.net <mailto:arc5 at mailman.qth.net>;
> milsurplus at mailman.qth.net <mailto:milsurplus at mailman.qth.net>
> *Sent:* Wednesday, November 30, 2016 12:44 AM
> *Subject:* Re: [Milsurplus] [ARC5] Carbon mic pops
>
> 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