[ARC5] Carbon mic pops

[Brooke Clarke]

Hi Wayne:

The DPST switch turns off the dynamic mike for security reasons. There was an "Infinity telephone tap" which was a hook 
switch bypass activated by a note from a harmonica.  See the movie "The Conversation" for an example.
The same attack could be made to a radio transmitter by pressing the PTT using some remote method and then the mike 
would be live. Another way would be to feed the mike signal to a secondary transmitter.  To prevent that the handset or 
microphone PTT switch also completes the microphone connection.  This is not done in the H-350 handset because it's 
intended for full duplex telephone applications done using a different U-229 pin so that substituting an H-250 for the 
H-350 will not fool the system.
http://www.prc68.com/I/H250.shtml - note Maintenance Instructions
http://www.prc68.com/I/H350.shtml - note Maintenance Instructions for D to F

-- 
Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
The lesser of evils is still evil.

-------- Original Message --------
> I have noted that it is common practice on aircraft now for a PTT to be DPST and to switch both on the transmit relay 
> line (grounds it) and the mike connection itself.  Although carbon mikes are not used much now, I have assumed that 
> preventing such pops was the reason. The intercom connection, using VOX, is upstream of the PTT.  I suppose another 
> reason for the DPST keying approach is that some radios use the actual mike connection to switch the relay on for the 
> transmitter and do not use a separate PTT line, although I don't think such radios are used on aircraft.
>
> Wayne
> WB5WSV
>
>
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