Question about converting military helmet

Hello I've not posted on rah before, but have lurked long enough to
realize that you guys have
a few electronics experts among you. What I have is a Gentex sph-4
military flyers helmet.
This is a single plug helicopter helmet. I've been told that the
microphone impedence for
military helmets is different (sorry guys I don't have the actual
values) from civilian helmets,
and I have tested this helmet in a helicopter. The earphone portion
works fine. My question is what must be done to this helmet to make
use of it in a civilian helicopter? Is it as simple as
replacing the existing mic with a proper resistance civilian mic? Or
is it something more tricky
and electronically sinister? Any thoughts would be appreciated, thanks
in advance!



Get yourself in the air, no matter how you get up there!

Wayne

In most cases if you have a David Clark style headset all you have to
do is change the mic to a civilian type. The electro fet M-7A is the
desired mic but the old M1 which is bigger will also work. It will
just plug right in to the same plug at the mic. You just have to
loosen two set screws at the mic to remove the plug.. I have some
friends that used to fly ex-military OH-58's and Bell 206 helicopters
also and they tywraped a second mic to the back of the military mic
and would rotate the mic around 180 degrees on the ball and socket mic
mount on their flight helmets and swap the mic plug to the other mic
to change between military and civilian aircraft. For fixed wing
aircraft you also need a single plug U-174/U to two plug adapter Y
cable. Several places sell these DC PN 18253G-05 including Sporties
for about $40.
David Clark sells a dual impedance headset H10-66 that has a switch to
change between the mil and civilian types. The addition is an
internal mic amplifier PN 18911G-01 and a switch.

The mil mic is a 4 ohm impedance dynamic (no built in amp].
The civilian mic is a dynamic mic with an internal amplifier so that
it looks like an old style carbon mic, the amp is powered by the
radio.
The earphones are about 150 to 300 ohms in either case.
John F
On 26 Feb 2006 16:16:13 -0800, wrote:

Hello I've not posted on rah before, but have lurked long enough to
realize that you guys have
a few electronics experts among you. What I have is a Gentex sph-4
military flyers helmet.
This is a single plug helicopter helmet. I've been told that the
microphone impedence for
military helmets is different (sorry guys I don't have the actual
values) from civilian helmets,
and I have tested this helmet in a helicopter. The earphone portion
works fine. My question is what must be done to this helmet to make
use of it in a civilian helicopter? Is it as simple as
replacing the existing mic with a proper resistance civilian mic? Or
is it something more tricky
and electronically sinister? Any thoughts would be appreciated, thanks
in advance!



Get yourself in the air, no matter how you get up there!

Wayne

Thanks a bunch for the info, but as I am only a helicopter pilot I
won't need the y adaptor. I knew Someone on this board would have an
answer!




Get yourself in the air no matter how you get up there.

Wayne

Originally Posted by

Hello I've not posted on rah before, but have lurked long enough to
realize that you guys have
a few electronics experts among you. What I have is a Gentex sph-4
military flyers helmet.
This is a single plug helicopter helmet. I've been told that the
microphone impedence for
military helmets is different (sorry guys I don't have the actual
values) from civilian helmets,
and I have tested this helmet in a helicopter. The earphone portion
works fine. My question is what must be done to this helmet to make
use of it in a civilian helicopter? Is it as simple as
replacing the existing mic with a proper resistance civilian mic? Or
is it something more tricky
and electronically sinister? Any thoughts would be appreciated, thanks
in advance!



Get yourself in the air, no matter how you get up there!

Wayne

Hi Wayne,

I am afraid that the impedance has to be matched to the station boxes fitted to your helicopter type. Normally civilain helicopters work on a tels impedance of 600 ohms and 5 ohm impedance mic for one headset, two headsets connected to the same box in parallel will be halved to 300 ohm. This is for LO impedance systems. HI impedance systems use 600 ohms and a 150 ohm impedance mic. Yours may well work but if someone else connects into the system the whole intercom will deteriorate. You will find the impedance information in the component manual of the station box or ICS box for the aircraft you are flying. The SPH 5 I believe is the civilain equivalent to your military SPH4.

Regards

lectricity