[ICOM] is there some sort of protector on the RX ANT, input of a 756 Pro?

[Diane and Edward Swynar]

Hi Gary,

I very quickly noted the absence of any "protection" of the receive antenna
port in my 751A the very day that I transmitted into a kilowatt amplifier,
with a 1500' Beverage antenna connected to the port: the RF feedback getting
into the transceiver made for all kinds of "...weird & wonderful" scope
patterns on my SB-610 monitor...!

The solution, of course, was the incorporation of an external 12 VDC relay
that automatically grounded the receive antenna port when transmitting---the
control voltage was secured directly from the multi-pin jack at the back of
the set.

I have no idea why Icom never did this "...de riguer", at the factory---but
they didn't, so one is forced to improvise in the field! Hi Hi.

~73~ de Eddy VE3CUI - VE3XZ


*****************************************


----- Original Message ----- 
From: "Gary P. Fiber" <gfiber at comcast.net>
To: "ICOM Reflector" <icom at mailman.qth.net>
Sent: Wednesday, February 09, 2011 2:04 PM
Subject: Re: [ICOM] is there some sort of protector on the RX ANT, input of
a 756 Pro?


> I am not sure of the newest Icom products, but I can tell you the
> earlier radios, IC-745, 751 / A, 761. 765, 781.... NONE of them had any
> sort of protection in the receiver input.
> If you removed the coaxial jumper and attached your receiving antenna to
> the Receive Antenna In jack, you were connected directly to the receiver
> front end.
> Use of some sort of protection or relay circuit to remove the RX antenna
> during transmit was always recommended at least by myself when I worked
> at Icom.
>
> Using the UHF (S0-239) connector there is a small relay between that
> connector and the Receive Antenna Out jack.
> During transmit that relay would open placing an air gap between the
> Transmitter output and the Receive antenna input so the receiver front
> end would be protected.
>
> I was never sure what the reasoning was for the ability to open the
> circuit to having an unprotected receiver input, maybe a preamp.
> So the long and short of it, if you attach a fine receiving antenna to
> the receive antenna input you need some sort of signal attenuator that
> operates when you transmit.
> Sending 100 or 1500 Watts out your transmit antenna will certainly cause
> you likely high millivolts of RF energy presented to the receiver input
> being picked up by your receiver antenna.
>
> Gary K8IZ.
>
> On 2/9/2011 9:56 AM, Carl N9GC wrote:
> > The 756 line has long had a history of that sort of failure. Burghardt
> > Electronics built a protection circuit for this purpose a number of
> > years ago. They only repair rigs now but may still have the protection
> > circuit available.
> >
> > 73, Carl N9GC
> > ----
> > Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
> > Icom Users Net: Sundays, 1700Z, 14.316 MHz
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> >
>
>
> -- 
> Gary P. Fiber K8IZ
>
> General Radio Telephone Operators License PG-19-6691,
> with Shipboard Radar Endorsement.
> Washington State Resident.
>
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> ----
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