ICOM 761 Question

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David Zelinski:
Good evening all.

I enrolled in the online ARRL operators course. After years of thinking about getting my license and callsign, I finally decided to do something about it. Hopefully I will pass the online course and pass my local exam by November and become a full fledged Elmer.

With that out of the way, here are my questions and sorry this may be long winded but I have to explain  to make my questions seem revelant.

Several years ago (1999-2001) while I was stationed onboard a U.S. Navy Expermental Craft "Baylander" (IX-514) based out of Pensacola NAS, I was introduced to the world of Ham Radio.

Onboard, we had a ICOM 761HF rig. I origionally thought it was a 751 but pictures recently have jogged my memories!

We had the rig onboard in case of Natural Disaster (Hurricanes) and we had to bug out for a shrimp fisher hurricane hole in the deep swamps of Florida or Alabama.

No one onboard knew anything about operating this unit and I was the only electronics trained crewmember onboard (Hardly as I am a CPO Electrician's Mate, not a Radioman or Electronic's Tech.) so the maintenance and operation of said radio became by default my responsibility. Typical military idealisms there.

I read the owners manual until I memorised it and even learned how to use the antenna auto tuner. I even tried a comm check one late night to test "my" rig's output capability after listening to HF chatter and talk for hours using the call signs provided by "Big Navy" - November, Hotel, Lima, Tango, Fiver, One, Four (NHLT514). Well needless to say, I got laughed off the civilian band net after I explained who I was and what I was trying to accomplish and why when questioned.

For some strange reason I still to this day remember one of our other crewmembers telling me that our radio was modified to use both military and civilian freqs across the bands. This though I could never verify.

QUESTION #1-
Is it possible that this ICOM 761 was in fact modified? If so, how was it done? There was no other external electronics cases other than the external power supply and external tuner and multiplexer (I believe) that I remember seeing.

Any ideas from the experts out there?

Not that I am trying to modify one but I do think that I am going to try to purchase a 761 as a first unit more for nostalgia reasons once I pass my test and manage to join the ranks of the world of Ham Operators.

QUESTION#2-

I understand wave propagation and antennas and grew up in a house with a parent who DXed CB on a old 23 channel General tube set with a large linear connected to a Moonraker 4 antenna back in the 60's and 70's and had a old US Army AN/GR 10 set up modified(all in my possession now) but if I remember correctly, our Navy shipbased ICOM 761 was connected to a large civilian style marine whip mast antenna. Is this possible? Will it work?

I live in a mobile home park and the thought of a large mast yagi style or box antenna will not go over well with my neighbors OR the park owners.

I am still learning everything here and would welcome any and all advice.

Feel free to e-mail me at my home e-mail if you wish- gocaps@cox.net

Thank you greatly,

Dave-              

 

:
Hi Dave,

I have and use an ICOM IC-761.  It's getting a bit old,
(like me) but still works great.
The 761 is like a super IC-751A,  better specs in general and an internal power supply.

Answer 1.
I believe it's possible to modify it to transmit almost anywhere in the HF space, (it already has general coverage receive.)So the unit you referred to was probably modified in that way.  

Answer 2.
You could certainly use any one of many vertical antenna with ground radials,  or some half-wave types that work without radials.  You could also use an antenna designed for mobile (automobile) operation, but with reduced performance.

I encourage you to keep working on getting your license.
Good luck,
John, KB1NO

Wyn Hughes:
I also have a IC-751A, although mine in fact is the IC-751AF which is the comemrcial version of the IC-751A (transmits on all frequencies in the short wave spectrum, not only within the amateur bands).

The way this restriction is achieved in transceiver equipment intended only for the amateur radio market is usually quite simple, something along the following lines. The logic unit chip, which controls many of the functions of the radio, has the amateur frequencies programmed into it in the factory. When the radio is tuned to a frequency outside the amateur bands, the logic unit detects that and applies a bias voltage to the driver which drives the PA unit, to disable the driver. This means that no drive can be applied to the PA, and therefore no RF output from the transceiver, outside the amateur bands.

Disabling this function is usually simple and crude, such as cutting the wire which supplies the voltage which would otherwise prevent the driver from working outside the amateur bands. There are modification details posted on various sites.

My IC-751AF will transmit anywhere above 1.6mhz when the 'general' or 'gene' mode is selected. When in 'amateur' mode it works just like a normal IC-751AF. Below 1.6mhz the low pass filter attenuates the output, effectively preventing transmission on the mediul and long wavebands, which would otherwise destroy the filters.

The 751AF was popular with the commercial fishing fleets in SE Asia during the 80s and 90s, before the advent of satellite phones.

73 and good luck.

VR2AX

Bob Lewis:
I believe you will find info on the "out of band" modification here: http://www.mods.dk  As I recall is is a simple matter of clipping one end of a diode.


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