[Collins] 51J series input coil antenna requirements.

[Bill Kirkland]

There is an archived letter from me that lists the differences between the 
51J(j-2), the j-3(r388) and j-4.  There are differences in the antenna 
circuit between the j/j-2 and the j-3/j-4 but not between the j-3 and j-4.  
As Jerry points out, the J-4 is a modified J-3 - mechanical filter module.

The 51j or 51j-2 uses a balanced (differential) input and is transformer 
coupled. There is
  no antenna trimmer cap.

The j-3 and j-4 use a capacitor coupled singled ended input and have the 
antenna
trimmer cap.


Bands 2 and 3 do NOT use the coils in the RF rack, but rather the coils in 
the IF rack.
Bands 2 and 3 use 1 conversion stage with the PTO as the mixer.  Band 1 is 
up converted
to a high band, and then goes through 2 more mixer stages.


>From: "Dr. Gerald N. Johnson" <geraldj at storm.weather.net>
>Reply-To: geraldj at storm.weather.net
>To: collins at mailman.qth.net
>Subject: Re: [Collins] 51J series input coil antenna requirements.
>Date: Thu, 01 May 2008 12:24:01 -0600
>
>On Thu, 2008-05-01 at 09:09 -0400, jeremy-ca wrote:
> > Could someone refresh my memory as to what antennas and impedances these
> > various models were designed for?
> >
> > I seem to remember that the 51J3/R388 and J4/R388A were different?
> >
> > Carl
> > KM1H
> >
>As far as I know, the 51J-3 and 51J-4 differ only by the mechanical
>filter module. And there was a conversion kit to add the mechanical
>filters to a 51J-3.
>
>I have downloaded military R388 manual and I own a Collins printed 51J-4
>manual. They both say 50 ohms 100 pf for a short whip. The schematics
>are IDENTICAL in that area. Even to the component numbers and general
>layout.As if the same draftsman drew both schematics or made one from
>the other.
>
>That's not to say there haven't been variations in that area. I know my
>round emblem J-3 (probably a new front panel on a R388) is squirrelly on
>the 3 and 4 MHz bands. Peaking the antenna trimmer makes the RF stage
>oscillate with a few feet of wire for the antenna. So the antenna isn't
>loading the RF stage. There may be a mod to improve that. I don't know
>that this receiver front end hasn't been modified since its previous
>owner used it a lot in the 3 and 4 MHz bands for MARS. He did put in
>product detector and AVC mods.
>
>I can't tell from the changes I downloaded
>from /downloads/collins/r388co/ at BAMA (bama.sbc.edu) whether there
>were antenna circuit changes that affected the input impedance.
>
>388C4-07.gif from that list shows an added RF triggered relay to
>disconnect the antenna input to protect the input coils from transmitted
>RF. Its a diode detector, a transistor amplifier and a relay. Probably
>not a benefit to intermod performance with the diode on the antenna
>connection. But surely would be a benefit to protect fragile and rare
>antenna coils.
>
>--
>73, Jerry, K0CQ, Technical Advisor to the CRA
>All content copyright Dr. Gerald N. Johnson, electrical engineer
>
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