[Collins] R-388/51J mods - BCB included

[Bill]

You may wish to take a look at the RF coils on a 51J as it
uses inductive coupling on its antenna input.  The RF schematic
is basically identical.  In the archives is a letter from me 
detailing some of the differences between a 51j, 51j-3 and 51j-4.

bk

-----Original Message-----
From: collins-bounces at mailman.qth.net
[mailto:collins-bounces at mailman.qth.net] On Behalf Of Bill Cotter
Sent: Sunday, November 11, 2007 9:33 AM
To: collins at mailman.qth.net
Subject: [Collins] R-388/51J mods - BCB included


NOTE TO PURISTS - DO NOT PROCEED ANY FURTHER - MODIFICATIONS


Here is a brief summary of some useful mods, plus the BCB 
improved-sensitivity mod. de n4lg
---------------------------------------------------------------

Date: Mon, 01 Oct 2007 16:14:41 -0400
To: Boatanchors at mailman.qth.net
From: Bill Cotter <n4lg at qx.net>
Mime-Version: 1.0
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Message-Id: <20071001201904.42A6F859C61 at mailman.qth.net>
Subject: [Boatanchors] Re: Collins R-388 BCB mods

After tiring of the BCB interference and overloaded signals on 
several bands of the R-388, I set about on a cure. The obvious 
problem was the high-Z coupling of the antenna to the 1st RF coil 
set. The capacitive coupling drastically reduces the Q of the LC 
circuit in the first RF stage.

I ended up removing the coupling capacitors (C233, C234, C235, C236 
& C237) from S101 to the high-end of each band coil (L101, L102, 
L103, L104 & L105). I replaced the capacitive coupling with link 
coupling by winding turns of #22 (as a link) on the cold end of 
each coil. I don't recall the number of turns on each, but it was 
like 2 to 5 turns (highest to lowest freq). The hardest part was 
removal of C-236 and soldering on the link wire. It requires a lot 
of patience DEEP into the bandswitch, with a very skinny iron and 
hemostats.

The results were a dramatic reduction of overload and BCB 
interference on all bands - when using antennas with 50Z feeds. 
There was no measured (or, observed) reduction of sensitivity. The 
increase in circuit Q by unloading the LC circuit made a major 
difference in listening on the lower bands.

Another useful mod for the BCB listener is to increase band-1 
sensitivity by increasing RF/1st mixer coupling. Parallel a .01uF 
across C117 (3pf).

73 bill n4lg

R-388 Schematics http://www.jptronics.org/radios/Collins/index.html



At 12:33 PM 11/6/2007, you wrote:
>On Tue, 2007-11-06 at 06:05 -0600, kingwood wrote:
> > Greetings to all,
> >
> >        Does anyone have any information on what was called the
> "Band 1 mod"
> > for R-388/51J's by Dallas Langford?  I have a receiver here
> that has way too
> > much gain on "Band 1".  I would like to find out what this mod
> consisted of
> > before I start tearing into the receiver.
> >
> > 73 de K5JV
> >
> > Lon W. Cottingham
> > 1110 Golden Bear Ln.
> > Kingwood, TX 77339
> >
> > 281-358-4207
> > 281-358-4234 FAX
> > 281-795-1335 CELL
> >
>http://www.geocities.com/amlogbook/newmembers/reprint.htm
>offers to sell the article in a CD-ROM collection.
>
>Says
>
> > M054  51J/R-388 Band 1 Mod and AGC Mod (1) Dallas
> Lankford.  Improvement of Band 1 (BCB) sensitivity is
> >       addressed, as well as a simple modification to improve
> AGC on AM.  7/90
> >
>referring to the DX Monitor magazine, apparently July 1970,
>published by the International
>Radio Club.
>
>http://www.nrcdxas.org/catalog/reprints/ calls it reprint R64 and
>offers
>to sell it for 20 cents.
>
>That would probably involve the extra mixer stage used for the
>broadcast
>band. Not what makes my 51J squirrelly.
>--
>73, Jerry, K0CQ, Technical Advisor to the CRA
>All content copyright Dr. Gerald N. Johnson, electrical engineer
>
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