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

[Dr. Gerald N. Johnson]

On Sun, 2007-11-11 at 09:33 -0500, Bill Cotter wrote:
> 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
> Content-Type: text/plain; charset="us-ascii"; format=flowed
> 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.

If they hadn't needed to operate without an antenna coupler with short
whips on receiving huts, I think they would have been link coupled to
begin with. Link coupling certainly matches most ham antennas far better
> 
> 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).

There might be a compromise value that gets some increased gain without
driving the mixer so hard and compromising the intermod range with
strong signals. I suspect the small coupling capacitor was chosen to
limit the effects of strong signals characteristic of domestic locations
on the broadcast band while giving sensitivity some better than the 5
tube AC/DC kitchen radio. I'm thinking 30 to 100 pf might give adequate
gain without hurting the strong signal handling too much.
> 
> 73 bill n4lg
> 
> R-388 Schematics
> http://www.jptronics.org/radios/Collins/index.html
> 

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer