[Boatanchors] Re: Collins R-388 BCB mods

Mon Oct 1 16:55:51 EDT 2007

Hello,

In my opinion, a lot better way to deal with BCB interference is to build a 
high pass filter with a cut off frequency of around 3 MHZ. An excellent 
article on this subject is written by Joseph Carr. It can be found at

http://www.dxing.com/tnotes/tnote06.pdf

I have built several of these units both the one on page 6 and the one on 
page 7. They are easy to build, parts are readily available and they work 
very well. This method does not require modifying your receiver, something I 
hate to do.

Best,
DW Holtman
WB7SSN

----- Original Message ----- 
From: "Bill Cotter" <n4alg at qx.net>
To: <Boatanchors at mailman.qth.net>
Sent: Monday, October 01, 2007 2:14 PM
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 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. 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 01:52 PM 10/1/2007, you wrote:
>>A 15' whip will have an impedence of roughly 35 Ohms plus ground loss at 
>>its 1/4 wave resonance. The impedence decreases rapidly as the frequency 
>>goes lower than the 1/4 wave point which is 15.6 MHz. As the frequency 
>>goes up so does the impedence.
>>
>>The 51J series has a reputation for poor sensivity which is partially due 
>>to the input design and the way many use it.
>>
>>In many installations a 51Jx, and R-390 series for that matter,  is part 
>>of a group of receivers. These are fed from an active distrubition panel 
>>which contains an amplifier(s) that makes up for the signal spitting 
>>losses as well as providing a constant impedence to the receiver. Typical 
>>antennas varied from 15-35' whips aboard ship to long wires and various 
>>directional arrays to rhombics and large LP's at shore stations.
>>
>>Whether the input is 300, 450 or 600+ Ohms makes very little or any 
>>difference to a receiver with a high impedence input so the balun ratio is 
>>not critical. Purists can easily construct one that can be switched in 
>>steps from 4:1 to 12:1.
>>
>>One potential problem with many BA's is signal overload from nearby BCB 
>>stations when a big (long) antenna is used. A CATV balun or splitter with 
>>a 5 MHz lower limit somewhat reduces that potential by having a built in 
>>negative "gain" (insertion loss) below the cut-off frequency. These CATV 
>>baluns could care less if the coax feed is 75 or 50 Ohms, the ratio is 
>>still 4:1 resulting in either a 300 or 200 Ohm output in a perfect system.
>>My 51J3 and J4 were loaded with BC birdies on the lower ranges until I 
>>built a BC reject filter. Published IMD figures for these receivers are 
>>fairly poor and nowhere close to a R-390/390A.
>>
>>Carl
>>KM1H
>>
>>
>>
>>----- Original Message ----- From: "Al Klase" <al at ar88.net>
>>To: "Glen Zook" <gzook at yahoo.com>
>>Cc: <Boatanchors at mailman.qth.net>; <glowbugs at piobaire.mines.uidaho.edu>
>>Sent: Monday, October 01, 2007 12:56 PM
>>Subject: Re: [Boatanchors] Re: GB> Collins R-388 vs 51J4
>>
>>
>>>RE:  9:1 Baluns
>>>
>>>Yes, older receivers, designed for random-wire antennas generally have an 
>>>input impedance in the neighborhood of 400-600 ohms.  This a good 
>>>compromise between the resonate (quarter-wave x N) and anti-resonate 
>>>(half-wave x N) case.  The R-388 is designed to work with a 15-foot whip, 
>>>an even higher impedance situation. TV baluns are a crap shoot at HF, 
>>>often loosing effectiveness below 10 MHz, and you probably want a 1:9 
>>>anyway.  It's easy to make equivalent devices, and the required cores are 
>>>very useful around the shack.  You guys have shamed me into updating my 
>>>website:  http://www.skywaves.ar88.net/ANTENNA/antsys.htm
>>>
>>>Regards,
>>>Al
>>>
>>>Glen Zook wrote:
>>>>The article that was in Electric Radio a while back
>>>>about using television baluns on the older receivers
>>>>can be found at
>>>>
>>>>http://k9sth.com/Page_2.html
>>>>
>>>>The link is about 2/3rds the way down the list of
>>>>links and is entitled "Using television baluns to
>>>>improve receiver sensitivity".
>>>>
>>>>Glen, K9STH
>>>>
>>>>
>>>
>>>--
>>>Al Klase - N3FRQ
>>>Flemington, NJ
>>>http://www.skywaves.ar88.net/
>>>
>>>
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>>
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>>$ For vintage radio info, see the HCI web site $
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>
>
> ----------------------------------------------------------
> Bill Cotter, N4LG             E-mail: n4alg at qx.net
> 173 Carolyn Lane               Home: (859) 887-5563
> Nicholasville, KY 40356-9340   Work: (859) 323-6474
>
> ARRL   ARCI #9878   QRPl #1646   QCWA #29393   FISTS #4535
> ---------------------------------------------------------- 
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