[Boatanchors] Re: Collins R-388 BCB mods

Mon Oct 1 16:14:41 EDT 2007

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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173 Carolyn Lane               Home: (859) 887-5563
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