[Boatanchors] Re: Collins R-388 BCB mods
Al Klase
al at ar88.net
Mon Oct 1 17:28:26 EDT 2007
The fact of the matter is that the R-388 and 51J-4 are hacked up
designs. The original 51J had proper low-impedance windings on the RF
coils. The later top-coupled scheme was done at the behest of the
military to better accommodate short whips. The R-390 has both kinds of
inputs.
Al
DW Holtman wrote:
> 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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>>
>>
>> ----------------------------------------------------------
>> 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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>> Assistance: dfischer at usol.com ** $ For vintage radio info,
>> see the HCI web site $ http://www.w9wze.net
>>
>
>
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> Assistance: dfischer at usol.com ** $ For vintage radio info, see
> the HCI web site $ http://www.w9wze.net
>
--
Al Klase - N3FRQ
Flemington, NJ
http://www.skywaves.ar88.net/
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