[R-390] Preselector

[Roy Morgan]

At 09:25 AM 8/19/2004 -0400, JMILLER1706 at cfl.rr.com wrote:
>The entire front end of a 390 is in essence one very high performance 
>tuned preselector, and probably does the preselection function very well 
>by itself.

R-390 Afficianados,

To support J's message, it would be an unusual situation where an R-390A 
needed extra front end selectivity or RFI rejection.  I can think of a few, 
however:

1) As J mentions,
>  So unless you have a particularly strong interfering signal in an 
> adjacent band needing additional attenuation, I couldn't see another 
> preselector as helping.

The R-390's were perhaps not meant primarily for broadcast band use, and 
some of us live near transmitters in that part of the frequency range.  For 
example, here at my QTH, there is a moderate power AM transmitter less than 
4 miles away, and a clear channel 50 kilowatter some 10 miles away.  If 
this is your situation, a filter or preselector of the sort used by MWL 
folks or beacon hunters may be all you need. If there is just one station 
causing trouble, try a simple series tuned trap at the frequency of the 
offending station from the antenna connector to ground.  Here are some 
links to start a search for filters and the like:

Long Wave Radio (UK)
<http://www.wireless.org.uk/index.htm>

the Beaconworld Website (UK)
<http://beaconworld.org.uk/>

the Longwave Club of America
http://lwca.org/

Altair's Lowfer Page
<http://www.altair.org/lowfer.htm>

Kiwa Broadcast Band Rejection Filter ($60)
http://www.kiwa.com/bcb.html

Bandpass and I.F. Filter Information
<http://www.qrp.pops.net/bandpass.htm>
(not suggested for BCI problems from the high end of the band)

2) If you are getting cross modulation of any sort:
A) Check front end and mixer tubes for weakness or incorrect tube 
types.  Many of our receivers suffered at the hands of folks who wanted to 
"make it a lot hotter" by putting in wrong tubes of higher gain, or 
cranking up the IF gain setting.
B) Check the IF GAIN SETTING.  Do the check from Chuck's website, and set 
the IF gain at a reasonable level. See:
http://www.r390a.com/
and specifically:
<http://www.r390a.com/html/gain.html>

3) Preamplifiers and active antennas lead to trouble if not carefully 
selected and used.  As J says:
>   If you have a less than optimum antenna, the preamplification (or maybe 
> an active antenna) could help overcome antenna loss, but you also run the 
> risk of overloading the 390 front end, although this is unlikely.

It's easy for BCI or other interference to exceed the ability of an active 
antenna amplifier to stay linear.  Even a properly working preamp can cause 
mixing of normal strength signals if it's not a really good one.  Look for 
product reviews and performance reports from the SWL and ham literature 
before you buy any pre amplified device.

The preamps built by Millen, RME and others, were intended to improve the 
performance of pre-war and budget receivers in use at the time.  The SX-28, 
for instance, is terrible at higher frequencies when compared to the R-390 
radios.  Using one of those preamps with a well tuned-up R-390 would be 
quite instructive.

4) The LF "mod":
One trick published some years ago to get the R-390 to operate below 500 kc 
is to feed the antenna to the set past the first RF stage.  This opens up 
the receiver to all sorts of distortion and overload.  At least one article 
mentions the use of a preselector if you are looking for signals below the 
BC band.  The 55G-1 RF Preamplifier made by Colllins for use with the 51S-1 
receiver might be good for this, but unfortunately the only one I know of 
sold in quite some time apparently went for over $2,000.  (On my to do list 
is a functional equivalent with a toobe instead of a FET.)

5) Antenna thoughts:
>  Sensitivity at the high bands might be improved with a pre-amp.

I can't quote correctly, but the very first ARRL Antenna Book mentioned 
that the most and least costly improvement in any station can be made by 
improving the antenna.  In most situations that advice still holds even 
though many decades have passed.

Happy preselecting,
Roy


- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Work: Voice: 301-975-3254,  Fax: 301-948-6213
roy.morgan at nist.gov --