[Drake] Noisy(?) R-4B

[Glen Zook]

Basically any receiver has a "noise floor", some worse
than others.  This is caused by thermal action and
happens in everything that is above absolute zero in
temperature.

This noise floor can be affected by defective
components including tubes.  Other times, there is
nothing that can be done about it.  One way of seeing
just how bad the noise is to attach a signal generator
and see just how low a level that you can receive a
signal.  Now, this will be affected by the bandwidth. 
The narrower the bandwidth, the less the noise.

In actual operation, the outside noise is virtually
always greater on the 160 - 10 meter bands than the
internal noise of the receiver.  Thus, the noise
brought in from the antenna is greater than that of
the receiver and any signals that can be copied are of
a signal strength greater than the atmospheric and/or
man-made noise that comes in from the antenna.

For CW operation, you really need a filter around 400
- 500 Hz in bandwidth.  Much narrower and it becomes
uncomfortable to listen to.  Much wider, and you start
getting noise and QRM.  The Collins 200 Hz crystal
filter that was factory standard in the 75S3 and 75S3A
receivers ($400 option in the 75S3B and 75S3C) is
great for really "digging" out a CW signal from the
QRM.  However, it is very hard on the ears to use for
very long.

The Heath 400 Hz filter as used in the SB-Line is a
very good filter, easy on the ears and still sharp
enough to eliminate most interference and noise. 
Collins had/has a 500 Hz CW filter that is also very
easy on the ears.

Most tube type receivers have a noticeable background
noise when the AM or SSB filter is used for CW.  This
becomes much less when a CW filter is used.  The newer
solid-state receivers often have a fairly low noise
level (but some of the older solid-state receivers
have a very high noise level), but this is also not
always the case, some of the newer receivers also have
a high internal noise level.  Also, a lot of the newer
receivers have narrower bandwidths available for CW
standard.

The older tube type receivers with crystal phasing and
variable bandwidths are usually very good in terms of
noise when operating CW when they are put in the
narrower bandwidth positons.  The reduction in noise
is definitely noticeable.

If you do not have a narrower filter in the receiver
for CW, I strongly recommend installing one if you are
a serious CW operator.  On SSB, the bandwidth of the
filter is normally "filled" by the signal.  But, with
a CW signal, virtually all of the bandwidth is open
for noise and QRM when the SSB filter is used for CW. 
Thus, the reduced signal-to-noise ratio of the weaker
CW signals.

The R4B is a good performing receiver.  You just have
to use the correct filter for best operation.  For CW
this means around 400 - 500 Hz and for AM 6 - 8 KHz. 
For SSB, the bandwidths are normally in the 2.1 - 2.4
KHz range (although some manufacturers receivers go as
wide as 3.1 KHz).

8 ohm speakers are fine.  8 ohm ("stereo") headphones
may introduce "hum" and other distortion if the
receiver is designed for "high impedance" (2000 ohm)
headphones.  Sometimes 8 ohm headphones work fine on
some receivers and sometimes they don't.  If you are
not getting any noticeable hum, then you shouldn't
have any problems.

Glen, K9STH


--- K4BEV@aol.com wrote:

The noise is not noticeable while receiving fairly
loud cw, but is definitely there when trying to dig
out a weak one. This is, of course, with the antenna
connected. Is this normal for this radio, or should it
be close to dead quiet?

=====
Glen, K9STH

Web sites

http://home.attbi.com/~k9sth
http://home.attbi.com/~zcomco

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