There are two basic approaches to making a simple CW RX: direct
conversion or regenerative. (They are actually quite similar, the only
difference is in the oscillator and detector circuitry.)
One thing to be cautious about, however, is in trying to reduce the
circuit to too
few components. This was a common approach in
the days of tubes, when an additional tube (and the required filament
current) was a significant addition to a circuit. But another 2N3904
transistor or two really doesn't add to the cost of a circuit substantially,
and by using separate transistors for each stage it is much easier to
trouble-shoot, or to optimize each stage for one purpose.
A basic regenerative or direct conversion receiver will include an
optional RF amplifier, an oscillator, detector stage, and one or more
stages of audio amplification. The oscillator determines the receive
frequency. In the case of a regenerative receiver the oscillator
and detector often use the same transistor, but separating the
two functions makes performance more stable.
One of the first things to learn about reading schematics is to break
them into individual stages: once you can do this, they make much
more sense, and seem less complex. It also allows you to see how
you can modify them: for example, you can use the oscillator from
one design, the detector from another and the audio stages from
something else entirely.
In fact, this is the way I build receivers: each stage is built using
"ugly construction" on a small piece of bare circuit board and wired
together. If I want to try a different oscillator circuit, I wire it up
on its own board and swap it out with the previous one. I keep all
the pieces around, so when I want to build something else I don't
need to build all the parts from scratch - I just dig out the common
stages from my junkbox and build the couple more that I need.
Once I get it working to my liking, I build a copy of it into a
case that suites the end application.
As an example, I had a direction-finding receiver published in QST
(September 2005) that was intentionally made from simple, common
parts, most of which could be salvaged from an old AM broadcast
radio if necessary. This was for 80m, and used a cascode RF amplifier
(two transistors), a diode mixer, one transistor oscillator, an audio
pre-amp, and an LM386 audio power amplifier. This gave good CW
reception using an 6" diameter loop antenna attached to the receiver.
You can get by without the RF amplifier if you have a full-sized
antenna, but it provides a convenient RF gain control. Each of these
stages was very simple, and used generic NPN transistors such as
the 2N2222, 2N3904, 2SC1815, BC547, etc. True, some of the stages
might be a bit better using an FET such as the MPF-102 instead, but
the bipolar transistors provided adequate performance. You can get
by with fewer stages, but none of them are particularly difficult to
build.
For each of the stages there are many different circuits that you
can use: I borrowed the bits I liked form this receiver design
and that one. You can do the same thing with a different circuit
for each stage and still get good performance.
Meanwhile, here are some resources:
http://www.qrp.pops.net/ is the QRP homebuilder web page. Lots of good
info on circuits, construction, etc.
http://www.intio.or.jp/jf10zl/ JF1OZL has a lot of simple circuits to try, as well
as tips on homebrew techniques. For example:
http://www.intio.or.jp/jf10zl/3trcw.htm is a 3-transistor CW receiver for 15m
(though the relay switching necessary to do this makes the receiver complex.)
http://www.intio.or.jp/jf10zl/cmc.htm shows how he builds each stage on a
piece of generic circuit board with pads.
