Here is my favorite crystal-controlled QRP transmitter - the "Pippin" by G3MY:http://www.qsl.net/g3pto/pippin.html
It is simple, cheap, uses commonly available parts, and can be adapted to a number
of different configurations. It can be used on any band up through about 20m or
so (as long as you have a fundamental frequency crystal for it). The only thing that
sets the band is the output filter, and you can build those for various bands and
switch them if desired.
The critical component, of course, is the crystal. 7.040 used to be the standard QRP
calling frequency in the US, but we've been shifting down to the 7.030 European standard
over time due to QRM. You'll probably have to order the crystal (unless you find an old
FT-243 crystal in the 6 MHz range that you want to try grinding upwards...)
The rest of the components are relatively non-critical. The 50pf variable capacitor in series
with the crystal is for fine tuning, but you can leave that out if your crystal is close enough.
The resistors can be +/- 50% or more - it may change the current draw, efficiency or output
power somewhat, but the rig will still work. (You might try optimizing them once you have it
running.) The two capacitors across the crystal set the feedback level - both are common
values, but you could use 47pf and 120pf or even 100pf if needed. (The ratio is more important
than the exact values.) This is a point where you would want to use good quality capacitors,
such as silver mica, NP0 ceramic or polystyrene.
Just about any general-purpose NPN transistor should work in this circuit, such as the 2N3904
or 2N2222. It doesn't dissipate much heat, so a small plastic type is adequate. The PNP
transistor can be a 2N2905 in a TO-39 case (which makes it convenient to fit a heat shink),
though you may find that a 2N2907 in the smaller TO-18 case, or even a plastic type such
as the 2N3906, doesn't get too hot in casual operation. (Or put several of the TO-92 plastic
types in parallel.) Again there are lots of options, including many that are commonly available.
And cheap, so you can blow up a few in your experiments and not worry about the expense.
The stuff connected to the collector of the output transistor depends on the operating
frequency. The capacitors C1 - C3 and the coils L1 and L2 form a low pass filter for the
desired band. There are standard tables for these. You can wind the coils yourself, either
on a toroid core or a toilet paper tube. The capacitors are standard values. You can replace
all of these (as well as the RF choke) with a parallel-tuned circuit: I'd probably use a loaded
Q of 1 (so both L and C have 50 ohms reactance), and then the L replaces the need for the
RFC. (You'd still want to couple the output through a series capacitor.)
How to build it? I usually use a prototype board for experimenting, then build the circuit
itself using "ugly construction", meaning that the parts are simply soldered to each other, and
to the plain surface of copper-clad board for a ground. Here is a good tutorial on this method
(from a site with lots of other circuits for homebrewing as well): http://www.qrp.pops.net/ugly.asp
Here is another method, from another site with lots of interesting circuits:http://www.intio.or.jp/jf10zl/cmc.htm
You can also cut pads into the circuit board with a file, hack saw, or Dremel tool: the transmitter
is easily built on a board with a 3x4 grid of pads.
With something this simple, getting it working on breadboard then rebuilding it for a final version
isn't difficult. For larger circuits I often build up each stage on a separate piece of circuit board.
For example, one receiver has a RF preamp, mixer, oscillator, audio preamp, and audio power amp
as individual pieces. That makes it easy to try a different circuit in one of the positions. I then
reuse the boards for other projects, because my final versions are build directly into a case of
a particular size. If you have enough space in your case you can just mount the individual pieces
of board and connect them together.