A thread on the radiokits mailing list recently discussed the Radio Shack Science Fair three-transistor shortwave radio kit. The manual for this kit–and many other P-Box kits–can be found at KA9VNW’s Sparktron website.
This is a cool radio, but another subscriber pointed to another that seems like it would be easier to build. Designed by Charles Kitchin, N1TEV, the schematic appeared in the August 18, 1994 edition of EDN magazine. The article is titled, “$10 receiver has microvolt sensitivity.”
This radio uses three 2N2222 transistors, while the P-Box radio uses one NPN transistor and two PNP transistors. The 2N2222s are much easier to come by, I suspect, and I already have a package of 100 in my shack.