This page is based on the article: "AM Radio Booster" by Gary McClellan, Radio Electronics, April 1972, page 25.
Figure 1 shows the antenna input circuitry of a typical AM radio. There may be some slight variations on this, but the circuit described here will compensate for them.
Figure 1: AM radio input circuitry before the modification
The author described assembling this circuit on a 1"*1" perf board, I actually laid out a small PC board with excellent results. In this era of surface mount components, I think a much smaller version can be laid out on a PC board, allowing this circuit to be put inside even the smallest radios, such as Walkmans. If I do such a board, I will put the PC board artwork here.
Figure 2: AM radio input circuitry after the modification
From the article: After the booster is built, open the radio and locate the antenna. Trace out the secondary winding (it does not go to the tuning capacitor!) and snip the leads. Connect these leads to the input of the booster. Run leads from the booster's output to where the antenna secondary went. You must finally connect up the power and ground and mount the board to be finished. When that is done, turn on the radio and tune in a weak station above 1400. Adjust the set's antenna trimmer and you are done. Happy Listening!
To the above I would add: