Balun core or any RF toroid can be used.
SWG 30 wire 3 strands twisted.
Both transformers are identical. Resistor is used to take the AF and audio is given to AF Filter..
How to wind the core ? Make a trifiliar wire. Three wire is taken and twisted for 5 to 8 turns per inch by handdrill. then wound the 15 to 17 turns. One side wire end are labelled as 1,2,3 and their other ends are labelled as a, b,c. Test for dc continuity. 1-a,2-b,3-c and no other pair should show the zero. If it shows then wire enamel is broken and it needs to be rewound using new wire. Connect 2and a and your transformer is wound ok. Becareful with scrapping the enamel.