This is a easy and quick way to make a feedback oscillator from old tapedeck. You just need to connect the rec input with output.
You can do it by soldering wires between the in and out lines (r in to r ou and l in to l out) Most homestereo tape players use rca connectors and you can use them as your outputs. You can also do this without soldering if you have proper wires to do the connections ( few rca wires and Y shaped rca adaptor for output from feedbackloop). Press rec/play to get it working. Frequency of the oscillator is adjusted by turning the tapedecks rec level knobs. The singnal is quite strong so I do not recommend you to plug this to your most expensive or fragile sound gear.