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This Crystal Radio circuit is outline to show tuning situation. You have variable capacitor and you do not know what capacitance it is and know the wound coil is surely long enough. Good antenna aerial, good ground, Crystal earphone, we just need the tuning for set to work. Old method was open capacitor as for high frequency and connect circuit with repeated cut and trail of coil winding length to find broadcast band making sure you can tune high range of band after final cut and connections.
Test tool is wooden handle, clip, like a Fahnestock clip, short steel needle (what a surprise to see the old style phonograph record needles so plentiful on the Internet). At end of wood handle, screw down clip and pinch steel needle between clip and wood to make probe. Connect wire from capacitor rotor to handle clip.
Our probe is connected to rotary part of variable capacitor and stationary part connected into rest or circuit. Top end of wound coil is unconnected. Listen on earphone as you push probe through thin wire insulation of wound coil to find stations you want to tune in. Leave few extra turns on coil and unwind excess windings and connect to end to rotor connection of tuning capacitor.
You might want to try method on other radio sets for different frequencies if bandpass filters do not block range you want to receive.
Second idea is make two probes and connect with short wire and place probes on each side of solder joint on working set. If noticeable improvement in sound, re solder that joint.
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