A way to check old variable capacitors with Insulation PCB

If you restore antique radio, you may need to check the variable air capacitor. However you've probably known it's not easy if the capacitor is old. There normally are two problems, 1st, the rotor partially touches the stator in the rotation, 2nd, it's not touching but gets too close, near touching somewhere, then the tuning linearity has been lost.

To check and find out the exact location, so that you can fix the part, High Voltage, more than DC 1000V, Insulation Checker Board is useful. Since 1000V can make approx. 1mm glow discharge in the air gap, you can see where and which part touching or near touching in the rotation. In this video DC 1200V Cockcroft-Walton Insulation PCB shows it with spark discharge. Though you'd better use glow discharge only needless to say.