Leaky capacitors are a common cause of problems with electronic equipment. By leaky, I don’t mean that the capacitors puke out their own viscera, but that they are conducting a certain amount of DC current. This leakage cannot be tested with a just a multimeter. Capacitors used in vacuum tube circuits are often subject to high voltages, and I’d like to test the caps under similar circumstances as in the circuits. This is where the high voltage PSU comes in handy.
Even very small leakage currents can be enough to cause problems, for example when used as bypass caps between tube amplifier stages. A 10µA current through C44 in this schematic would cause approx. 5V over R23 (500kΩ) and into G1 of the EL3, enough to completely mess up the bias point of the output stage.
Therefore the instrument must be able to detect currents in the µA range. The current draw to the capacitor under test can also be fairly high, both when charging large electrolytic caps, and when they are very leaky. These currents must not destroy the ammeter, so the test instrument must have a current limiter and selectable ammeter range, and luckily the DIY tester has both features.
A video demonstration, showing the given capacitor under test:
At power-on, the instrument starts in Capacitor Discharge mode. I change the mode to Capacitor Test, and the instrument changes both current limit and ammeter range to 5mA. Then I gradually increase the output voltage. The ammeter doesn’t show much impact on the 5mA range, so I change the range to 500µA and continue increasing the voltage. At 350V (still less than the voltage rating of the capacitor), the ammeter shows a leakage current of 260µA. With this bypass capacitor, powering up the amplifier stage shown in the schematic might destroy the output tube or the output transformer due to unrestrained current draw.
There was no doubt that the given capacitor had to ble replaced.
Other usages for this device:
Retronics 
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