We used to make those out of a 6.3 volt transformer and a couple of resistors.

Later I figured out a neater one for caps. You need a 1 KHz square wave of 400 mV with as close to 350 ohms source impedance as possible.

I can check caps with it from about 0.47 uF all the way up. If you can manage to, have the 400 mV square wave ride on 200 mV DC and it will also automatically detect shorts.

It is way faster than any ESR meter out there, even the old Wonderbox. Once you learn how to read it that is. And it is easier to see than almost any analog or digital display.

One of these days I plan on doing a writeup on it but I want to get a bunch of pictures depicting the readings on good and bad caps of a bunch of different values.