Thanks for the link. That meter seems well built but it only reads ESR.
I´m buying and recommending the other one because it reads lots of other stuff.
Why do you say it can´t read ESR in circuit? I see a cap attached to the ZIF socket. I think it´s possible to attach test leads instead and use them to test the caps directly on PCBs...
I built one years ago and still use it. The principle is very simple. It consists of a square wave oscillator running at a fixed high frequency. Then it goes a through a passive lowpass filter (or some diodes, can´t remember) to fake a sinewave. The osc output then goes through a small mains 220/12 transformer used for stepping the signal down to a very faint level. (If the cheap ebay meter outputs a test signal as low as this, it will be capable of reading in circuit).
Then the return signal is read by a microamp meterand that´s it.
Caps are reactive. The higher the frequency the more they will appear as a short circuit to AC. You can regard DC as just an infinitely low frequency AC. The rest is simple: to calibrate the uA meter we short circuit the test probes and move the needle to maximum Amperage. This equates to zero Ohms, or the lowest ESR possible. Then we use a 1 ohm resistor, and the needle should move bit to the left, then a 2 ohm resistor and the needle moves another bit, and so on. Simple and effective. Of course the uA scale is now an ESR scale and must be redrawn, but I´m not very good with graphics. I just use it intuitively and just KNOW when a cap is good or not by looking at the scale.
I can post some pictures of it later.