Quoted from La_Porta:
Vid, can you explain to me what an ESR meter is? What ESR stands for? Does it test resistance between the two sides of the capacitor, or how does it work?
As electrolytic caps age, their ESR (Equivalent Series Resistance) increases.
The higher the temps and ripple, the faster the caps age.
Of course, the higher the resistance, the higher the temps, so the problem becomes a downward spiral.
As you could see from the C26 example, even though a cap's capacitance value is within spec, the high ESR tells us that the cap is not functioning properly.
99% of the time, the ESR meter can read a cap while still installed in the circuit. Although, when you have a bank of caps installed parallel to each other (like in an unregulated CNC stepper driver power supply), you have to pull one leg.
An ESR meter is not perfect. It will not read really tiny caps (like anything below .01uf), and there is not a perfect go/no-go line. You quickly learn the acceptable ESR ranges for different cap values. Just probing through your junk box will give you a crash course in ESR in about 15 minutes. Bad caps will often just say "Leaky" on the display.
As far as which ESR meter to buy, there are many that cost more than the Atlas ESR70+, but I don't think any have the all the features that it has. If you repair lot's of boards, it is a great tool. If you just poke around a few times a year, you can buy lots of replacement caps for the $129 that the meter costs...