I just repaired a sewing machine control box. It didn't turn on when connected to mains 220V. At the beginning I didn't notice any severe problem. No burns, no blown fuses, bus dc voltage was present. I started to look into the switching power supply circuit. Everything looked well. However, just for curiosity I replaced two electrolytic capacitors 100uF/50V and 47uF/50V, and the machine started to work normally. My question is.. the old capacitors physically looked good, no bulky, and the multimeter showed the right values. But they were bad. So it caught my attention that even they are not out of value, they are faulty. Why is that? Could be they get dried and this affected the switching circuit? What is happening inside the capacitor that makes them bad, but hard to notice at first insight?

 

Electrolytic capacitors can change value with age, in this case it may take operating voltages to detect any anomaly that may occur when stressed under power, loss of capacitance etc.
Max.

 

The ESR can also increase in old (or dieing) capacitors, essentially rendering them unable to perform. The easiest way to check the ESR is with an ESR meter. Here's more info:

https://www.allaboutcircuits.com/te...uivalent-series-resistance-esr-of-capacitors/