Tiido wrote on 2020-08-09, 13:21:
You cannot test power related parts in circuit for the most part, since they are in parallel with many other capacitors on the same power rail, giving you bogus results.
This is generally true. If a single electrolytic failed on a power rail, and all the other ones are fine, you are unlikely to find it in-circuit.
In practice, if only a single of many electrolytics (e.g. tantalum caps) failed (and did not fail short!), the system usually keeps working fine. It's not the healt of every capacitor that keeps the system working, but the ESR at any point of the system must not exceed a certain threshold. While it is true that stray inductance makes capacitors far away less effective, this applies to ESR measurement in a similar way to system stability. Again, it is true that ESR measurement is usually done around 100kHz, whereas high-frequency spikes have spectral components up to 50MHz, but on the other hand, these high-frequency components are not bypassed by the electrolytics, but by the 100nF ceramics scattered over the whole board. These rarely fail.
If many electrolytics are paralleled next to each other (like on PSU output or VRM input), they are likely to fail at the same time, as wear and tear is similar to all of them. Even if one fails slightly, it increases stress to the other capacitors, so they will soon follow.
So while it is definitely correct that you can not probe a single capacitor in-circuit, measuring ESR in-circuit usually is a sufficient indicator whether there might be ESR-induced problems in the circuit. The cheap and handy all-in-one component testers seem to be quite useless for that, though. The interesting measurement is the ESR (or often the impedance which is assumed to be due to the ESR), which is displayed on all-in-one-testers only if the tester deems the circuit behaviour to be capacitor-like. More often than not, you get a "1.5V diode" or some resistance reading when you probe a power rail on a PC component. Dedicated ESR testers, like the Bob Pease one or the EVB one (I own the latter) work fine for the first-level in-circuit capacitor inspection in my experience.