mpe wrote on 2021-04-21, 07:24:
The device advertises in-circuit capacitance/ECR testing (given by using 100 kHz pulses and low voltage). Will see how it goes! In worst case I can always use it to test desoldered parts.
This is doubtful, at least in a general sense. Most of these apply a very low current across a discharged capacitor to measure instantaneous voltage rise (prior to significant charge buildup on the cap). In theory, the involved voltages and currents are so low that semiconductors won't activate and sections of the circuit will remain electrically isolated. The device can still be confused by passive components (capacitors, inductors, resistors) in electrical proximity or by leaking semiconductors not fully isolating a section of circuit.
An even more sophisticated device could potentially detect and measure impedance deviations via reflectometry. Usually this is only done on wires and RF circuits which have fairly distinct characteristic impedance, but it's theoretically applicable in more general conductors over a wider frequency range. Measuring impedance over a wide frequency range would also allow capacitive reactance to be isolated from resistance and inductive reactance, but the sampling performance required means it's likely only possible in a laboratory setting for the moment.
On the plus side, this device does operate at 100khz which is the typical point of self resonance for electrolytic caps (where inductive and capacitive reactance is equal in magnitude). That's the correct measurement frequency for lytics, although it's not necessarily correct for plate caps, ceramics, or other products intended for higher frequency application.
Miphee wrote on 2021-04-21, 06:11:
So you either start removing and measuring elcos one by one or trust the readings you get in-circuit. If I had to choose a method (strictly talking about XT-S3-era boards) then I'd choose to replace them all. The usual 10/33/47/470 uF elcos are extremely cheap.
The analogy isn't exact, but the two perspectives I alluded to earlier in the thread are similar to the debate over predictive vs preventative maintenance. Determining which approach works best is usually very situational and will probably be different for a person starting out in repair vs someone who has experience.
7F20 wrote on 2021-04-21, 15:29:
It's absolutely enough to only desolder 1 side of the component; you don't have to fully remove it.
For this kind of job, it's fine. For completeness I'll just point out that your world gets a bit more complex when dealing with RF circuits.