Mike_ wrote on Today, 15:48:

shevalier wrote on Today, 13:28:

In PSU without an APFC, the main converter itself is the only high-frequency load. The APFC is essentially a step-up converter. […]
Show full quote

In PSU without an APFC, the main converter itself is the only high-frequency load.
The APFC is essentially a step-up converter.
Thus, this capacitor is sandwiched between two high-current sources of high-frequency interference, and it often ends up in a bad state, unlike in older topologies.
When it fails (due to capacitance and ESR), the APFC (MOSFETs, diode, and often the controller) goes burn, making the repair of such a PSU cost half as much as buying a new one.
The primary capacitors in this topology fail completely without any outward signs.

If you’ve taken apart a PSU with an APFC, you always need to carry out 2½ steps:
- Measure the voltage across the large capacitor. For a 400V capacitor, the reading should be 380V ± 2–3V (for other ratings, check the voltage dividers connected to the controller. However, it definitely cannot be less than 375 volts)
- Measurement of ESR
- Measurement of capacitance, but this will be calculated automatically from the previous step.
According to current trends, 1 watt ~ 1 μF.

P.S. Alas, this is now one of the most likely points of failure. 🙁
We need to check them and wait for the price of these components to drop tenfold.
https://eu.mouser.com/ProductDetail/Vishay-Ro … kERvpWojg%3D%3D

I don't have an ESR meter, so that's out of the question...

I have a Chieftec with a 390μF Teapo capacitor.
This capacitor has a capacitance of 360μF and... an ESR of 4Ω.
Although it should be less than 1Ω.
And, interestingly enough, this capacitor looks in perfect condition.

So I apologise, but this isn’t a question of appearance or brand. 🙁
Unfortunately, the ESR does need to be only measured.
There are indirect ways of measuring the ESR, for example using an oscilloscope calibrator.
But that’s such a faff.
Personally, I measure it using a sound card.
I’m also into audio, and I need the impedance curve itself, not just a single figure.

https://deforg-free-fr.translate.goog/Zmeter. … &_x_tr_pto=wapp

shevalier wrote on 53 minutes ago:

I have a Chieftec with a 390μF Teapo capacitor. This capacitor has a capacitance of 360μF and... an ESR of 4Ω. Although it shou […]
Show full quote

I have a Chieftec with a 390μF Teapo capacitor.
This capacitor has a capacitance of 360μF and... an ESR of 4Ω.
Although it should be less than 1Ω.
And, interestingly enough, this capacitor looks in perfect condition.

So I apologise, but this isn’t a question of appearance or brand. 🙁
Unfortunately, the ESR does need to be only measured.

Well I don't have the equipment to measure ESR so that's not an option no matter what. The options are either to leave current cap as it is, or just in case replace it with the unused chemi-con SMQ 330µF cap in the picture.