Chieftec APS-650C owners beware

[mitchw]

My trusty old Chieftec APS-650C decided to emit a pop and some smell few weeks ago. This happened when I pressed the power on button on the case.

Upon examination I discovered:
- No mains fuse blown, still drawing some little current from the mains
- TNY278PN MOSFET almost shorted (about 90 ohms drain to source)
- Primary of the 5VSB transformer open
- 470uF 400V TEAPO mains smoothing capacitor reading only 5nF

I managed to repair it. First I repaired the blown transformer, the tiny wire coming out of the winding to the pin on the transformer was blown. I was able to solder some new wire to it. Next, I replaced the TNY278, and the last, the 470uF 400V cap.

I cracked open the 470uF cap and one of the internal connections failed.

As I read from some foreign forums, this failure on these power supplies is quite common. The 470uF cap goes open, and then usually the TNY278 fails short which either blows the primary of the standby transformer or it blows the 0.22ohm resistor next to it. Sometimes it can also blow more stuff.

So if you are an owner of a PSU like this, and it still works, you better replace the 470uF 400V cap before it fails.

[stj]

Re: Chieftec APS-650C owners beware

the cap was probably destroyed by the APFC circuit - that's starting to become a pattern in all modern psu's!
it's only there to please the power company to make up for deficiencies in their networks!

[mitchw]

Re: Chieftec APS-650C owners beware

I am not sure why it failed. Either the APFC switching action combined with slowly increasing cap ESR due to age. Or it failed just from old age, as there is a trace eaten away. The cap wasn't bulging. I had old 1960s caps from vacuum tube era fail the same way. They were measuring okay in the early 00s, but 15 years later they were O/L, despite sitting not used the whole time.

Testing the capacitor directly revealed it still had decent capacitance, but ESR was a bit high - could be that my leads were not touching properly and this tester is not to be trusted 100% really.

[ChaosLegionnaire]

Re: Chieftec APS-650C owners beware

the use of a 400v cap is the issue on apfc circuits. 400v is too close to the working voltage of the apfc circuit of 390v. sometimes due to spikes or brownouts on the power grid, it can even overshoot the 400v rating of the primary cap, causing it to fail.

to prevent this from happening again, uprate the voltage of the primary cap to 450v or 420v if there are space constraints. this should prevent it from happening again.

teapo is considered a junk cap brand here anyways, so im not surprised it failed, much less last as long as it did.

[mitchw]

Re: Chieftec APS-650C owners beware

I temporarily replaced it with a lightly used Hitachi HU series, which has the same ratings.

It lasted almost 11 years (from June 2010 to Mar 2021), so this isn't that bad for a cap running at almost 100% of its voltage rating

[PeteS in CA]

Re: Chieftec APS-650C owners beware

the use of a 400v cap is the issue on apfc circuits. 400v is too close to the working voltage of the apfc circuit of 390v. sometimes due to spikes or brownouts on the power grid, it can even overshoot the 400v rating of the primary cap, causing it to fail.

to prevent this from happening again, uprate the voltage of the primary cap to 450v or 420v if there are space constraints. this should prevent it from happening again.

teapo is considered a junk cap brand here anyways, so im not surprised it failed, much less last as long as it did.

Does your experience suggest that the reliability issue is with running the caps close to their rated voltage? Could it be that these types of parts were originally developed for 50/60 Hz usage and handle higher switch frequency ripple much less well? Obviously the answer could be "Both".

[momaka]

Re: Chieftec APS-650C owners beware

the cap was probably destroyed by the APFC circuit - that's starting to become a pattern in all modern psu's!

The first time I saw that failure, I knew those APFC circuits are up to no good. Surely some last longer than others, especially the ones build with plenty of caps (or just big enough caps) to handle the higher ripple current. But in the end, I don't think we will see any APFC circuit last as long as the oldschool method of caps straight-up filtering 50/60 Hz mains.

It lasted almost 11 years (from June 2010 to Mar 2021), so this isn't that bad for a cap running at almost 100% of its voltage rating

True.
But the problem is, I think manufacturers/designers of these PSUs have now well understood exactly how long those parts are going to last. And to me at least, this just seems like another form of planned obsolescence. Yes, 10 years is a pretty good run for a PSU... but the fact that I have 20-year-old Deer PSUs with completely junk brands on their primaries, like JEE, CapXon, Viva, and etc, and none of those yet failed... that should tell you something about how unreliable APFC circuits have made things.

I had old 1960s caps from vacuum tube era fail the same way. They were measuring okay in the early 00s, but 15 years later they were O/L, despite sitting not used the whole time.

Caps not sitting used at all is about the 2nd worst thing to do to them after caps being abused with too much heat, ripple current, high voltage (or a combo of all three.) Electrolytic capacitors will last the longest when used on a stable voltage supply (lower ripple) that's not too far below their rating (2/3 of their rated voltage tends to be optimal) and in a device where temperature swings are fairly controlled (temperature not too high or too low.) The further you move from these, the less the cap will last - especially with the last one: that is, e-caps certainly don't do well with high heat.

ITesting the capacitor directly revealed it still had decent capacitance, but ESR was a bit high - could be that my leads were not touching properly and this tester is not to be trusted 100% really.

Perhaps there was flux on the leads? Or perhaps one of the leads inside the cap got snapped due to chemical reactions with the flux or improper assembly/design at the factory. I suspect the cap did go open-circuit, though, because otherwise the TNYswitch likely wouldn't/shouldn't have blown on its own.

Obviously the answer could be "Both".

Indeed.
I think the answer is that both the high running voltage and high ripple current are contributing factors to the failures.