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Old 12-02-2017, 08:06 AM   #1
Behemot
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Default Bad NCC SMQ 220/400 in 2006 Seasonic S12II

So when I am running out of these 450W (and sometimes even 400W units lol), I am slowly getting to repair (or to quickly try to repair; no time to waste with lost cases) all the ATX PSUs I have here. Got to this 400W Seasonic S12II, most likely, there is no sticker and fan in the lid.

Discovered shorted freewheeling diode in the PFC and fuse of course. I also tried to measure the NCC SMQ 220/400 caps and whooa! It has ESR, but three different meters I have all show no capacitance, and the AVR tester shows it is a 2.59ohm resistor What could happen to a cap to be like? I guess that is why the diode failed in here…

ADD// found the lid, it's a 380watter, SS-380GB. After replacing the diode and cap with some ancient Nichicon GK 270/400, it did turn on without fan and still on a light-bulb-in-series.
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Last edited by Behemot; 12-02-2017 at 09:19 AM..
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Old 12-02-2017, 08:34 AM   #2
Stefan Payne
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Default Re: Bad NCC SMQ 220/400 in 2006 Seasonic S12II

Normally its overvoltage that kills primary caps and not much else.
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Old 12-02-2017, 09:56 AM   #3
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Default Re: Bad NCC SMQ 220/400 in 2006 Seasonic S12II

Seems that I was lucky and indeed only the diode and the cap were bad. Diode was 2 A/600 V, replaced with 3A one. Used fuse only rated 4 A as it will only see 230V mains (and about 50% load anyway with FX-6300 and GTX 650).
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Old 12-15-2017, 05:04 PM   #4
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Default Re: Bad NCC SMQ 220/400 in 2006 Seasonic S12II

It's possible that the capacitor failure caused the diode to fail due to excessive ripple current or something like that.

I haven't seen it myself yet, but I've heard of rare occasions where the PFC circuit in active PFC PSUs can go unregulated and send lots of voltage to the filter cap. You can usually tell when this happens by how the capacitor vents - the top is usually split open wider if the failure was more violent.
(Compared to the classic bad caps which leak out the top over the course of a few months, not violently in a few seconds. Well, except for Sacon FZ!)

-Ben
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Old 12-16-2017, 04:32 AM   #5
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Default Re: Bad NCC SMQ 220/400 in 2006 Seasonic S12II

This has not vented yet, not even been bulged, but if it had no capacity than the whole internal structure must have failed. The PFC output was than pretty much unfiltered which put excessive load on the forward switchers, and also increased load through this diode…it was luck only the diode failed, as it was smaller in this 380W version. They already put beefier ones in the 430 and higher rated, it seems. So the switchers could explode in such case. They usually take half of the circuitry with them, like all the SMD diodes, resistors, tiny transistors in the FET gate and feedback circuits, plus the controller. With all that I rarely succeed in repairing - I usually do not even try anymore.

Quite lucky I did measure it though, I wanted to replace it anyway as they put quite small caps in and unlike FSP, they usually do not pass ATX requirement for hold-up time. But if I did not and than put it elsewhere, could have been interesting show. And than looking for the cause of the failure…

Last edited by Behemot; 12-16-2017 at 04:37 AM..
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Old 12-16-2017, 01:48 PM   #6
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Default Re: Bad NCC SMQ 220/400 in 2006 Seasonic S12II

Looks a lot like the failure in my ThermalTake M850W PSU - short-circuited primary cap (and blown fuse... though everything else was okay in my unit).

Just more proof suggesting APFC PSUs stress the primary caps way too much. On my ThermalTake M850W, I added a 0.047 uF, 400V poly film cap in parallel with the main cap in hopes of that one "eating" up the high frequencies and any voltage spikes. I might up that to 1 uF, though.

I swear, the next APFC PSU I get with a blown primary cap, I'm going to rip out the APFC and retrofit it with a classic voltage-doubler (if PSU is used on 110/120V AC) or a single 400V cap (if used on 220-240V AC) right after the rectifier.

Last edited by momaka; 12-16-2017 at 01:50 PM..
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Old 12-16-2017, 02:07 PM   #7
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Default Re: Bad NCC SMQ 220/400 in 2006 Seasonic S12II

I'd just ditch that for parts lol. Have kinda too many such units lately, actually four bad units in a row which do not work after being refurbished - way too many for my taste! Just today another CWT GP platform stand-by rail exploded on me (got two different such units already bad before), seems like some series problem. (Guy said he tested everything in a comp before, wonder what the problem was than; I just replaced all the crap caps ).

If it has combo driver than no way and even if not, there could still be some feedback for error amplifier.
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Old 12-16-2017, 02:58 PM   #8
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Default Re: Bad NCC SMQ 220/400 in 2006 Seasonic S12II

Well, you could retrofit those PSUs with a board from a 5V, 2A power adapter, lol.

But you are right, probably not worth the trouble unless you have nothing better to do. Personally, I'd just look for more OEM 250-350 Watt PSUs without PFC that just need a recap. Way easier to work on and more reliable than modern crap. Even Deer start to seem attractive now, no?
(As in, Deer/Allied PSUs, lest someone new here think I have some kind of animal fetish or something )

Last edited by momaka; 12-16-2017 at 03:00 PM..
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Old 12-16-2017, 03:04 PM   #9
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Default Re: Bad NCC SMQ 220/400 in 2006 Seasonic S12II

Two of the units crapping on me are passive PFC. Some Huntkey 400watter and FSP400-60THN-P (BlueStorm OEM). One shuts down pretty much immediatelly, the other one with load above 100 W or something like that. Plus these old units have dozens of small caps on several boards and are filled with yellow crap glue. So many things to go wrong.
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