1. Home
  2. Electronics Repair
  3. Electrolytics blow after 1 hr

Electrolytics blow after 1 hr

Another day in bad caps hell. I fish a Dell GX520 out of the pile at the local recycler and proceed to try and raise the dead. I've done this many times with good success, but not this time. Upon initial inspection, I find the usual bulging electrolytic caps. There were 4 identical caps, 2200uF 6.3VDC 105C. I replace them with 2200uF 10VDC

105C caps (Panasonic ECA-1AHG222). Note the increase in working voltage. These are brand new caps from Digikey. I run the unit overnight and find that two of the caps now have bulging tops.

Assuming that I may have had some marginal caps in the bag (unlikely but worth testing), I replace the two that bulged with 2200uF 10v 85C caps (Panasonic ECA-1AM222) also from Digikey. This is somewhat of a downgrade, but should work. I also replace the power supply, for no better reason than I had a spare.

Same problem as before. The two caps blew their tops within a few hours, leaking caustic goo.

The two large black caps in the foreground are the surviving 105C caps, while the blue caps in the background, are the bulging 85C caps.

Using my finger and a thermocouple thermometer, I determined that the bulging caps are not getting warm or at least are not any warmer than other nearby components.

I've replace literally hundreds of caps but I've never seen this before. It really makes me wonder if the bad caps problem might be caused by something other than defective caps and/or marginal capacitor voltage ratings.

Duz anyone have a clue as to what is causing this problem? I haven't had time to probe the board for power supply bus noise or excessive current quite yet. What I could use is some speculation as to the probable culprit.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com               jeffl@cruzio.com
# http://www.LearnByDestroying.com               AE6KS
Reply to
Jeff Liebermann
Loading thread data ...

Bad rectifier letting AC through to the caps?

Reply to
William Sommerwerck

Try using Low ESR caps in that circuit... Most switching signals need caps with virtually no R in them.. Otherwise, they'll heat up in side a bit and vent, or partially get there.

Jamie

Reply to
Jamie

(snip)

Jeff, I've been reading your posts long enough to conclude you know what you're doing, generally in spades.

But ... did you check the polarity? I ask that because there are cases where the PCB screen legend was arse-about, causing grief to cap-replacers who didn't record the "outgoing" cap polarity and match that against the screening.

Otherwise, if heating isn't an issue then what WW said is a possibility.

Reply to
who where

"Jeff Liebermann"

** Errr - I hate to tell you this pal ........

but the pic shows all 4 caps installed with reverse polarity.

A smaller cap ( C310) is clearly installed correctly and so are all the others I can see.

The markings on the PCB are a bit unusual with the white triangle indicating positive.

.... Phil

Reply to
Phil Allison

Typo alert! Would you believe "what WS said"?

Reply to
who where

I think that triangle is actually a "+" sign, and if so, I agree

Arfa

Reply to
Arfa Daily

"Arfa Daily"

** I see a white circle outlining each electro, then a solid white triangle making a chord across the same circle plus a large white "+" sign right next to the triangle.

The bulging electros are in all in backwards .....

..... Phil

Reply to
Phil Allison

The caps are in backwards.

tm

Reply to
tm

AAAAAAARGH! I can't believe I did that. I usually assume that the silk screened marker was matches the negative stripe on the electrolytics. Very embarrassing.

Thanks for catching my mistake (as I crawl back into my hole and sulk).

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com               jeffl@cruzio.com
# http://www.LearnByDestroying.com               AE6KS
Reply to
Jeff Liebermann

Whats the problem? It all fits into the URL:

formatting link

Reply to
circuit

Maybe not very commonly known anymore, but its always been a standard design practice through the decades, when a board is built, that all the polarities of electrolytic caps face the same direction. Have seen a few cases the last couple of years where this is NOT followed though.

Reply to
circuit

Frightening. I had the right idea ("AC getting through" is a subset of "wrong polarity"), but I never would have guessed that. How can a manufacturer so blatantly ignore a standard that's been around for I-don't-know-how-many-decades?

Reply to
William Sommerwerck

That's easy, there's so many standards to choose from.

Jeff

Reply to
Jeffrey Angus

Well, "many" standards is equivalent to "no" standard!

Years ago, when I was working in Ecuador at a NASA site, we had to replace some blown power rectifiers. I -- and everyone else -- assumed that power rectifiers always had the cathode on the bolt end. Uh-uh. These were backwards -- anode-to-bolt -- so our replacements kept blowing.

Reply to
William Sommerwerck

Jeff Liebermann wrote in news: snipped-for-privacy@4ax.com:

I reversed an electrolytic once,it blew and sprayed goo on the side of my face,and foil in the instrument.

it's not as bad as my cutting through a power cord (missing GND pin,needed replacement) that was still plugged in. :-( Big flash and loud KAPOW,hole in diagonal pliers,and boss nearby was filling out my performance review,and was working on the part about following safety prodcedures. I'm glad the pliers had insulated handles.

--
Jim Yanik
jyanik
at
localnet
dot com
Reply to
Jim Yanik

You're lucky you didn't get molten metal in your eye. I once deliberately shorted out an AC line with a scredriver. The result was not at all pretty.

Reply to
William Sommerwerck

An instructor told me, ass-u-me anything and you will make an ASS of U and ME

Reply to
N_Cook

In article , Phil Allison writes

Bingo.

--
Mike Tomlinson
Reply to
Mike Tomlinson

What makes a cap pop? Heat. Voltage/current surges. Figure it out.

--
Live Fast, Die Young and Leave a Pretty Corpse
Reply to
Meat Plow

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.