Mains Line Filter Cap Failures on PSU Test Equipment - Page 1
Forum section: electronics / repair

author: BH
author_rank: Contributor
timestamp: July 21, 2015, 03:28:48 pm
content: Hi all,Recently I've experienced two separate incidents in the lab where the X2 suppression capacitor (which is strapped across the mains input) has failed in catastrophic fashion.The first incident happened about 3 weeks ago on a very old dual rail PSU (it was some obscure manufacturer and I forget the name). The mains capacitor suddenly caught fire (flames could be seen bursting through the enclosure) and the entire lab was filled with smoke - it was horrendous!!! We threw the PSU away.The second incident happened today, again on an old PSU, but this time it was a HP 6634A. Again, the mains input cap went kablooey and filled the lab with smoke. This is a decent PSU so I'm going to replace the cap and put it back into service.Anyway, I wanted to run this by the other forum members to see what experience other people have had with these capacitors. Has anyone else experienced it as a common failure mode? The component in question is quite high risk because it's literally strapped across the mains input and, if it goes kablooey, it's a significant fire hazard.In both my incidents the supplies were very old, so perhaps age of the caps has something to do with it. Any comments about that?I'm also tempted to try and test the quality of the mains input just in case we're getting frequent high voltage spikes which could explain the failure of these components. I am wondering if I can set up some sort of long term data logging system to take the measurements over a few weeks. I have all the kit I need around me to do some data logging. Does any one have advice about capturing evidence of mains spikes?

author: Andy Watson
author_rank: Super Contributor
timestamp: July 21, 2015, 03:37:26 pm
content: Were these input filters part of Schaffner filter unit? These filters are infamous for having used a batch of dodgy capacitors. The problem would appear to be one of shelf-life rather than usage. The common factor is that the capacitors all date from the same period in the 1980s.

author: wraper
author_rank: Unknown
timestamp: July 21, 2015, 03:56:21 pm
content: Guess RIFA caps in transparent case? They were used in Schaffner filters too.

author: BH
author_rank: Contributor
timestamp: July 21, 2015, 04:43:37 pm
content: Thanks for the replies Andy and wraper!Andy:The filter in question is made from discrete components fitted to the PCB; it's not one of those mains input sockets with built in filters. It's basically one large X2 strapped across the mains, and then two smaller caps which go between live-earth and neutral-earth. It's the bigger cap that has gone in each case.wraper:It is indeed one of those RIFA caps in a transparent case! Where did those photos come from? Your personal experience of failures?Both:From what you've both said it does sound like the problem could be a bad batch of caps. When they die, they certainly do seem to go out in style. Our lab was literally filled with smoke; we had to abandon the place and wait for the smoke to clear! I'm not sure what exactly is in these caps but none of us wanted to breathe it in.

author: edpalmer42
author_rank: Super Contributor
timestamp: July 21, 2015, 04:58:12 pm
content: How much of this is due to age and how much is due to voltage rating?I think I've seen a few reports of this from here in North America (110VAC) but most of the incidents seem to be from 220VAC (or higher!) countries. In one of the pictures, I see a rating of 250V. I would be nervous about using a 250V capacitor on a 220 volt line.Ed

author: wraper
author_rank: Unknown
timestamp: July 21, 2015, 05:12:23 pm
content: Quote from: BH on July 21, 2015, 04:43:37 pmGoogle Rifa explosionand look at the pictures. Though I've personally seen one exploding itn the front of my eyes, when repairing some stuff which wasn't powered for years. There were a lot of Rifa caps with cracked case, one of them exploded. Actually all of such Rifa caps of those years, I have seen, were with cracks. I've read a conclusion somewhere, that they absorb moisture and go BOOM!.QuoteThis is not bad batch, all of those caps ever made are intended to go kaput.

author: SeanB
author_rank: Super Contributor
timestamp: July 21, 2015, 05:17:27 pm
content: Depends on time, crazing of the case from age allowing moisture in and then it is a ticking bomb. I just replaced a filter, not a Schaffner, but a Belling Lee, which did not have the capacitor fail but the common mode choke went foof.They fail mostly after about 10 years, or after being unpowered for over a year, so old equipment that is used infrequently is the most common problem.Cure is to place a fuse directly on the input before the cap( time delay so the switch on surge will not blow it) for safety, which is done in the fused IEC sockets, where the fuse is directly connected to live. Second fix is to look, and replace any transparent RIFA units with WIMA yellow blocks, those pretty much only die from gross overvoltage, or from being cooked till the case melts.

author: wraper
author_rank: Unknown
timestamp: July 21, 2015, 05:18:13 pm
content: Quote from: edpalmer42 on July 21, 2015, 04:58:12 pm~250V of X2/X1 safety rated capacitors is not usual DC rating. They are indeed meant to be used at mains voltage.

author: BH
author_rank: Contributor
timestamp: July 21, 2015, 06:39:37 pm
content: Thanks everyone for the responses.Here is a good forum post on this very issue:http://www.vintage-radio.net/forum/showthread.php?t=69128

author: tautech
author_rank: Super Contributor
timestamp: July 21, 2015, 08:13:33 pm
content: Quote from: BH on July 21, 2015, 06:39:37 pmThese failures are not uncommon, I've have a few over the years, just like wraper has shown.Domestic appliances, TE and power tools.Every few months somebody posts like you, but this is nothing new on EEVblog.Old Schaffner filters have the worst reputation, maybe their products are OK now?Many prempt these failures and replace either the caps or the complete filter assemblies in their precious TE.

author: KJDS
author_rank: Super Contributor
timestamp: July 21, 2015, 08:28:16 pm
content: What really annoys me is that it takes about a week before the smell leaves the lab. Sometimes I see one go each week, for a few weeks in a row, usually in winter when there is more moisture and I can't open too many windows due to the cold.

author: BH
author_rank: Contributor
timestamp: July 22, 2015, 08:42:47 am
content: @tautechSorry for the repeat post; I did try to search for the issue beforehand but I couldn't find anything.Regarding prevention, we're going to do the same and replace all the suppression caps on our older test equipment.@KJDSYeah I agree, the smell lingers for a long time afterwards! The first time it happened we had to abandon the lab for over an hour, and the smell lingered for a couple of days.

author: tautech
author_rank: Super Contributor
timestamp: July 22, 2015, 11:08:02 am
content: Quote from: BH on July 22, 2015, 08:42:47 amNo apology required.At least your thread has a good title name that should be easier to find for future searches on this subject.

author: veedub565
author_rank: Frequent Contributor
timestamp: July 22, 2015, 01:51:43 pm
content: It's a common problem on the BBC Micro PSU too, especially as they are so old and most have been stored in the loft for 20yrs.I replaced them on one I bought recently as a precaution, they hadn't blown yet but looked really crazed and cracked. I didn't even try switching it on until I'd replaced them.One solution was to replace the original 250v ones with new 270v ones.

author: Shock
author_rank: Super Contributor
timestamp: July 22, 2015, 03:31:22 pm
content: This seems to be an age related issue especially if you just crank up old gear that has not been in use for a while, they appear to heat up and blow after a short time after reuse.I believe I've just had one fail myself and haven't even opened the case, I can hear something venting anyway.

author: BradC
author_rank: Super Contributor
timestamp: July 22, 2015, 03:59:12 pm
content: Quote from: KJDS on July 21, 2015, 08:28:16 pmIt's even worse when your "lab" is the corner of your living room and you have to explain to your wife what the smell is and consequential brown stain on the wall that takes forever to remove.

author: KJDS
author_rank: Super Contributor
timestamp: July 22, 2015, 04:04:03 pm
content: Quote from: BradC on July 22, 2015, 03:59:12 pmHaving you considered trading the wife in for a new scope

author: SeanB
author_rank: Super Contributor
timestamp: July 22, 2015, 05:28:47 pm
content: Hard to get that trade, most people with a new scope for swap already either have a wife or are recovering from having had one.Me, no wife, and not intending to get one either. New scope however.............

author: Kleinstein
author_rank: Super Contributor
timestamp: July 22, 2015, 06:40:26 pm
content: It's a common problem with X2 capacitors. They are supposed to low without to much smoke, but some types are not that kind. Usually X2 caps a re supplosed to be behind the switch, not directly at the cable - there you should have higher quality X1 type caps.Way back this was a big problem with old TV sets catching fire from blows caps. There where a few bad types, not just the transparent RIFA ones. They had to recall lots of them. Today the caps usually don't start a fire at least, but still a blown X2 cap is a common defect.

author: codeboy2k
author_rank: Super Contributor
timestamp: July 22, 2015, 06:55:37 pm
content: Quote from: tautech on July 21, 2015, 08:13:33 pmI'm putting that on my todo list right now

author: tautech
author_rank: Super Contributor
timestamp: July 22, 2015, 08:08:08 pm
content: Quote from: Shock on July 22, 2015, 03:31:22 pmNot only age related. I'm not sure what process cause X rated caps to crack but some types do.The Rifa caps can just crack and equipment not used for some time will absorb atomospheric mosture and whem repowered ....... fiz.....boom.I've seen this twice in Kenwood Chef mixers.IMHO failures are probably less if equipment is left in a powered state, not ideal I know, fire risk and all.

author: Stray Electron
author_rank: Super Contributor
timestamp: July 23, 2015, 02:27:08 am
content: Quote from: SeanB on July 21, 2015, 05:17:27 pmThat's been my experience also. A few years ago I got a bunch of the old HP E and F computers with the front panels that contained switches and lights and I was restoring them and I had a number of capacitors of type type explode. Several while I had the PSU out and open. They made abrilliantflash like a Flash Cracker and aloudband. I don't remember the brand but all were made of that same looking translucent material. The funny thing was that the computers never hiccupped. They just kept right on running!

author: Shock
author_rank: Super Contributor
timestamp: July 23, 2015, 05:19:37 am
content: Quote from: tautech on July 22, 2015, 08:08:08 pmHah! I have a Kenwood mixer on my repair shelf with this exact same symptom, I thought it might be motor related, but now you mention it I bet it's the same problem. Sweet.

author: tautech
author_rank: Super Contributor
timestamp: July 23, 2015, 06:07:47 am
content: Quote from: Shock on July 23, 2015, 05:19:37 amAlso check the snubber on the speed control circuitry.

author: BradC
author_rank: Super Contributor
timestamp: July 23, 2015, 09:08:46 am
content: Quote from: KJDS on July 22, 2015, 04:04:03 pmHell no. It took me 20 years to find one that entertains my thing for tools. If I can justify it, I'm allowed to buy it. She even comes along shopping some times to look at all the shiny things.The only behaviour I've had to "adjust" is the "it's broken, let's buy a new one". I much prefer, "it's broken, let me spend 12 months planning out how I might fix it and save yet another item from landfill". That usually winds up with the compromise that I'll fix it now, but I need this new tool to do itEveryone wins.

author: wagon
author_rank: Frequent Contributor
timestamp: July 23, 2015, 11:41:50 am
content: I've replaced those caps in all manner of stuff. Not a complete list, but here goes.Kawai organsYamaha organs(with the organs the customer will report a 'bang'and sometimes a stink. They are used to supress 'pops' on power up or off. The instrument works OK without them, but I change them anyway)Clipsal 'C-Bus' lighting controllersDaken M70&M100 electric fence energisersVarious sewing machine pedalsWhatever that crap is that comes out of them, it's very hard to clean up. Acetone, carby cleaner, thinners, isopropanol, etc. all move some of it, but not all.

author: wagon
author_rank: Frequent Contributor
timestamp: July 23, 2015, 11:52:35 am
content: You could say the problem is 'Rifa' with these capacitors.

author: nfmax
author_rank: Super Contributor
timestamp: July 23, 2015, 12:09:56 pm
content: My score of failures of these capacitors:A 1980's vintage ASTEC SMPSU in a computer I built back in the dayA Wyse WY-30 terminal for the aboveAn HP 3312A function generator (much cracking & popping, but it kept on working all the while)A Neff dishwasher (this one, on the other hand, apparently took out the circuit RCD and killed a PIR light controller, though it may have been a mains transient that did for both)The capacitor design is supposed to be fail safe, i.e. it does not fail short-circuit. To be fair, none of my adventures ended that way

author: Kleinstein
author_rank: Super Contributor
timestamp: July 23, 2015, 03:03:07 pm
content: Not failing by short circuit did not allways work. I had a Makita drill with the cap blowing - not just the cap, but also tripping a 16 A fuse and blow the next larger 35 A one. To be fair I pluged in the machine a second time after it hat tripped the 16 A braker before.In this case likely to many sparks from a broken motor braught the cap down.

author: Smith
author_rank: Frequent Contributor
timestamp: July 25, 2015, 10:01:52 am
content: I had the same experience with an almost brandnew and expensive Bosch drill. The cap was shorted, and blew the 16A fuse of our house. There wasn't much to see about the cap itself. Replaced it, and I had a free drill.Philips equipment PMxxxx scopes and some HP 54xxx scopes where known for failing and exploding mains caps.