Safety capacitor dielectrics used in modern Schaffner mains filters - Page 1
Forum section: electronics / repair

author: matthuszagh
author_rank: Frequent Contributor
timestamp: September 09, 2022, 04:03:11 pm
content: Many people on this forum (and others) have recommended that the Schaffner mains filter be replaced on old equipment based on the presence of old (often RIFA) metallized paper (MP) capacitors inside, which undergo the typical failure mechanism. People then recommend replacing these with modern Schaffner offerings. However, Kemet still sells MP safety caps (alongside film versions (MKP)). To my knowledge, Kemet has never acknowledged this failure mechanism for MP caps, so it's not clear the issue has been fixed and I always op for MKP replacements, which I don't believe have this failure mechanism.Has anyone opened up a modern Schaffner mains filter? Do we know they're using film safety caps instead of the paper variety?A related question: since the failure mechanism is with the capacitors themselves, is it generally possible to open the filter and simply replace the MP caps with film ones? This would also address the challenge of finding modern, mechanically-compatible replacements for the filters. I guess in the worst case the lid could be cut open and soldered back after, though it would be nice to find a solution that's less destructive.

author: james_s
author_rank: Super Contributor
timestamp: September 09, 2022, 05:37:59 pm
content: I opened one from a TDS3000 scope I replaced because the housing was all rusty, it had modern style film capacitors inside, not the old transparent RIFA ones.

author: factory
author_rank: Super Contributor
timestamp: September 09, 2022, 06:12:10 pm
content: I've only opened one so far, from a Tek 24xx series scope (can't remember which), it took some violence as it was potted inside, I found two Wima MP3 caps & another unidentied white cap. The Wima MP3 is also paper based rubbish.As you can see from the pictures, there is no chance of reuse.The smell when one failed at work was so bad, that I never want to experience thisagain. Along with RIFA/Kemet madness, Schaffner madness are a change on sight part for me. They get changed for other brands, problem some are integrated with voltage selectors & fuses, I tend to avoid used TE with these, due to the expense of replacements.David

author: james_s
author_rank: Super Contributor
timestamp: September 09, 2022, 06:17:23 pm
content: Seems a bit extreme to avoid gear that uses them. I've only ever changed that one Schaffner filter and it was only because it was rusty, I have numerous others in service. I've only ever had one of those RIFA capacitors fail and it was 40 years old, I probably have some of those in service still too. Anything can fail, I haven't seen those to be particularly less reliable than anything else. I live in 120V land though so stresses on such parts are lower. I also shut off power to my bench when I'm not using it.

author: factory
author_rank: Super Contributor
timestamp: September 09, 2022, 06:22:32 pm
content: Yes with the 240V to 250V we have here, they are more of a liability, anything 30+ years old needs to go.I edited my last post, the reason for avoiding certain TE (such as Solartron/Schlumberger 7150), is the Schaffner madness with built-in voltage selector/fuses are too expensive.David

author: matthuszagh
author_rank: Frequent Contributor
timestamp: September 09, 2022, 06:31:22 pm
content: Quote from: james_s on September 09, 2022, 06:17:23 pmFor what it's worth, I've never seen a RIFA or Schaffner fail in any of my equipment and I've got quite a few pieces of gear at this point, most of which is constantly plugged into the wall. I'm also in 120V land. But, all the horror stories (even if they're generally confined to 220+V land) do make me a bit concerned since I'd really like for this gear to work for me for decades to come.

author: wraper
author_rank: Unknown
timestamp: September 09, 2022, 06:36:56 pm
content: Quote from: matthuszagh on September 09, 2022, 06:31:22 pmThe explosion usually happens after equipment was not powered for a while or imported into other country from US and plugged into 230V (was powered from 110V previously).

author: iJoseph2
author_rank: Regular Contributor
timestamp: September 09, 2022, 08:11:47 pm
content: I replaced a Schaffner mains filter on a Racal 2101 recently, and I had to use another Schaffner because there just wasn't any space to do anything else (and it had incorporated voltage selector and fuse).. even the tabs on the filter were bent because it was right up against the transformer.I just picked the cheapest same shape one I could find as it was about half the price (I think I used part number ending -6 instead of -2 off the top of my head).

author: iJoseph2
author_rank: Regular Contributor
timestamp: September 09, 2022, 08:16:08 pm
content: Quote from: matthuszagh on September 09, 2022, 06:31:22 pmProbably worth visually inspecting them.. if they're showing signs of cracking or bulging you know you should replace them ASAP.

author: srb1954
author_rank: Super Contributor
timestamp: September 09, 2022, 10:17:09 pm
content: Quote from: iJoseph2 on September 09, 2022, 08:16:08 pmYes, but this only applies for individual capacitors. You can't visually inspect capacitors potted in a filter module.An insulation resistance test can sometimes show up failing capacitors before they explode. Failing Y capacitors are easily detected with an insulation resistance test but the X capacitors have to be really faulty before the insulation resistance test shows up a problem. In potted filters the X capacitor is generally paralleled with a 1Mdischarge resistor and this dominates the insulation resistance value until such time as the failing capacitor drops below a few M.I annually test all my equipment for insulation resistance and I find that for many items the insulation resistance drops slowly over time, even for equipment that hasn't been powered up. However, if it has been a very dry year, the insulation resistance for some items occasionally improves a litttle. In my tests I find that is is Schaffner filters and RIFA capacitors that are the main culprits but I have also found some TDK filters deteriorate over time as well.

author: med6753
author_rank: Super Contributor
timestamp: September 11, 2022, 08:10:13 am
content: In 120V land it seems Schaffner mains filters rarely fail. I've never had one fail but I've seen plenty in 240V land go out as shown in examples here. The capacitors themselves are rated at 250V so it makes sense due to the added stress.Any equipment that shows up on the bench with individual RIFA capacitors get changed on sight. Theyalwaysdisplay cracking which will eventually lead to magic smoke.

author: matthuszagh
author_rank: Frequent Contributor
timestamp: September 12, 2022, 09:16:09 pm
content: I reached out to Schaffner directly and they were very quick in responding. They said the following in reply to my inquiry:QuoteGood news then; modern Schaffners should not suffer from this fault. It therefore seems that the recommendation to replace old Shaffners with new ones is well-founded.

author: srb1954
author_rank: Super Contributor
timestamp: September 12, 2022, 10:08:51 pm
content: Quote from: matthuszagh on September 12, 2022, 09:16:09 pmIt would be nice to know at what point they stopped using the rubbish capacitors they used to use so we don't have to needlessly replace all Schaffner filters on sight.I have had so many older Schaffner filters fail that I try to replace them with another brand but sometimes that is difficult due to space limitations. Other brands are often just a little bit bigger than the Schaffner filters and can be very difficult to squeeze into very tight spaces.

author: james_s
author_rank: Super Contributor
timestamp: September 13, 2022, 12:07:46 am
content: Quote from: srb1954 on September 12, 2022, 10:08:51 pmWell the one I took apart was from a TDS3014 (not B) and it had modern good quality capacitors in it. Not sure when that was made, early 2000's?

author: matthuszagh
author_rank: Frequent Contributor
timestamp: September 13, 2022, 02:46:00 pm
content: Quote from: srb1954 on September 12, 2022, 10:08:51 pmI posed this question to them. The individual I'm speaking to says he has no recollection of Schaffner using paper caps (unsurprising, since a lot of this gear is 20+years old), but that he might be able to look into it a bit more with a part number. I've never had a Schaffner failure and never opened any of my Schaffners up, so I can't confirm they used paper. But, if you have one or more part numbers with confirmed MP caps, I'd be happy to forward them along.

author: srb1954
author_rank: Super Contributor
timestamp: September 13, 2022, 09:06:39 pm
content: Quote from: matthuszagh on September 13, 2022, 02:46:00 pmI have had Schaffner FN323 and FN326 series filters fail explosively. The date codes on the faulty units indicated that they were manufactured during the 1980s and 1990s. I haven't opened any up to confirm the capacitor type because of the extremely revolting smelling goo that oozes out of a failed filter.I haven't yet had any failures from filters manufactured from 2000 onwards (maybe because I have very few examples of equipment from this era). However, I am wondering if it just a matter of time before gradual moisture ingress causes these to also fail dramatically.

author: matthuszagh
author_rank: Frequent Contributor
timestamp: September 15, 2022, 03:02:26 pm
content: Quote from: srb1954 on September 13, 2022, 09:06:39 pmHere's the reply I gotQuoteThat doesn't give a precise date. But, your early 2000s filters should be fine.

author: Gnarflord
author_rank: Newbie
timestamp: October 19, 2022, 10:28:38 pm
content: Maybe some data points help, so I'll add my little Schaeffner explosion I got last weekManufacturing date was '89, installed in a nice Tektronix 2445B in 240V land. But I guess the thing was partially faulty before releasing its smelly goop as it would trip my breaker randomly a few weeks apart. I couldn't find the device at fault at first, because it happened when I switched on the power strip to all the test gear and after resetting the breaker everything seemingly worked again. Well, at least the smell made it easy to find the fault this time around; also replaced the PSU caps while I was at it

author: Swainster
author_rank: Frequent Contributor
timestamp: October 20, 2022, 07:03:40 am
content: 2 more data points here, both HP (a 3314A and a 8656B) and both Schaffner FN370-2.2 inlets. Both resulted in the instrument blowing the previous owners' circuit breakers and resulting in me getting the units at a knock down price. On the flip side, I had to buy new inlets, which are still available new from the usual catalog suppliers, however not that cheap. Local mains is 240VACEdit: Both instruments are late 80s. The 3314A is a '87 design release, and the 8656B has '89 date codes inside

author: wraper
author_rank: Unknown
timestamp: November 02, 2022, 07:30:30 pm
content: Today while repairing lab balance PSU found two PME271Y Y2 caps, one of them has crack on a side. PSU was made in 2005, so I assume Rifa caps were not fixed at that time.

author: alm
author_rank: Super Contributor
timestamp: November 02, 2022, 08:56:48 pm
content: Quote from: wraper on November 02, 2022, 07:30:30 pmThere's no doubt that Kemet still sells metalized paper capacitors, but the question is when Schaffner stopped using them in mains filters.

author: factory
author_rank: Super Contributor
timestamp: May 18, 2025, 11:33:25 am
content: Quote from: matthuszagh on September 09, 2022, 04:03:11 pmWell I've opened a newer one, I bought an item with what seemed like intact cal seals from the late 90s and bought a replacement expecting to find a 30+ year old Schaffner inside, turns out the cal seals could be removed intact (because they stacked several) and the filter had already been changed at some point.It was however worth me changing as I found a bad crimp connection, where Solartron/Schlumberger had put a resistor & wire into the same crimp terminal, the wire fell out as it wasn't inserted correctly when crimped.Anyway I ended up with a 2003 vintage Schaffner filter left over and thought lets open it, this was much easier than the older one, as no potting compound was used, just had to cut the glue between the can & the plastic housing and desolder the ground connection, inside are the ceramic & plastic film capacitors described in the reply from Schaffner in reply #16. This could be easily be put back together & reused too.David

author: factory
author_rank: Super Contributor
timestamp: August 30, 2025, 12:41:45 pm
content: Schaffner madness denied again, this one is a 1994 vintage FN9260, capacitors are 2 ceramics and an ERO F1772 MKT (93 datecode), no paper based crap here. It will be reassembled & refitted to the Ultrawave cleaner it came from.David