Rifa Again - Page 1
Forum section: electronics / repair

author: David Hess
author_rank: Super Contributor
timestamp: June 12, 2025, 01:11:16 pm
content: The Rifa capacitor in my 52 year old Husqvarna Viking 6030 sewing machine popped and filled the room with stinky smoke. Instead of any sort of electronic speed control, it has a universal motor and controls the field winding with a reostat in the foot petal, so there is no damage other than the Rifa capacitor.

author: the Chris
author_rank: Regular Contributor
timestamp: June 12, 2025, 03:04:28 pm
content: Just yesterday I spotted hairline cracks in RIFAs on a PSU that reminded me to take the time and go round and replace them in all of my equipment before something happens. I already had too much luck considering the amount of old instruments I operate.I created a list with all instruments and started to search for information what instruments are affected and what sizes are to be ordered so I can order all caps for all instruments at once. It made me think that a list created by the collective user base of his forum that contains such information about as much instruments as possible could be a very valuable source as everybody that is into older instruments will be affected by the plague at some point in the future. What do you think?Christian

author: David Hess
author_rank: Super Contributor
timestamp: June 12, 2025, 03:20:58 pm
content: Quote from: the Chris on June 12, 2025, 03:04:28 pmI think it will be an awfully long list, especially if you include 52 year old sewing machines. It never occurred to me that a sewing machine that lacks an active circuit for speed control would have a Rifa capacitor.

author: tggzzz
author_rank: Super Contributor
timestamp: June 12, 2025, 03:26:05 pm
content: Quote from: the Chris on June 12, 2025, 03:04:28 pmFrequently it is unnecessary to replace withexactlythe same capacitor, but the relevant X/Y class is mandatory.Occasionally it is necessary to replace with exactly the same RIFA capacitor, since they are packed in too tightly to allow anything else. Yes, HP, I'm looking at the HP8562B PSU. Buteverythingis crammed tightly into that spectrum analyser series - it is a mechanical marvel, with the biggest and most complex hinges I have ever seenhttps://www.eevblog.com/forum/testgear/hp-vs-tektronics-design-philosophies/msg3153112/#msg3153112

author: David Hess
author_rank: Super Contributor
timestamp: June 12, 2025, 06:10:59 pm
content: Quote from: tggzzz on June 12, 2025, 03:26:05 pmA Y capacitor can replace an X capacitor, however Y capacitors are typically found with lower values and be physically larger.An X capacitor is suppose to fail without causing a fire. A Y capacitor is suppose to fail without causing a fire, and fail open to prevent an electrical hazard.

author: David Hess
author_rank: Super Contributor
timestamp: June 12, 2025, 07:26:31 pm
content: I temporarily replaced the bad Rifa 0.1 microfarad capacitor with a 0.1 microfarad 600 volt DC Sprague Orange Drop from my salvaged parts supply. It runs fine without the capacitor, but I want to see if the Orange Drop survives.

author: Haenk
author_rank: Super Contributor
timestamp: June 12, 2025, 07:50:06 pm
content: The Rifa caps are likely very different from that Sprague - they are self healing (until they explode); and there is quite some noise from the motor on the cap, so I would call this Sprague cap "unsuited", maybe even "risky".

author: Gyro
author_rank: Super Contributor
timestamp: June 12, 2025, 08:00:22 pm
content: Quote from: David Hess on June 12, 2025, 07:26:31 pmA non X certified capacitor in an across-the-line application can make a failing Rifa look tame. The metallization on an orange drop capacitor will be significantly thicker than on a X cap.'Temporary' can very easily become more permanent. The bleed resistor across the cap suggests that it is positioned before the switch, meaning that it poses a fire risk whenever it is plugged in but not in use / unattended. Just don't do it.

author: David Hess
author_rank: Super Contributor
timestamp: June 12, 2025, 09:08:20 pm
content: This capacitor is neither across the motor nor across the line; it is across the foot pedal, although it does see the full AC line voltage. If it shorts, then the motor runs, which is what happened with the Rifa capacitor.While the service documentation for this 1973 Viking sewing machine shows electronic speed control, this one has no electronics; maybe the electronic speed control was optional? The foot pedal modulates the motor field somehow. The capacitor is there only for EMI reduction.

author: floobydust
author_rank: Super Contributor
timestamp: June 12, 2025, 09:38:28 pm
content: The Rifa's with metallized-paper dielectric... at work I'd seen some new ones sitting in a parts drawer, bulged and cracked. They are not dimensionally stable with age. As well, they are from an era before mains transients were quantified, and class X and Y defined.But what I find strange is the PME271's are still manufactured (Kemet)They've gotten widespread use in EMI filtering, some kind of marketing voodoo they have.I remember firing off an email to Kemet about WTF and they never responded. Enough burnt cooked pictures, class lawsuit material.

author: TERRA Operative
author_rank: Super Contributor
timestamp: June 13, 2025, 03:51:59 am
content: Quote from: David Hess on June 12, 2025, 07:26:31 pmDon't do this, just use the correct rated parts.

author: Gyro
author_rank: Super Contributor
timestamp: June 13, 2025, 08:55:14 am
content: Quote from: David Hess on June 12, 2025, 09:08:20 pmIf the capacitor is not directly across the line then it indeed does not need to be a Class X.X caps need a certain level of energy to be available in order to evaporate the thin metallization and self heal. Sometimes sufficient energy is available from the charge in capacitor itself but in cases of small capacitance and low mains voltage (US) it may need to draw a fault current from the mains to achieve self-healing. Clearly, in your case, the motor is sufficiently high impedance to prevent this from happening.Even though the capacitor does not need to be Class X rated, you should check the datasheet for flammability approval and breakdown voltage. The other issue is the danger (sudden fast moving needle, fingers etc) of the motor suddenly starting at full speed with no warning. Clearly having an X cap didn't do the job (I think that counts as a product design fail). You might find that a polypropylene film X cap might open more easily but there's no guarantee, or even, sensible way to test.This might be a case for substituting the highest value Y cap you can find and seeing if it still provides sufficient interference suppression. Failing that, go for the highest breakdown voltage cap you can find (which will almost certainly be Polypropylene rather than paper) that will fit in the space.

author: David Hess
author_rank: Super Contributor
timestamp: June 13, 2025, 07:45:13 pm
content: Quote from: Gyro on June 13, 2025, 08:55:14 amI plan on adding the proper capacitor to my next Mouser order. The better options were all polypropylene.Quote from: floobydust on June 12, 2025, 09:38:28 pmI noticed the same things; NOS Rifa's bulge and crack sitting in storage, and these specific capacitors are still sold. I assume they still make them because some customers refuse to qualify different parts.

author: Haenk
author_rank: Super Contributor
timestamp: June 13, 2025, 09:37:08 pm
content: Quote from: David Hess on June 13, 2025, 07:45:13 pmI assume the issue is the vintage plastic itself, which seemingly is unstable and eventually cracks and lets moisture in. Chemistry has come a long way since the 70s/80s, so maybe the material is better now...I don't think the paper/metal construction is per se bad. It's just the moisture+paper that let's them explode.

author: floobydust
author_rank: Super Contributor
timestamp: June 14, 2025, 01:24:56 am
content: Rifa PME 271 goes back to 1971. But some Australian AEE type same p/n 1977, not sure what that's about.So back then plastics were not so great, maybe that's why the paper dielectric was used.

author: Haenk
author_rank: Super Contributor
timestamp: June 14, 2025, 09:06:53 am
content: Quote from: floobydust on June 14, 2025, 01:24:56 amI can only assume that's due to the standard plastic foil being flammable, while the paper could be cheaply impregnated to become non-flammable. And maybe it's a matter of pricing, as Teflon foil would likely do the job, but is expensive, and I'm not sure, if foil even was available back then.

author: Gyro
author_rank: Super Contributor
timestamp: June 14, 2025, 12:17:24 pm
content: Quote from: Haenk on June 14, 2025, 09:06:53 amThe reason for paper is that it has a higher dV/dt withstand than Polypropylene (and other lesser plastics). I don't know about the dV/dt of teflon, probably very good, but it is too bulky and expensive to use. Unfortunately the moisture ingress over the years causes them to bulge (rather than the self-healing) and the epoxy encapsulation is rather brittle, making the problem worse.P.S. As an aside, all X caps,including plastic film ones can be made to bulge. I did an experiment a while back with my 10kV 200uA limited insulation tester in non-trip mode. You could hear them repeatedly tick as they self-healed and they slowly expanded.

author: floobydust
author_rank: Super Contributor
timestamp: June 14, 2025, 06:51:12 pm
content: Do you mean oil-filled paper? That is still a decent dielectric, common in substation power transformers, laser HV pulse caps.Only Kemet is making these metallized-paper X,Y caps, and they have SMT versions as wellSMP255so they've bet the farm on them.

author: Gyro
author_rank: Super Contributor
timestamp: June 14, 2025, 07:36:53 pm
content: I meant the PME 271. The dV/dt spec of the 271 is significantly higher than equivalent MKP X caps.I hadn't seen those smd ones. It makes sense, there are precious few smd reflow capable films. Let's just hope they have the encapsulation sorted.