- VCF South West - June 14 - 16, Davidson-Gundy Alumni Center at University of Texas at Dallas
- VCF West - Aug 2 - 3, Computer History Museum, Mountain View, CA
- VCF Midwest - Sept 7 - 8 2024, Schaumburg, IL
- VCF SoCal - Mid February 2025, Location TBD, Southern CA
- VCF East - April 2025, Infoage Museum, Wall NJ
-
Please review our updated Terms and Rules here
Power supply first
- Thread starter iz8dwf
- Start date
blackepyon
Veteran Member
Class X and Y line-filter capacitors tend to dry out on older machines, and they fail spectacularly. I'd replace them right away regardless of whether you smell the magic smoke or not, because it's not a matter of IF, only when.
I'm not too fond of tantalums in new designs, just because when they go, they fail short-circuit and often leave skid stains. Obviously, people like them because they're more compact, and when refurbishing an old machine, I'd still replace like with like, just to keep it true to the old design.
Japanese branded electrolytic capacitors (Panisonic, Nichicon, etc) are usually quite reliable. I've replaced oodles of electrolytics back when they had that huge run of cheap capacitors with the pirated formula, and motherboards were dying left and right. They fail open-circuit, and USUALLY don't leave skid-stains.
As far as skid-stains go however, it's a tossup between decrepit Ni-Cd electrolyte and cat pee. Both are quite destructive if left too long.
I'm not too fond of tantalums in new designs, just because when they go, they fail short-circuit and often leave skid stains. Obviously, people like them because they're more compact, and when refurbishing an old machine, I'd still replace like with like, just to keep it true to the old design.
Japanese branded electrolytic capacitors (Panisonic, Nichicon, etc) are usually quite reliable. I've replaced oodles of electrolytics back when they had that huge run of cheap capacitors with the pirated formula, and motherboards were dying left and right. They fail open-circuit, and USUALLY don't leave skid-stains.
As far as skid-stains go however, it's a tossup between decrepit Ni-Cd electrolyte and cat pee. Both are quite destructive if left too long.
Chuck(G)
25k Member
It's not a matter of drying out; often it's a case of moisture intrusion. Film caps (X and Y-rated safety caps are examples of these) have a rather more complex failure mechanism. But the failure rate exhibits the typical "bathtub" curve.
https://arxiv.org/pdf/1607.01540.pdf
blackepyon
Veteran Member
Does the insulation crack from age and let moisture in, or does moisture ingress and crack the insulation? I'd still replace them as a mater of course.
Interesting info there. Thanks. Gonna make a printout of that.
Chuck(G)
25k Member
I suspect it's the former a bit. It's uncommon to see a brand-NOS Rifa X-cap that's bad, so I'm guessing that it's a matter of self-heating, combined with the elevated temps in a PSU. I think that it's interesting that the audiophile community seems to like the Rifa non-safety caps and don't seem to have a problem with them.
blackepyon
Veteran Member
Apparently they get really uppity about replacing caps on their tube amps so long as they're still working. Gotta have that sound.
Back to topic.
Step, the First: Thou shalt always check voltages!
Hugo Holden
Veteran Member
This might require some consideration.
On point 1,` as Chuck points out, the class X & Y capacitors don't "dry out" (That is the province of the electrolytic capacitor). If you want you can leave the X and Y capacitors out. Mostly the amount of RF suppression is very limited due to the very low internal impedance of the mains(line) power as a source.They fail as many of the Rifa ones are metalized paper and when the casing cracks they absorb atmospheric H20 and expand. Or the metalization is eaten away by voltage spikes and they lose capacity.Or they can short out and smoke. They are really pretty hopeless compared to vintage but much larger oil filled capacitors or even good high voltage poly caps, but they do not have the official fire rating.
On point 2, You really don't know how lucky you are to have the Tantalum capacitor. It is true that early examples have a penchant for shorting out, but modern ones are a lot better. Infinitely better than surface mount electrolytic capacitors which spill their electrolyte all over a pcb creating multiple faults and destroying the components such as the films on surface mount resistors and the fine copper tracks on difficult to get at areas under IC's. A tantalum cap does not do that very often.
blackepyon
Veteran Member
I'll have to take your word for it. Unlike many on this forum, I do electronics and computers as a hobby rather than as a full time profession, so there are gaps in my knowledge where my experience runs out. That said, I appreciate the corrections ('cause then I don't have to be wrong any more).
Hugo Holden
Veteran Member
Actually apart from the issues with the X2 capacitor, Rifa do make really great capacitors in general. They did a range of high temperature electrolytic caps with special seals and rated to 125 deg C, the type of thing used in under bonnet automotive electronics and those capacitors still remain the best electrolytics I have seen.