PSU line filter failures...common?

[genie_mac]

Hi,

I thought I'd post this here as it isn't really machine specific (if there's a better place for it, mods, fell free to move it).

Well I've had three line filter caps (EMI suppressors) fail in spectacular fashion (including that lovely lingering smell) in the last few weeks. These were all PSU's from late 80's to early 90's and hadn't been used in a long time (i.e. years). I think the caps are the metallised paper type, so I was wondering if this kind of thing has happened to others. Are these getting to an age where the dielectric has totally dried up and they are shortening out?

I managed to find a replacement for two of them (your typical modern X2 metallised film cap) but on one of them (IIcx PSU) I couldn't read the capacitance as it was totally destroyed. It just seems strange to me why different PSU's would have such different size caps when they all essentially have quite similar power ratings and are trying to filter similar noise frequencies between the lines. So how critical is the capacitance value here?

Any thoughts?

 

[genie_mac]

So would anyone know the value of the Rifa cap that's connected across the lines (live and neutral) straight after the fuse in an IIci / IIcx psu?

 

[mcdermd]

I guess I should have slipped a PSU into that box I sent over the pond.

 

[genie_mac]

Ha too late now But seriously, the IIcx psu only blew up two days ago, otherwise I would have asked you. I got the other two fixed, so not too bad.

I'm just hoping that Plus is not crossing the big pond on a 787, more electrical problems than even the oldest Mac

 

[James1095]

The value is not terribly critical, but it's normally selected based on the switching frequency of the power supply. All it does is prevent electrical noise from leaking out of the power supply back into the wiring and interfering with radio reception. You can run the computer without it on a temporary basis, but it's best to replace it with something when possible.

I've seen a number of them fail. Usually smokey and smelly, but the X2 caps are rated as such in that they are highly resistant to actually catching fire.

 

[genie_mac]

Thanks James1095, yeah was running it without the X2 for a while for testing purposes. I guess it did fail safely; it didn't catch fire and could have gone either open (no filtering) or short circuit (blown fuse) if I hadn't pulled the cord within two seconds.

Btw value of the cap is 0.47uF, did a bit of detective work after I removed it

 

[CelGen]

The line filter caps have been the most recent ones to start failing. Thankfully they are a lot easier to replace and have modern substitutes (polyester).

 

[techknight]

Yup, they just form the common mode chokes/Filters to stop back-EMF from getting into the power line, AND stop power line EMF from getting into the power supply itself, and possibly affecting its stability/regulation, even though with todays SMPS controllers is virtually impossible. But back then, hard to say..