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I pulled this out of an old IBM 5154 EGA monitor from the 80's. Clearly both are bad (though the monitor itself works anyway...). I know they're capacitors, but what kind? What would a replacement for these be?

capacitor

I'm also curious about why the monitor would work despite two out of two of these things being in such terrible shape.

Edit: here is a picture of the top

enter image description here

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What you have there are metallized impregnated paper capacitors made by KEMET.

Here is a datasheet of the parts.

The most important thing you have to look for is the capacity rating which should be on the top of the capacitors, which we can't see in your picture (see page 9 of the datasheet).


With the added information, I'd say this is your replacement part: PME271MD6100MR30

This is a X1 rated capacitor, but X1 > X2 in terms of safety, so it should be fine. Actually - scrap that, Mouser links to the wrong datasheet.

Make sure to measure the distance between the legs, this one has 22.5mm which seemed to be the spacing on the old ones based on the information of the datasheet, but better measure that.


For a spacing of 20.3 mm, the PME271M610MR30 sounds right.

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  • \$\begingroup\$ I added a picture of the top. It says 0.1uF X2 \$\endgroup\$ – Sydius Jun 30 '18 at 23:08
  • \$\begingroup\$ @Sydius based on that I proposed a replacement part. \$\endgroup\$ – Arsenal Jun 30 '18 at 23:18
  • \$\begingroup\$ Thanks! The spacing on the pins is ~20mm. There are holes in the PCB for 15mm and 20mm, though. \$\endgroup\$ – Sydius Jun 30 '18 at 23:22
  • \$\begingroup\$ @Sydius so it's probably the second part I linked. Those are actually X2 rated capacitors, mouser links to the PME271E series and not PME271M series... Never trust distributors... \$\endgroup\$ – Arsenal Jun 30 '18 at 23:28
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    \$\begingroup\$ Oddly enough, the paper-epoxy dielectric has been sold as the premium material because it self-heals better than polyproplyene. However, the epoxy seems to go through an aging process that result in failure. The Rifa series has gotten the worse reputation for delayed failures, but that might be only because it had exceptional popularity. There is some suggestion that other paper-epoxy brands might have this problem. \$\endgroup\$ – Robert Endl Jul 20 '18 at 12:36
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It's a line filter capacitor, an X2. That series has a reputation for failure after a number of years and you can find other pictures like yours on the internet. The dielectric is paper soaked with epoxy and the failure is probably due to the epoxy ageing and cracking. Because they always fail open (I think) the monitor will still work. I think that part is still being made. If not, there are lots of equivalent replacements. Look for 0.1 uF, type X2 with the same lead spacing.

Better yet, replace it with an X1 if one will fit.

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  • \$\begingroup\$ I added a picture of the top. It says 0.1uF X2 \$\endgroup\$ – Sydius Jun 30 '18 at 23:09
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They are bidirection line noise suppression caps are designated as X caps across line and Y caps to ground.

It must attenuate (filter) lightning transients from outside and SMPS noise going out.

enter image description here

These are the typical values you can copy but must be X caps line rated on left (C1, C2) or Y cap rated on right (C3, C4).

Look under Plastic Film Caps.

Most likely 470nF >=3kV rated X caps

Blimey, you must have had a lightning surge greater than the rating. Dont leave it plugged in during a storm and replace with better quality caps.

Are you near Florida?

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  • \$\begingroup\$ Thats not lightning, it is the classic failure mode of those parts and is basically an age thing. The RIFA version of that part has a nasty reputation for failing resistive and stinking the place up as it slowly cooks, takes ages for the smell to clear out. If you ever buy 80s vintage equipment, have it open and have 'em out and replaced BEFORE applying power (I have a HP VNA that still smells slightly). \$\endgroup\$ – Dan Mills Jan 17 at 14:58
  • \$\begingroup\$ @DanMills Fascinating how they specify the liability of this insulation test. ( guaranteed for one test only and they did it. "The 100% screening factory test is carried out at 2,150 VDC. The voltage level is selected to meet the requirements in applicable equipment standards. All electrical characteristics are checked after the test. It is not permitted to repeat this test as there is a risk to damage the capacitor. KEMET is not liable in such case for any failures." \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 17 at 15:07
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They look like mains filter capacitors, and if that is the case there is probably a designation on them somewhere like "X3", which is a safety rating. It is important that they be replaced with one of the same or better safety and voltage rating. The monitor would indeed still work, but its rejection of mains-born transients and noise would be compromised.

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  • \$\begingroup\$ I added a picture of the top. It says 0.1uF X2 \$\endgroup\$ – Sydius Jun 30 '18 at 23:08
  • \$\begingroup\$ I think there is a specification for an X3 but I have never seen one. \$\endgroup\$ – Robert Endl Jul 1 '18 at 2:03
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KEMET PME271M, Metallized Impregnated Paper

The PME271M is constructed of multilayer metallized paper encapsulated and impregnated in self-extinguishing material meeting the requirements of UL 94 V–0.

Here’s the datasheet

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RIFA PME271m 40/085/56 Looks like it's availabe on ebay and mouser, however, do note that there are no markings visible indicating the capacity, you need to figure that out somehow. Id's suggest taking a look at the board it came out of, if you can determine how it is marked on the board, you should be able to find out exactly what model it is.

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  • \$\begingroup\$ I added a picture of the top. It says 0.1uF X2 \$\endgroup\$ – Sydius Jun 30 '18 at 23:09
  • \$\begingroup\$ Not sure I would buy "New, old stock" of these from ebay, and not just because buying safety critical parts and ebay is not a happy mix. The things are known to have age related issues (the cases crack then water gets absorbed, next power on, bad things happen). \$\endgroup\$ – Dan Mills Jan 17 at 17:59
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I searched up "PME271M"(which was a marking on the capacitors shown in the photo) in google and found that it is a "metallized paper capacitor". Here is the datasheet. You can buy replacements here. As for your second question, you can conclude that the functioning of these components is non-crucial to the functioning of the monitor. Most likely it is for filtering some signal in the monitor, though I am not completely sure.

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  • \$\begingroup\$ given their position ans the X2 rating the connect directly across the mains before and after the common mode choke. \$\endgroup\$ – Jasen Jul 1 '18 at 5:55

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