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RE: Wima MP3 caps

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In Reply to: RE: Wima MP3 caps posted by Rob F on April 30, 2010 at 19:54:12

Here's a copy of a post from another forum by Jonathan Carr which might help those (including me) who don't happen to own an o-scope in determining the outer foil orientation. Haven't tried this but I will soon.

"As long as the film cap value is not too big, you can use a precision AC millivoltmeter to check the polarity. Ground yourself, hold the cap body in your bare hand, hook up the millivoltmeter input to the cap leads (first with plus to one lead, minus to the other, then flip the leads) and look at how much the needle swings (you may need to switch the range setting on the millivoltmeter). The polarity that gives the lower needle swing is the "right" one.

Mark the film cap, and use it thereafter in the same orientation that you would use an electrolytic cap. Most film caps sound smoother and a little darker when the outer foil is connected to ground, or the signal output side (the side at the lower voltage potential).

In practice, if I have unmarked film caps that I plan to use, I will test them all in one sitting. It doesn't take that long."

regards, jonathan carr
___
http://www.lyraconnoisseur.com/, http://www.lyraaudio.com

Follow Ups:

Visual check with MP3 suppression caps. - Al Sekela 15:15:28 05/05/10 (2)
The Wima MP3 suppression caps are encapsulated in transparent epoxy. It is possible to see which lead connects to the outside of the core by inspecting with a good magnifying glass. There is some sort of bonding agent present around one of the leads, and I assume it is on the outside of the metallized paper winding.

These caps have the printing on a foil piece that covers, but does not connect to, the core. The AC voltmeter test gives identical results no matter how I hold and connect the leads, and I believe the foil piece acts as a shield.
- RE: Visual check with MP3 suppression caps. - Rob F 21:06:54 05/05/10 (0)
  They definitely measure differently depending on orientation.I do not use Jonathan Carr's technique though. I wind wire around the case and attach the caps to a variac, crank it to 50v and measure the voltage on the uninsulated wire for both orientations. It is a pain, but works.
- RE: Visual check with MP3 suppression caps. - rwiley 18:35:15 05/05/10 (0)
  Yes, I apologize any confusion I might have caused in this regard. I do not have any of the MP3's on hand and so assumed they "looked" like other Wima capacitors, having a vertical black line imprinted on the right hand side of the cap. I've always assumed this line indicated the outer foil. So...I don't know what to conclude regarding the correct orientation when using Wima caps.
  
  FIW...Jonathan Carr's method is a relatively easy and consistent way to identify the outer foil and seems to work well on all other caps I've tried.
  
  Bob
RE: Wima MP3 caps - rwiley 15:01:48 05/02/10 (0)
Jonathan Carr's method works (as I suspected). Just to clarify...in the orientation which produced the lowest meter reading, the outer foil is located on the side connected to the (-) lead of your VOM.

Bob

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