I have used over 1000 of these Russian K40-Y PIO coupling caps in the past 5 years. Not one (to my knowlege) has ever failed. These are military grade capacitors made to very strict tolerances. They were made by Russia during the cold war in the late '70's to early '80's.
I have often wondered what one looks like inside so today I took one apart for a closer look. They have plenty of "oil" on the inside impregnated into a heavy paper. I don't know what kind of oil it is but I did get some on my hands and found out that it did not wash off easily. The oil has no odor that I could detect. Inside the metal case is a thin cardboard that is used probably to insulate the inside of the cap from the outer metal case. Once this removed there appears to be a "jelly roll" of paper on the inside (the jelly) and a very thin sheet of aluminum foil on the outside (the roll). I cannot be sure that the metal is aluminum but it looks like aluminum. Whatever it is, it is non magnetic.
Last edited by Bob Latino on Tue Nov 30, 2010 5:14 pm; edited 4 times in total