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Posted by David Sherman [24.32.202.83] on Monday, June 07, 2010 at 15:54:42 :
In Reply to: It's called "Infant Mortality" posted by Chriscase [76.224.243.231] on Monday, June 07, 2010 at 14:53:15 :
Yes, the failure rate of electronic components follows what's called a "bathtub curve" -- lots of failures initially, then a long period of few failures, and then an increasing rate of failures at the end of expected life. That's why space and military gear is "burned in" in order to get past the first part of the bathtub curve.
Have you ever disassembled a distributor condenser? What I've found is that inside the metal can is a small modern capacitor of the sort installed on printed circuit boards. The shape of the can is a relic of the days when the condenser was made out of rolled up oil-soaked paper and aluminum foil that filled the can, but nowadays the capacitors are a lot smaller and they just mount a standard tiny one inside the can. The typical capacitance is a few tenths of microfarads. My point is that they usually used the cheapest capacitor available, which probably only costs a few cents in quantity.
Instead of replacing the whole assembly, it might be better to use a good quality modern capacitor -- one that's resistant to high temperature, voltage spikes, and vibration, and either mount it inside the traditional can or simply hook it across the points without the can. I have not tried this, but if I had trouble with condensers repeatedly failing, that's what I'd do. I have quite a pile of military electronic gear in my bone-yard, and I'd probably scrounge a .1 or .22 microfarad hermetically sealed capacitor rated at 200V or better out of something. They have metal cans with glass end seals around the leads and probably cost the government an insane amount of money. They are absolutely moisture-proof, and glass and metal aren't going to deteriorate over time like plastic and wax. They are rated at the military temperature range, which is -55 to +125 degrees C, far more extreme than the commercial temperature range of 0 to +70 degrees C.
If I didn't have an electronic bone-yard and had to just go buy something, I'd get a 200 volt multi-layer ceramic cap and mount it in the old metal can with some stuffing around it so it didn't vibrate on it leads. The leads tend to crack off of ceramic caps if they vibrate. Polyethylene and polypropylene capacitors have better electrical characteristics than ceramics but they are not so good at high temperatures and a distributor doesn't need the advanced electrical characteristics.
Frankly, good capacitors are not expensive these days, and it's a shame that the "condenser" manufacturer's stick the cheapest one they can get in the little tin can and sell it at a 100X markup. For that matter, even if a mil-spec capacitor cost $10 versus 3 cents for the cheap ones that they use, I bet a lot of owners of old vehicles would be willing to pay for a top-quality unit that would eliminate the regular routine of putting in a new condenser just in case that was maybe the problem.
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