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Posted by David Sherman [72.47.9.228] on Saturday, January 29, 2011 at 14:57:29 :
In Reply to: Condenser question posted by Tim Holloway [69.54.28.229] on Saturday, January 29, 2011 at 08:57:31 :
This might be a bit of a stretch, but it could be that the old condenser had a developed a short inside it, which it cleared when it went "pop". If you take apart a modern condenser it simply has a small commercial capacitor such as is used on circuit boards stuck inside the oversized old-fashioned metal can. The original ones, though, filled the whole can with a roll made by laying out two strips of aluminum foil and two strips of paper, with one strip of foil hanging over one on edge of the paper and the other strip hanging over the other edge. The strips are rolled up tightly. The foil sticking out one end of the coil contacts the can and the foil sticking out the other end is attached to the lead wire. Sometimes the paper is soaked in oil or wax. On the old ones, the end is sealed imperfectly with wax or tar.
Over time, these "paper" condensers can develop shorts internally. Moisture can get in. The foil can corrode or the paper can rot or wear through. One advantage of these foil condensers is that if there's a short through a pinhole, quite often a small arc will vaporize the foil near the short and clear it. Perhaps that happened and that's the "pop" he heard. Or perhaps the condenser opened up entirely. The engine will generally run okay with no condenser. The condenser is there to keep the points from arcing and burning.
I repair old tube radios from time to time. Invariably the owner of the radio tells me "it probably just needs a tube". The reality is that it usually needs all the capacitors replaced ("Condenser" is just the automotive term for "capacitor"). Tubes do not usually go bad with time. Tubes wear out as they're used, but when they're cold, nothing deteriorates. Capacitors, on the other hand, contain oil that leaks out, paper that deteriorates, electrolytes that dry out, and so on. The radio that "had a great tone" before grandma last turned it off in 1970 is unlikely to work at all if you plug it in and turn it on today. In fact, if you just plug it in and turn it on, there's a good chance there will be a sizzle, bang, or smoke from some capacitor that's gone bad in the intervening years. I always power them up slowly with a variac to give the power supply capacitors a fighting chance. Most of the time, they all need to be replaced, though.