I have a novelty bill collector/coin changer in my basement bar. Yesterday it stopped taking bills so I opened it up and saw that the current limiting resistor in series with the mains input burned up. At first I thought maybe the resistor was just underspec'ed (at 2W) and gradually burned up over time, so I replaced it with a 4 W--Still burned up almost immediately. Then I thought maybe the diode across the mains was shorted or leaky, so I replaced that, still the 220R burns up. My only other (layman) thought as to why that resistor would burn up is if the LED in the opto were shorted. NOTHING else in that area shows signs of burning but that resistor. Are there any other conditions that would make that resistor burn up (I measured the mains voltage at 118VAC)? I've never actually seen a regulator like this that uses an optocoupler to regulate mains input...very strange...
http://www.fairchildsemi.com/ds/4N/4N26.pdf
http://www.fairchildsemi.com/ds/4N/4N26.pdf