this is true of most parts, particularly about not necessarily blowing immediately when "abused". components often fail some time after some initial abuse or repeated abuse. this is very common in the case of voltage surges and lightning etc. i've repaired a lightning damaged answering machine, had more parts fail within hours, replaced those and had others fail the next day before i gave up and realized it had taken a big enough hit to marginalize most or all of the parts. i've seen this with computers as well when only part of it (say a modem port) seems to have failed at first and then more problems crop up weeks or months later. =20 it's also painfully easy to make the same mistake over and over when inserting parts, i've done this once or twice, in one case it took a month for me to figure out why all of the apd modules were "bad", when in fact i kept putting in a wrong resistor on 3 different days over a 2 week period, and i had built plenty of these boards before. it simply did not occur to me that i was using too small a resistor to terminate a line and that was why the signal was weak, no one else spotted it either, i was most embarrassed a month later when i figured it out, fortunately we had not thrown the "bad" units away and they were easy to fix (since the apd modules were about $300 each..). it's possible for a board to be marked incorrectly or confusingly as well. =20 when in doubt use brute force trouble shooting, i.e. use a meter and a scope and don't assume anything is "right" (and look at the connections on the board and the signal/power flow to make sure they make sense if you designed it). i've seen commercial boards with incorrect polarity markings before as well in consumer gear where they just put the part in correctly even though the marking was wrong so it's important to check before replacing parts that the board actually agrees with how things are inserted and/or that it is correct in the first place. =20 most equipment failures i've seen come from components that are slightly over stressed continuously or intermittently and just take years to finally fail. tv repair shops usually know which parts "always" fail on a given chassis, i did this for awhile and it is amazingly accurate.=20 certain parts just wind up being slightly over stressed because things weren't properly calculated or because there are "surge" or startup or abnormal but common operating conditions that weren't considered when the parts were speced out. i've seen slightly abused resistors that burned in half but still looked nearly normal and the ends would touch, most of the time creating an intermittent. parts do sometimes fail in atypical ways. i've seen a ceramic cap fail under very modest operating conditions with no obvious physical problem until i swapped it out in desperation (i was swapping all the parts of that stage at that point, if it made sense or not, and ceramic caps nearly never fail). sometimes brute force is necessary, particularly if it's something you just designed. sometimes other people won't even be able to find mistakes that will make you feel terribly silly when you do finally figure it out, don't feel bad, we're human, we make random mistakes sometimes, all of us. Herbert Graf wrote: ------- > FWIW electrolytic's can be both cranky and very forgiving at the same > time, and just because one blows at a certain time doesn't mean the > event that caused it to blow happened WHEN it blew. --------- --=20 =93Cowardice asks the question: is it safe? Expediency asks the question: is it politic? Vanity asks the question: is it popular? But conscience asks the question: is it right? And there comes a time when one must take a position that is neither safe, nor politic, nor popular- but one must take it simply because it is right.=94 : Martin Luther King Jr. 1929-1968 _______________________________________________ http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist