As another fellow just said above: "in this era of counterfeit components..." One of the biggest problems with the counterfeit components is that there are many of them that will pass a quick test, but fail miserably well under the voltage or current that is specified for an authentic component. In the case of a common diode, one ampere, counterfeit ones will pass a simple DMM test, but WON'T tolerate even 400 mA for more than a fraction of a second, and Puff! This is a terrible problem, and the pirate industry is becoming too good in respect to appearance and finish, making it very difficult to identify the fake ones from a legitimate one, even with low power loupes... I am starting to see all kinds of fakes, from large power supply electrolytic capacitors to expensive power transistors. Those were actually made from cheaper dies (similar to old 2N3055) and put into new TO-3 cans, relabelled to look exactly like Motorola´s MJ15003. A simple test with a DMM will show those are OK, but will run at lower power levels and fail at less than half the power rating. Only detailed testing with curve tracers will show them to be fakes. Amclaussen.
That reminds me of a time I spent hours trying to interface a computer to a printer. The printer acted as though the CTS signal was not working. Using my breakout box I continuously tested each side, printer and computer, and the signals all seemed to be there.
Finally, in desparation, I tested the cable. And found the just-removed-from-packaging cable did not have all pins working.
That combined with another time I found a lot of power supplies not working, being traced to the outlet itself putting out 70 volts instead of 120 (one of the three phases had not been connected when they installed it), made it my rule to aways question my assumptions and always check the simple stuff.
3drob - One trick I learned over the years is to (loosely) tie a knot in the cable of defective items. It can easily be undone but is a flag that something is probably broken. Wire/cable cutters are useful for 'marking' a truely defective item you do not repair.
In an era of counterfeit components it is very important to persue the source of any suspected bad components. In my first case of counterfeit parts we got tubes of ICs with a good part at each end of the tube and bad parts in the middle. Grabbing a fresh tube solved the problem...briefly.
If you get a bad part out of stock you must check all the remaining stock and warn the purchasing agent.
Most of the time QC tests some representitve sample of a group of parts, unless that part is an extremely tight tolerance part. Those 2 diodes may have been the only bad ones in the batch, and just happened to be not in the selection set for QC to have tested. And he makes no mention of any more scopes with this problem. 2 diodes out of thoushands? Not bad, actually...
tekochip wrote: "Fortunately there were transistors in the same family with a higher breakdown voltage. I replaced the finals with the higher breakdown transistors and the amplifier served the band as a monitor for another 20 years."
I always replace semiconductors (and sometimes electrolytic capacitors) with the highest rating available in the same family. I figure that the designer was budget-conscious and used the lowest one that would work. Line voltage at the upper limit and/or high ambient temperature may have killed the one I'm replacing. Why not buy a little margin for a few cents?
It took me about six hours of trouble-shooting to find the defective TRIAC in my washing machine. Prices from Mouser for replacements were:
No, no, no. This problem was NOT solved. Attributing the two bad diodes to bad luck is unworthy of an engineer. ALL the diodes in stock should have been checked by QA and the problem reported so other engineers (who might be similarly plagued) can benefit.
I can speculate that the QA component testing could have been faulty, the diodes might have been mismarked, or perhaps the diodes were counterfeit. (I know, they are very cheap, but still it needs checking).
Debera, you made me laugh. More in frustration than humor though.
One of the Engineer's where I work would discover a bad probe or cable while working on some project. Instead of throwing it away he would just leave it on the bench for someone else to pick up. Or worse, throw it back in the bin with the other (good) probes! And he would never waste his time marking the probe or cable as bad. He had no intention of fixing it (but perhaps too cheap to toss something that might be fixed); just clueless on the cost of another Engineer's wasted time. AAAAACKKKK!
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