This is quite a difficult problem. We've seen this before in a Gadget Freak. A component is identified as faulty. It's replaced and that solves the problem. Never in a million years would guess that the new component was also faulty. Maddening.
I agree, Rob - maddening. The only good thing to come out of that type of situation is the realization that it is actually a possibility, no matter how remote. I've been there, done that, got that t-shirt of spending a lot of time troubleshooting a board after replacing a component - only to find the replacement component was also bad. But afterwards I never wasted half a day again before verifying that the replacement component was good...that falls in the same bucket as the time I spent thirty minutes troubleshooting a circuit without verifying first that the power supply was working to the board...oops, did I admit that online?!!!! Of course that happened when dinosaurs still roamed the earth, but both incidents have stayed with me to this day...
Yes, Nancy, I can see how this would become a one-time problem. Once you've discovered that the replacement part for a faulty part may also be faulty, it's bound to stay with you from then on. This is likely a one-time incident.
This is a lesson for anyone who fixes anything -- all the way to replacing a light bulb. We always assume that if we replace a part and it still doesn't work, there must be another cause. I suppose the lesson is that where's there's smoke, there's fire. If one part is bad, there's a chance that other parts in the same lot could be bad, too.
Yes one should analyse the actual problem other than just solving it and becomming happy as in we have done a lot of work .I remembered once when i was in university and was making one of my unmanned ground vehicle after a particular circuitry was done i tested the circuit and it wasnt running i digged out that the problem was with one of my IC which got burnt i removed the IC and replaced it again the circuit was not working after testing same issue was comming with the circuit i again removed the IC and replaced and thrice circuit wasnt running after 3 hours i came to know that i was attaching the Same burnt IC again and again thats why my circuit was not running and my time got wasted .It was a very bad plus a funny experience too .
Moral of the story is always throhe burned componenets instead of using them again and again.
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!
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.
Rob, the problem of component quality and test is one that has been around for a long time. It is a numbers game. Many years ago, my father was in charge of component procurement at a government electronics lab. Parts had a shelf life, and after that they were discarded. Employees could pick them out of the bin, but they could not be sold or given intact. Nedless to say, I got a lot of them to "play" with implementing and designing circuits. Of course there were bad components, but it was not a large percentage. Since I was just doing this to learn and for hobbist purposes, it had little effect. On the other hand, the engineers who depended on them would waste lots of valuable time if they had use them.
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).
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.
You make a great point, Eric - especially for a large company like Tektronix that has the resources to fully address these types of issues - they probably have a large stock of those diodes and they certainly have a QC department that can handle the testing. It may be faulty parts coming from a certain supplier and taking the time to find the root cause could possibly prevent not only QC failures at the factory but operating failures in the field, depending on the mortality rate of the part in question. In the case of the scopes - it is very frustrating when using a piece of test equipment and having it fail while testing or troubleshooting.
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.
BrainiacV -- from these comments, I get the feeling that this problem is not uncommon at all. I guess troubleshooting should come without any assumptions, including the assumption that the parts -- even new ones -- are not faulty.
I would worry about what was shorting the diodes. Parts don't usually fail without some sort of mechanism, especially a diode shorting. If a replacement blew right away I would be suspicious that something was making it blow.
I had an audio power amplifier that lost its finals. You can always assume there was some sort of abuse at the output, so I simply replaced the transistors. Everything worked well, but then I cranked the output to clipping with the output lightly loaded, the output transistors instantly shorted. I did a little digging and found that the VCE breakdown on the transistors was being exceeded when the amplifier wasn't loaded. As long as there was a load the brute force power supply would sag enough to prevent breakdown, but in absence of a load, the higher voltage swing would cause the transistors to exceed their breakdown. 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.
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:
Another variable to consider when seeking a root cause, Tekochip, is the source of your parts. I was building a microcontroller based wind rose for a project in college and I had an LED display that would light 4 different colors of LEDs to indicate 4 different ranges of wind speed, with the LEDS trailing within four circles to indicate wind direction. So...I had four circles of LEDs, each a different color. The smallest circle was yellow with a 0-5 mph range, the next circle was green...hopefully you get the idea. the outer circle was red and indicating over 20 mph. I etched my own circuit board, carefully drilled the holes - it was an intensive labor of love. I went to the local electronics store that all of us students bought our supplies at. After painstakingly soldering in all of my LEDs and beginning running tests - there was a huge variance in the color of my red LEDs - some showed up a very orangy-red which was very distracting to the balance of the display. I desoldered the ones that were orange and returned to the electronics store which often bought "surplus" parts. This time I brought a battery and tested the LEDs right at the shelf until I found enough red LEDs that were truly red, to complete my project. This taught me that when you buy at a store that carries surplus or nonstandard stock, to be especially careful to test the parts you are using to make sure they are operating within spec (or in the case of the red LED, what you expect).
It might be heat from the soldering iron. I remember those ceramic terminal strips, and several components might be soldered into those silver-plated notches. It might require a lot of heat to overcome that much thermal mass to melt the solder and replace the diode.
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...
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.
Agreed, this is the type of fake I've seen. Maybe the die bonds are weak or the temp. specs. don't live up to the real part, but sitting on the bench in the lab they pass all the tests and make the purchasing guys happy.
Chiming in not as an expert but as an enthusiast of the classic 500-series Tek 'scopes: The Type 547 isn't a storage 'scope, perhaps the author meant the Type 564? I've owned both and still have the 564, which still stores a trace! Not bad for a 50-year-old instrument.
Some of the mor4e frustrating repair projects come from assuming that a new part is good because it is new. Out of tolerance capacitors and wrong marked resistors can lead to an extended diagnostic chase, that is certain. Mismarked transistors also, particularly when a PNP is found marked as an NPN. So for some things that need to be correct the first time they are switched on, validating the "ggodness" of every part winds up being a real time saver.
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