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Sherlock Ohms

Bad Parts Plague Oscilloscope

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Larry M
User Rank
Platinum
Re: Root Cause
Larry M   4/30/2013 9:30:02 AM
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:
  • 120 Vac: 34 cents
  • 240 Vac: 36 cents
  • 480 Vac: 39 cents

Guess which one I bought.

kf2qd
User Rank
Platinum
QC May have been fine...
kf2qd   4/30/2013 9:48:16 AM
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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...

SherpaDoug
User Rank
Gold
Re: Quite a maddening problem
SherpaDoug   4/30/2013 11:08:56 AM
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.

rrietz
User Rank
Iron
Re: Quite a maddening problem
rrietz   4/30/2013 12:49:19 PM
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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.

BrainiacV
User Rank
Platinum
Re: Quite a maddening problem
BrainiacV   4/30/2013 12:50:41 PM
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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.

Life has gotten less frustrating because of that.

Amclaussen
User Rank
Platinum
Re: QC May have been fine... But maybe NOT!
Amclaussen   4/30/2013 1:17:06 PM
 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.

Rob Spiegel
User Rank
Blogger
Re: Quite a maddening problem
Rob Spiegel   4/30/2013 8:09:17 PM
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Hey Naperlou, how long was the shelf life? Do components deteriorate on the shelf? What makes them deteriorate?

tekochip
User Rank
Platinum
Re: QC May have been fine... But maybe NOT!
tekochip   4/30/2013 9:10:34 PM
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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.


Nancy Golden
User Rank
Platinum
Re: Root Cause
Nancy Golden   5/1/2013 10:58:37 AM
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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).

Nancy Golden
User Rank
Platinum
Re: Quite a maddening problem
Nancy Golden   5/1/2013 11:11:49 AM
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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.

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