In the good old days of the 1960s -– when everyone was gainfully employed –- military equipment designers implemented amplifiers from discrete transistors, resistors, and capacitors from the approved list, rather than simply plugging in an op amp, as we would probably do today.
While prototyping a circuit for a Navy flight-line ECM tester, I encountered an unusual problem. An amplifier circuit that was too simple to fail was not working correctly. My prototyping technician reported that the stage couldn’t support the signal levels and was clipping badly.
I left my cube, sat in front of his bench, and checked the scope. Sure enough, the output was clipping. Yet there didn’t seem to be any obvious errors in the layout or connections. Nevertheless, the biasing of the transistor was way off. We prototyped using 1/4w 5 percent military RC05 resistors that were clearly marked. I checked the DC decoupling capacitor values and leakage, and there was nothing amiss. I pulled the transistor from the board, and it tested fine.
Then I measured the voltage at the base-bias divider network. It was very low, despite the divider consisting of 330k and 33k to give roughly 9 percent of the supply rail. Finally, I unsoldered the two resistors and measured them. The 33k was actually 330 ohms! Clearly, the 33k resistor had been mismarked orange-orange-orange by the manufacturer and shipped as 33k. In reality, it was 330 ohms. The third most significant orange band should have been brown.
So much for the minutia of the lesson learned many years ago. Yet it’s relevant today. When prototyping and when troubleshooting, we must never assume the components are the correct value, whether they be chip resistors, capacitors, or larger leaded components. Prototyping and test personnel should be instructed to measure each component before installing, as annoying and time-consuming as that may be. Each part should be verified to make sure it is correct per the parts list and schematic.
Then when problems arise, you will be certain that it is not a design issue, but rather a fabrication issue. It should be noted that wrong-value components apply equally to modern designs using op amps and chip components. Assume nothing is my mantra in troubleshooting, and it should be yours.
This entry was submitted by Michael Neidich and edited by Rob Spiegel.
Michael Neidich is a circuit and system designer in disciplines ranging from DC to Lightwaves, from discrete Germanium to GaAs, from ICs to VLSI, and from consumer to military equipment. He has worked at semiconductor firms such as Anadigics, Analog Devices and Zoran Corp., General Microwave Corp., and Sharp Electronics Corp.
Tell us your experience in solving a knotty engineering problem. Send to Rob Spiegel for Sherlock Ohms.
I believe that those of us who went to school in the analog days, when there were labs such as those of which you speak (old style scopes and voltmeters) acquired skills which can't be duplicated in today's digital world. Possibly a lot of the stuff we learned was subjective; the way I put it is, if you've never dealt with 60-cycle hum, how can you begin to understand more complex methods of troubleshooting?
When I was in engine school, some four decades ago, I went to Genral Motors Institute. GMI was well-funded by General Motors, and had a whole team of technicians who made sure that every piece of equipment in the lab was in tip-top shape.
I then transferred to a smaller, less-well-funded engineering school. We were taking a Motors course, and were making some unexpected discoveries, like energizing the field on a motor does not affect its running.
It turned out that this school did not have the large lab maintenance team that I was used to from GMI, and about a quarter of the patch cords we used to hook up the motors had opens in the wires.
Our standard routine soon became starting each lab session by grabbing a handful of patch cords and measuring them with an ohmmeter. After that, our motors started performing much more like how the books said they should.
Rob, I have written for Design News before, when Karen Field was the editor. So if those records are available you can learn a whole lot about me.
I can certainly expand that story, because those two detailers did indeed give me quite a few interesting experiences. In a similar manner, our technical writer created some interesting explanations on several occasions..
But it will probably wait until next week since this week has been filled up by others who made their requests earlier.
William Ketel (you will need my name to find out what I have written.
I'd like to use your story here for a Sherlock Ohms blog. It's a tad short, but it's a good story. Perhaps you could flesh it out a bit to get it up to about 300 words.
If you're interested, please let me know at: rob.spiegel@ubm.com
I could also use a short bio to include. Twoor three sentences would be fine.
Several years back I worked at a place where we had detailers to turn our sketches into drawings. One project that I had built up on proto board worked very well, and would be a large coast reduction which also improved reliability.
So after the prototype was optimized I gave the sketch to the detailers, who were a couple of young guys very impressed with themselves. The drawings were then passed to the techicians who wired up the first board, and attempted to calibrate it. After they were unable to get the circuit to function at all, I was called back into the production area to find the problem. I checked some voltages and they were way off. Then I looked at the drawing, and saw that the resistor values just would not work. Back into my office and got my sketch, and found that several of the lower valued resistors were way to high, 27K ohms instead of 27 OHMs. Then I realized that the detailers had assumed that I had forgotten to put the "K" om several of the resistors, and since they were so very smart, they fixed the dumb engineers mistake. I marked up the drawing for the technician and once the correct values were in place, the design worked well. Then I went and explained to the detailers that if they ever thought there was a mistake, they had to come to me and ask about it, rather than make changes themselves. I am not certain that they learned anything though.
I had something very similar happen when I (a ham and commercial licensee) was fixing CB sets for other members of the Rolling Meadows Civil Defense [ESDA] organization. A very common fault was the carbon resistor in the emittor or cathode of the audio output/modulator active device getting fried so that its value was about 10 to 100 times what it was supposed to be. In the process, the multiplier color band got fried so it looked like the correct one for the measured value of the resistor. In the case of one 5-watt transistor walkee-talkee the resistor was supposed to be 33 ohms [orange-orange-black] and it was fried to look like orange-orange-brown = 330 ohms. The symptom in both tube base stations and transistor walkies was low volume and much clipping. Some reasonablness checks and voltage measurements solved the problem. Always make voltage measurements and check for reasonable values.
I never liked SMT resistors. You can't read them and when you pull one off to measure as soon as a meter probe touches it It’s gone like a tiddlywink never to be seen again. Tape them down helps. My first time seeing a (or not seeing) a SMT resistor I ordered one from the material guy at work he handed me one in a clear bag. The next day I asked for another and mentioned the bag was empty. It was there.
Now the new color code resistors are very bad. Years ago I purchased my son one of those breadboard kits with springs where you can make a dozen things. I had to get out my DMM and measure the resistors browns violet and reds were especially hard to tell what was what. He quickly lost interest.
The surface mount components are probably the biggest ptoblem, both because you pretty much can't hand "wire" a prototype (or limited production) board and also because it's so difficult to read the numbers on them (it there's even room for numbers).
But today's leaded components - particularly axial leaded resistors - have their own problems, relating to the color codes. Some of the resistor manufacturers (sorry, China) seem to have a very odd conception of what the colors are supposed to be (or maybe it's the fault of a heat-curing process for the color bands). In any event, I have seen orange (and red) that looks like brown and other similar problems. Getting out the magnifier helps a bit, as does comparison with other resistors ("yes, that's probably orange, not brown"), but ultimately, one has to get out the ohmmeter. Better a little time spent up front, than time spent troubleshooting and removing and replacing components...
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