I work at a National Laboratory, and was asked to put together some special instrumentation which needed three PMTs (photomultiplier tubes). This little side project had zero budget, so I proceeded to gather the necessary parts from the plentiful store of used equipment we have available.
The PMTs are exquisitely sensitive light detectors, but each one runs at a slightly different optimum value of high voltage. Since these were used tubes, it was necessary to "characterize" them, running a plot of their output vs. high voltage input to determine the best operating point.
I set up a rack of test gear and proceeded to characterize each tube, powering them with a small high-voltage supply that had recently been taken out of service from another instrument setup that was no longer needed. The supply had a rather small analog meter for reading the output, too small to trust for reading the values that were going into my spreadsheet. It did have a rotary range selector and three digit turns counting dial, however, and experience had shown that these worked well. The tubes all responded reasonably within expectations, and soon I had the response curves I needed.
I obtained two more high-voltage supplies and set up all the tubes, carefully setting each supply to the optimum operating point. To my surprise, only one tube was reading back: the one connected to the original high-voltage supply. I swapped cables a few times and found that all three tubes did indeed still work, but only when connected to the first supply. I measured the output of both of the other supplies and found that they were spot on, exactly the values requested by the range selector and turns counting knob. The coaxial high-voltage connectors I was using made it impossible to directly measure the output voltages when connected to the tubes, but the supplies should have had ample current capability to drive these loads.
After multiple checks to make certain I was seeing what I thought I was seeing, I decided to step away and come back to it later. I was thinking that perhaps I could find a coaxial "T" that would allow me to measure the supplies under load.
The following morning, a thought occurred to me. The only supply I hadn't measured was the one that was working! I fired up the rig when I got to work and found that, indeed, that first supply had a problem which caused it to deliver about 60 percent more voltage than was being requested. The operating points I had measured were actually way too low, and the voltages the other two supplies were providing were insufficient for the tubes to operate.
So the supplies that were "not working" were fine, and the one that was "working" was broken!
The main confounding factor was not having data sheets for the tubes, which meant their approximate operating voltage was unknown. Also, the misbehaving power supply had recently been in service but was not calibrated there. It was simply run up until the system began working without noting the knob settings. Hindsight is always 20/20.
This entry was submitted by Todd Johnson and edited by Rob Spiegel.
Todd Johnson worked at Fermilab for 29 years, first as an accelerator operator and then as the operations specialist for the recently retired Tevatron.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
Another fine Sherlock Ohms story where the answer to the dilemma can only be solved when the on-the-scene Sherlock drops all assumptions. Perhaps that's the most important quality of a good Sherlock -- that nothing is assumed.
That reminds me a bit of the first three or four steps in troubleshooting electrical appliances, like my stereo system: 1) Is the power cord plugged in? 2) Is the main unit switched on? 3) If so, is the cord frayed? If the first two are true and the third is not, then you start suspecting incorrect connections among components, and if those are OK, you may have a faulty stereo component. But this Sherlock almost sounds like one of these first questions could not even be asked, or at least not correctly answered.
This is another of those stories where it appears more obvious in retrospect. As I've said in response to previous Sherlock Ohms stories, I'm amazed by the tenacity of our readers. I never would have thought to check the "working" supply.
This is an example of what happens when we assume that an item that we pull from "stock" is OK, meaning that it meets specifications. It also points out the need to check calibration stickers, at least when accuracy is required. I wonder if the supply in the story had a calibration sticker, and if so, how far out of the calibration time it was.
But it is an interesting tale, and I have come across other instances where the assumption that some piece of equipment was functioning correctly was wrong. The result is that items not "brand new" now always get checked prior to use. This often helps preserve sanity and avoid a lot of frustration.
I'm definitely with you on the assumptions causing problems!
I once installed a sound system in a local church. A few weeks later I got a call that they were having trouble with noise. I fairly quickly narrowed the problem to a small pre-amp I'd had to add to get the microphones up to the desired volume. Since it was a "zero budget" job and the preamp was simple I decided to repair rather than replace. As I remember, the whole amp consisted of two transistors and a few passive components, powered by a 9 volt battery.I had no test equipment with me, but did have a pretty good assortment of parts, so my troubleshooting approach was to just replace the first transistor. No change! Nor did replacing the second transistor!
Puzzled, I started trying to isolate the problem by shorting signal to ground with a spare electrolytic cap. Shorting the input had no effect. Shorting the output of the first stage reduced the noise, but did not kill it completely. Jumping around, I found that putting the cap across the power supply (the 9 Volt battery) killed the noise! The cure was to replace the battery, and for good measure add a large electrolytic across the battery.
Never before or since have I found a noisy battery, but that one taught me not to assume that a battery is always a clean power source!
This is a good story, OldGeek. I'd like to use it as a Sherlock Ohms posting. It would need to be a tad longer, but I'm sure you could flesh that out with a bit more detail.
If you're interested in the fame that comes with Sherlock Ohms let me know at rob.spiegel@ubm.com
Also, we would like to include a brief bio of two to four sentences.
I think every engineering student should hear a version of the following fable -
A grouchy king imprisoned 3 subjects who displeased him: a mathematician, an engineer and a commoner. He told them of his anger and said if they could not free themselves by daybreak tomorrow, they would all be executed.
The mathematician looked at the cell door lock and began to compute the number of possible variations, trial time for trying all variations in order to work out a survival strategy with his cellmates.
The engineer looked around the cell and began to inventory the possible materials he could use for a MacGuyver door key.
The commoner walked over to the door and tried the handle. It was unlocked!
Sort of like checking that the AC cord is plugged in. Verify your problem before you try to find a solution.
My Dad, an ME PE and experienced hyralic machine designer and company owner contracted to buy for his customer several zink injection casting machines at auction. All the machines were inspected working before he offered the customers winning bid. All were delivered during my Christmas break from college, so I asked Dad if he needed some help finishing the job he had begun earlier in the week. Since it was a union shop and all their employees were given the Saturday before Christmas off he said yes.
Arriving early Saturday at the install site we both went to work on seperate machines. Lunch time came as I compleated my last assigned machine so I asked Dad if he wanted some help on the machine he was working on for the last hour. Saying yes, I asked the problem he was having. He remarked that "the machine was working yesterday when he left the plant and he could not understand why it didn't work now!"
As he was just about finishing assembly of the hydraulic pump for the third time I waited til he finished and asked him to turn it on. He did and I said turn it off an back on after the motor stops. He did and I told him go have a soda and I'l fix it. I VERY cautiously chuckled while he had his Coke and I rewired the 3 phase feed to the machine.
After a call to the plant manager we were told the assigned union electrition was told not to report on Monday since the owner's in house people would do tne clean up. And, by the way, those motor casting rotation arrows are there for a purpose.
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