By Sheldon Greaves, Ph.D.
Photo by the author.
Author's Note: All pretensions of ignorance and general clueless in what follows are absolutely genuine and are for educational purposes only.
In my long and halting quest to learn more about electronics, working with an oscilloscope has always struck me as a kind of threshold moment. Although I don't know much about them, they have always fascinated me. As I've messed around with this or that small project, it became clearer and clear that at some point I would need one of these beasties, even though they seemed beastly complicated.
Long ago I made what I affectionately called a "hillbilly oscilloscope" that was really just a circuit that took an input and with help from a standoff transformer turned it into static or hums delivered through a pair of headphones. Remarkably, you can learn to tell a lot about what a circuit is doing from such simple clues. But this is to an oscilloscope what a stethoscope is to an MRI scanner.
Last year at one of the DeAnza Electronics Flea Markets in Cupertino, CA I saw many oscilloscopes for sale, most of them out of my very modest budget. But on the last market of the season, I saw a smallish oscilloscope hooked up to a generated and turned on, so I could see the trace. In other words, it more or less worked. It came with a manual, probes, and cover. They wanted $40, but since it was getting close to closing time, they let me have it for $30. The manual is dated 1971, so I assume the instrument is of similar vintage.
Sadly, I didn't have a place to set it up and use it until recently when I replaced the two flimsy pine shelf planks I've used as a "workbench" with a very nice school table I got at a garage sale for $5. When I moved it into position, almost the first thing to go on it was the oscilloscope. This evening I decided to play around with it and see what I could learn about it. For a while I sat looking at it a bit nervously, sort of like Indiana Jones trying to decide just how to swap the golden head for a bag of sand without everything coming apart. The manual offered some clues and a whole lot of jargon I didn't grasp. To make matters worse, based on the alternate spellings, I assume this is a British device, so the jargon might not match up to how we say things this side of the Pond.
I needed the manual just to find out how to power it up. The brilliance knob also doubles as the power switch. Thirty seconds after clicking it on, that magical, mostly horizontal trace appeared. I spent some time messing with some of the dials. "Trace rotation" tips the line to a mild slant up or down. "Focus" sharpens the line, as does a knurled wheel labeled "astig" which I assume has something to do with astigmatism--this is an analog device after all. There are two channels. Turning them both on shows two lines. There is a "position" knob that moves the trace up and down. Another knob controlled the rate of the pulsing trace across the screen, and still others (one for each channel) appeared tied to the voltage of the signal one intended to measure.
My hope was to take one of my crude crystal rigs and see if I could see the signal on the oscilloscope screen. I managed to reassemble one of them in time to get a really annoying talk radio program. Although it doesn't have a variable resistor (yet), it does have a tapped coil that tuned to a sports news show that was merely tedious.
Connecting one of the probes to channel 1 and turning it on, I played around with touching the probe to different parts of the radio to see if I could get a trace that seemed to reflect what I was hearing in the earphone. Playing with the input voltage changed the shape of the line, and sometimes caused the line to disappear. Touching the outside edge of the front panel also blanked the screen. At times I heard a loud hum as I was probing about. Part of it was from an overhead light. When I turned it off there was still a hum which I suspect came from the oscilloscope itself.
So no, I did not crack open the secrets of this intriguing device during the hour or so I spent playing with it. I plan to spend more time, and will take a run through the manual to see how much I can glean from it. People who know tell me that while I can't explore the phenomena discernible with a gigahertz capable device, and while 10 Mhz barely puts me into ham radio territory, there is still a lot one can learn from something like this. Ideally, I'd like to find someone who can look over my shoulder and show me some stuff in real time; I welcome any tips and suggestions from readers.
Oscilloscopes: Electronic test equipment used for observing wave shape, with clear, easy to read LCD screens. Available from Circuit Specialists, Inc. with unique features and functions compared to your average digital storage oscilloscope.