Yes, you read that title correctly. …and no, I have not been in the sauce yet today. A mechanical oscilloscope. Just like spinning disk scanning television once existed, so too, did mechanized oscilloscopes.
An oscilloscope is a truly amazing device. Those who suspect they’ve never seen one probably have–in the form of a heart monitor in the movies. An oscilloscope is a time dependent voltmeter. It displays the change of voltage over a period of time. Imagine taking a repetitive voltage measurement at a predetermined time interval and then graphing the results on a piece of paper. Now imagine a device that can do it in real time and display it on a screen as a graphical trace. That’s it. That’s what it does.
Such scopes, regardless of type, require a time base to repetitively sweep the trace across the screen at the desired rate and a means to deflect the trace upward or downward in response to the applied voltages. A positive voltage will push the trace above the centerline, a negative one below it. A circuit called a vertical amplifier allows you to scale the input to a level that allows for easily visualizing it on the graphical screen. So the two most basic controls are for the sweep rate, often expressed as the number of seconds per screen grid division along the X-axis…and the amplitude of the display, often expressed as volts per screen grid division along the Y-axis. When everything is properly calibrated, you can in fact measure right from the screen and be able to tell the exact values. Obviously modern (and many not-so-modern) scopes have all sorts of other functions to enable specialized triggering or other attributes needed to capture and display a pulse or wave in a manner that is suitable for the task at hand. Some have multiple time bases and multiple input channels to allow more than one waveform to be observed at the same time or one in response to another, and some also allow for computations between the waveforms to be made.
Apply a smooth alternating current from a conventional generator and the result will look like a sine wave. This is because the rotating windings in the generator build current in one direction as they approach the magnetic field pole and then it levels off and decreases, only to build in the opposite direction as the other magnetic pole is approached in the machine. The speed of rotation will dictate the frequency of the wave. The strength of the magnetic field will dictate the amplitude of the wave. And you thought 8th grade math class was useless!
There. Oscilloscope 101 condensed into less than 440 words.
Obviously modern computers can provide the display, although many of us prefer the real CRT type due to the persistence of the fading glow of the trace as it goes by. A modern digital phosphor oscilloscope can do pretty good job of simulating this, but at a cost that is beyond the budget of most of us. I have several old scopes. My first was a Tektronix 551, a massive lab grade scope that had a roll-around cart and separate power supply. It was a tank! It also had TWO beams…..not two channels….but two completely separate electron guns in the CRT that could be controlled independently…..with swappable vertical preamps for each beam, I could put in multichannel units if so desired–I once had 8 traces going on it at one time. I had gotten this one when I was about 13 years old…bought from my friend’s dad…….it cost me 50 bucks! It would have been a bargain at twice the price as they say. I no longer have that scope, but still have a special place in my heart for Tektronix scopes of that era and I presently have 3 or 4 of them. They were the best of the best in their day. Beautiful workmanship. A couple of them still get used for real work too!
Now the neon mechanical scope…
Prior to the widespread use of CRT tubes, way before television. Other methods were tried to perform a similar function. One such device was the “Tattelite” …a neon tube based display intended to stand in for a CRT in an early oscilloscope application. CRTs existed in 1935, but they were not cheap…and the driving circuits were complex. For a simple project like monitoring a basic power or modulation pattern the “Tattelite” was a viable option. And it was something that could be put together at home and at low cost. The special tube needed was only a couple of bucks at the time. The October, 1935 issue of QST (the ARRL’s monthly publication) detailed the basic construction process for the neon oscilloscope.
Neon Oscilloscope instructions
The tube used is similar to the familiar neon lamp that many an indicator or night light was made from. The difference is the electrodes are in a line rather than parallel. If a DC current is applied, only one electrode will glow…when the polarity is reversed, the other will glow……furthermore, as the applied voltage is increased–a greater portion of the gas by the electrode will glow.
If you were to connect this to an alternating current source, the appearance would be two glowing areas at each electrode. To the naked eye, this would appear constant. Just like your night light. Now if you could effectively increase your visual shutter speed so to speak, you could catch the increasing amount of glow…followed by the decrease…and then the increase upon the other electrode and so forth…..all within the 60Hz line frequency applied to it. That is where the spinning mirror comes into play. A mirror, mounted atop a shaft parallel but slightly offset from the tube axis allows for a visual effect that resembles a screen with the orange glow rising and falling as it appears to sweep across the field of view. Just like a common modern oscilloscope!
The unit described in the 1935 QST article was intended primarily for monitoring the modulation levels of an AM transmitter. It would do an admirable job of showing over or under modulation, an imbalance, as well as general envelope shape. The construction details are fairly straight forward and in the article such common everyday parts as a simple electric fan motor, or even the motor from a klaxon horn off of a car can be used.
Shortly after the article, a company began to mass-produce the mirror and tubes for sale so that others could build their own unit. Actually “mass-produced” is likely not really accurate. Although I have heard of this technology being used in the film industry to check for sound defects in their audio systems, I have never seen a commercial unit of the sort. In all my travels and research, I have only seen one of these in person. And it is right here. I obtained it from a fellow collector and decided it was a worthy project to complete and get running.
This example had apparently been started with the intention of being able to measure audio as well as RF signals. Possibly it was completed at one time and then robbed of parts….or perhaps it was just never finished. Finding that the tube was functional saved me the step of making a replica for it in my shop. The fan motor appears to be from a Polar Cub or similar style of fan. Controls and audio transformer appear to be common mid to late 1930’s parts. Because I was anxious to see how this thing would work, I somewhat hastily connected a power source through a Variac and current limiting resistor for the tube and plugged in the power for the motorized mirror. Adjusting the voltage applied and the mirror speed allows viewing of the 60Hz line frequency waveform as shown in the photos and youtube video. WOW! Now that is just cool!
Obviously a proper housing and a darkened room are needed to properly use this, and more careful speed control of the motor is critically important. I may make a better video later, but even so it was really neat to be able to capture the quick video to share here.