Commercial Cathoderay Oscillographs

We now are ready to consider the structure of the cathode-ray oscillographs offered for commercial use. Let it be known that it is our intention to discuss those devices which are being offered for use by the radio servicing industry for radio receiver design and servicing, public address design and servicing and to the amateur radio fraternity, for the adjustment of "ham" transmitters. There are in use a large number of cathode-ray units intended for the industrial field and college laboratories. In many respects these devices are like the ones to be described in the pages following, but since we are catering to a limited field, we deem it best to devote very little space to the application of the tube to fields and industries other than those named. At the same time, we recognize that many of the industrial applications are similar to those found in our fields, because the method of utilizing the cathode-ray tube is to convert mechanical and other physical impulses into electrical impulses and to interpret the cathode-ray tube image accordingly. As such we hope that some of the information contained in this volume will be of value to the men, who will employ the cathode-ray tube in the industrial field.

To analyze properly the structure of the commercial cathode-ray oscillographs available at this writing, for use in the fields named, we must of necessity classify the instruments in basic types. Perhaps others may not agree with our method of classification, but since no definite standard classification is available, we take the liberty of establishing our own.

1. The basic unit is the cathode-ray tube with its power supply.

2. An elaboration upon this basic unit, which forms class 2, the most popular is the cathode-ray tube with its power supply and to which has been added amplifiers for the vertical and horizontal deflection plates and a linear (saw-tooth oscillator) sweep circuit

These are the two basic divisions. Elaboration upon basic units 1 and 2, are accomplished in several ways, dependent upon the nature of the service involved. Basic unit 1 can be converted into basic unit 2, thereby greatly expanding upon its field of application, by the addition of the component circuits stated.

Elaboration upon basic unit 2, is also possible by the addition of some means, generally known as a frequency modulated multi-waveband oscillator covering the i-f. and broadcast band, sometimes the high frequency band, whereby a frequency modulated signal is available. (By frequency modulated signal is meant a constant voltage output which changes over a pre-determined frequency band, as a result of some action associated with the oscillator, or as a result of a separate unit intended to perform that function.) This is the unit intended for visual alignment work.

Investigation of the cathode-ray instrument market brings to light the fact that certain instruments intended solely for such visual alignment and not possessing controlled sweep circuits, are available. It is possible that this type of unit constitutes a third classification, but we feel that it is not justified. Recognizing the tremendous versatility of the cathode-ray tube as used in basic unit 2, the addition of equipment for alignment is just an expansion of its functions, particularly so when a unit, such as that stated as basic unit 2, can be expanded to include alignment by means of simple external equipment. Consequently, we openly state that it is preferable to expand basic unit 2 to include alignment than to expand a cathode-ray system, developed solely for alignment, to include the functions and capabilities of the tube equipped with power supply, amplifiers and linear sweep circuit. We make this statement with the full knowledge that it is possible that several visual alignment cathode-ray units, intended solely for such work may make their appearance upon the market. As such, they fill only one need. When expanded upon, as stated before, the ease of operation, so much required, is not achieved.

It is true that certain types of units of limited utility are suitable, if they fill certain needs—when it is definitely known that the needs considered are all which will ever be required. Thus, the basic unit 1 is suitable for the modulation percentage checking of transmitters, without any additional equipment. But as will be shown later in this volume, there is much more to the adjustment of a "ham" transmitter than merely determining the percentage of modulation, so that basic unit 2, is by far preferable.

No serviceman, after analyzing the servicing field, can say that the only function he can see for the cathode-ray tube is its use for visual alignment. With this thought in mind, we will embark upon the discussion of the various types of cathode-ray oscillograph units available in this country and the structure of each. It is not the purpose of this volume to compare the merits of each of the instruments described herein. All that we shall do is to discuss the structure of each. Decision to buy one or the other must come from the prospective purchaser. The only aid we can give is to describe the operating capabilities of the devices presented.

It might be well at this time to call to your attention something which you may or may not notice as you examine the wiring diagrams of the commercial units. The examples of spot displacement, cited in an earlier chapter, showed all four deflecting plates free and with separate connections. This is contrary to the electrical connections actually found in the usual run of three and five-inch tubes. As a general rule, one plate of each set is joined to the other and connected to anode number 2, so as to keep these two plates at ground potential. This gives rise to a slight amount of distortion of the pattern, in the form of a departure from a perfect square when equal amplitude voltages are applied to the two sets of plates. However, this amount of distortion in no way interferes with the normal operation of the tube when making tests of the type outlined in this volume.

National Union Model 3-5 Cathode-Ray Oscillograph

Various views of this unit are given in figure 54. The schematic wiring diagram is given in figure 55. Referring to the mechanical structure of the device, the unit will accommodate either a 3-inch or a 5-inch tube. The major variable operating controls are located upon the front of the panel and the switching controls, to be outlined, are located in the rear of the metal housing. Incidentally, the construction just mentioned is undergoing a change during this writing. Advice has been received that the connecting terminals, now located upon the rear, will be mounted in front with the variable controls.

Referring to the variable controls, they consist of a rough frequency adjustment of the linear sweep circuit; a fine frequency adjustment of the same circuit and an amplitude control, whereby the length of the sweep beam across the screen is controlled. Also a synchronizing control, for synchronizing the sweep voltage with the frequency of the voltage under observation and the control which adjusts the intensity of the spot. A spot positioning control, operating along the "X" axis is mount-ed on one side and is accessible through a hole in the housing. From information gathered at the time of this writing, the new units will also

Photograph at left courtesy National Union hudio Corp. Fig. 54. Left, panel of the National Union oscillograph. The cross-section tines are ruled on a transparent shield that clamps over the screen of the tube. Middle, chassis showing the 5-inch cathode-ray tube in position. Right, rear panel to which connections are made to the deflecting plates.

Photograph at left courtesy National Union hudio Corp. Fig. 54. Left, panel of the National Union oscillograph. The cross-section tines are ruled on a transparent shield that clamps over the screen of the tube. Middle, chassis showing the 5-inch cathode-ray tube in position. Right, rear panel to which connections are made to the deflecting plates.

possess a "Y" axis positioning control. A focusing control is also available through the side of the metal housing. It controls the anode number 1 voltage.

Referring to the schematic wiring diagram, the unit consists of the cathode-ray tube, a full-wave rectifier power supply for all of the voltages, a thyratron sweep circuit oscillator and two single stage re-sistance-capacity coupled audio amplifiers. By means of pin plugs and pin jacks, designated as A, D, S, D and A upon the wiring diagram, it is possible to arrange the single stage amplifiers in such manner that voltages to be observed and applied to the vertical and horizontal plates, are amplified. These amplifiers are rated at a voltage step-up of 35 for each stage. It is possible, if so desired, to feed the sweep circuit through one amplifier to the horizontal plates. It is also possible to connect the sweep circuit directly to the horizontal deflection plates; in which case, both amplifiers may be connected in cascade to amplify the signal being applied to the vertical deflection plates.

By means of these pin jacks, plugs and terminals, direct connection may be made to the horizontal and vertical deflection plates without passing through the amplifiers or without involving the sweep circuit. Each of the amplifiers is independent of the other, so that either one may be used in connection with the horizontal or vertical deflection voltages, without making it necessary to employ the other amplifier.

Provision is made by means of the connecting terminals to employ internal or external synchronizing pulses. According to the manufacturer's rating, the amplifiers used in the unit are linear from 20 to 100,000 cycles. The range of the linear sweep circuit, according to the manufacturer is such that frequencies up to 20,000 may be studied. (Editor's Note. By using the 5-inch tube, frequencies up to about 100,000 cycles have been studied, although a more reasonable limit would be about 80,000 to 90,000 cycles.)

Fig. 55. The schematic diagram of the National Union Model 3-5 oscillograph. The terminals within the dotted square and the two amplifier gain controls are on the rear of the instrument.

Perhaps it would be well to say a few words about the sweep circuit used in this instrument. When this sweep voltage is applied to the horizontal deflection plates without going through the amplifier, the position of the spot, with the amplitude control set at minimum, is to the extreme right end of the screen. As the amplitude control is varied, the spot moves towards the left and the sweep voltage results in a line, which can be lengthened until it extends across the screen. The operation of this system, may prove puzzling, hence this short description. When the unit is placed into operation with the sweep amplitude control set to minimum, the potentiometer adjustment across the thyratron control grid bias battery is such that the tube is fully ionized and is, in effect, short circuiting the frequency control condensers. With the horizontal deflection connection plug in the pin-jack S, the thyratron cathode potential is approximately that of the plate and since the free horizontal deflection plate is joined to the cathode, this plate is positive with respect to its associated horizontal plate and the beam is deflected towards the free plate and away from the center of the screen. During this state, the voltage developed across the frequency control condensers is substantially zero. It is conceivable that a small amount of voltage is being built up across these condensers, or that the amplitude of the sweep voltage is a finite value, rather than zero, but it is more convenient to think of the sweep voltage at zero, since the spot is not moving at all and does not appear to be larger than originally adjusted with the focusing control.

As the amplitude control is varied, a higher voltage is necessary between the thyratron plate and cathode, in order that the tube trip or flash. This means that a finite value of voltage is being built up across the frequency control condensers. As a result of the electrical connection of the free horizontal plate to the cathode of the thyratron and since the cathode is connected to what is the negative side of the frequency control condensers, the voltage developed across the frequency control condensers nullifies a portion of the original potential difference existing between the two horizontal plates. Also since the free horizontal plate now is not at the same positive potential with respect to its associated plate, the spot moves away a certain distance from the free plate. With this adjustment of the amplitude control, the tube trips at a definite rate and each time that it flashes, it returns the spot to its original starting position. As the voltage builds up across the frequency control condensers, the spot again moves across the screen. This is repeated and the result is a line part way across the screen representative of the sweep voltage.

As the amplitude control is increased, a greater value of voltage builds up across the frequency control condensers and the spot is moved further across the screen and away from its original position, thus making the sweep voltage line longer. To aid in the movement of the spot past the center of the screen, the thyratron plate receives a positive voltage as a result of its junction on the power supply voltage divider. The sum total result is that by means of the sweep circuit amplitude control, it is possible, as has been stated, to increase the sweep voltage until the line spreads across the entire screen. This sweep circuit has its potentiometer sweep analogy in the sweep circuit shown in figure 29.

As is the case with all thyratron circuits, the linearity of the sweep voltage is influenced by the amplitude. The lower the amplitude, the more linear the saw-tooth wave output. More about this later. Referring to the amplifiers, both have variable gain controls. Synchronization of the sweep voltage and the voltage waveform being observed is accomplished by feeding a small portion of the voltage fed to the vertical deflection plates into the control grid of the thyratron. A potentiometer controls the magnitude of this synchronizing pulse, obtainable internally or externally. The letters V, H and G designate the terminals; for the free vertical plate as V, for the free horizontal plate as H and the common ground as G.

In the operation of the sweep circuit, the rough frequency adjustment is accomplished by varying the condensers, five being used and selected by means of a switch. Fine frequency control is accomplished by varying the resistor in series with the frequency control condenser bank. As has been stated in connection with sweep circuits earlier in this text, adjustment of the amplitude control has an effect upon the frequency, so that when any adjustment is made upon the amplitude control to spread the image, a supplementary adjustment will be required in order to stop the image, so that it is stationary upon the screen. The fine frequency control is used for this purpose. The instrument does not contain a sinusoidal sweep.

Much more can be said about the synchronization control, but perhaps it is best to wait until we discuss the practical application of these devices, since the effect of the synchronization adjustment is general with respect to all units which possess such a control.

Dumont 145 Cathode-Ray Oscillograph

The details given in connection with the National Union model 3-5 cathode-ray oscillograph are applicable in their entirety to the Dumont model 145 instrument. See Appendix for Dumont Model 148 Oscillograph.

RCA Model TMV-122-B Cathode-Ray Oscillograph

Various views of this unit are shown in figure 56. The schematic wiring diagram is shown in figure 57. As far as components are con-

Courtesy RCA Mfg. CoInc.

NOTE: C-21 not included on all instruments.

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