National CRM Oscilloscope

The Type CRM Oscilloscope employs the little RCA-913 tube having a one-inch screen. In spite of its small size, this new equipment is thoroughly practical and is quite satisfactory for routine measurements in the amateur station. The circuit includes a power supply with controls for brilliancy and focus, a potentiometer for controlling the amplitude of the horizontal deflection, and a built-in 60-cycle sweep. The latter is particularly convenient as it permits checking transmitter operation with no connection other than a pickup coil.

Most amateurs, and particularly those interested in phone transmission, recognize the cathode-ray oscilloscope as an extremely useful apparatus which makes it possible to get the utmost out of a short-wave transmitter. Until just recently, however, such equipment has been both complicated and expensive, the latter item particularly preventing the amateur from enjoying its advantages. Recently, however, cathode-ray type tubes have appeared on the market at cost no higher than a low power transmitting tube, and with this tube as a basis, simple oscilloscope apparatus which will answer almost any question arising in the adjustment or operation of an amateur transmitter has been made possible. It is the purpose of this booklet to describe and show how to use the National type CRM oscilloscope, the outstanding features of which are its simplicity, compactness and low cost, and which will do practically everything that the more complicated oscilloscope will do.

Tubes: The rectifier tube employed is the type 6X5 operated as a half-wave rectifier, only one plate being used. The cathode ray tube is the RCA-913. The life of the 913 is the same as any of the common receiving tubes; that is, approximately 1000 hours. The life of the tube will, of course, be shortened if it is subjected to overloads or if it is operated at improper setting of the focusing and brilliancy controls.

Controls: The oscilloscope is put into operation by turning on the A.C. switch, located near the center of the front panel. This switch controls the primary circuit of the power transformer. The two knobs at the bottom of the oscilloscope unit are used to adjust the size and brilliancy of the spot; or, properly speaking, the clarity and brilliancy of the pattern. Of the two knobs at the top, the right is a switch and the left is a potentiometer. The switch controls the horizontal sweep circuit and has two positions. When the switch pointer is to the right the horizontal deflection plates are connected to the two right hand binding posts. When the pointer is turned toward the left the 60 cycle A.C. sweep is connected to the horizontal deflection plates, the external sweep being disconnected. The two binding posts on the left hand side are connected directly to the vertical deflection plates. One of each pair of binding posts is marked "GND" and is permanently connected to the cabinet.

Operation: After the tubes are properly installed and the cover plate fastened in place, the oscilloscope may be turned on. The focusing and brilliancy controls should be turned as far as they will go counterclockwise. The switch should be turned so that the 60 cycle sweep circuit is connected, and the potentiometer turned about one-third of the way on. The tubes should be allowed to warm up for 15 or 20 seconds. The focusing and brilliancy controls are now advanced slowly until a horizontal line appears on the screen. The length of the line may be controlled by the potentiometer. The width of the line is determined primarily by the focusing control at the left of the front panel and its brilliancy by the right hand control. When properly adjusted, the line will be about one-thirty-second inch wide. The apparent brilliancy will depend largely upon the amount of light that falls upon the screen, the clearest and brightest patterns being obtained when the screen is in shadow. The operator is cautioned not to operate the oscilloscope unless some voltage is being applied to one of the sweep circuits. If the beam is allowed to become stationary (in other words, to produce a spot on the screen) the screen itself may be damaged or, in extreme cases, the glass may be punctured. If the nature of the work requires that the beam be adjusted to a spot, the room illumination should be kept low. The spot will normally be near the center of the screen or, in the case of the horizontal line, the screen will be divided by the line into two equal parts. The actual position of the figure on the screen is, of course, unimportant. When installing a new 913 tube, the socket may be adjusted so that the horizontal sweep is actually horizontal by simply loosening the mounting screws and rotating the tube as may be required. The position of the tube is adjustable from front to back by moving the socket mounting as a whole.


The operator is cautioned not to touch any of the exposed terminals or socket connections inside the oscilloscope cabinet when the unit is in operation, since the 913 is supplied with 500 volts D.C. One precaution, especially important where the oscilloscope is to be used for checking a powerful transmitter, is to prevent stray r.f. voltages from getting into the supply circuits via the a.c. line. Usually two 0.01-ufd. condensers, connected in series across the line with the midpoint grounded to the cabinet, will be completely effective. In some cases, however, it may be necessary to increase the size to 0.1 ufd. The condensers must be mounted inside the cabinet where they, themselves, will not be in the field from the transmitter.

Net Price $11.10 without tubes

Source: Notes on Amateur Radio Transmitter Design by James Millen

Back to the Miscellaneous National Product Line