Patent GB-502143-A (GB)
Title: Improvements in and relating to electrical testing apparatus
IPC: H04M1/24
Filed: 1937-09-06
Abstract: 502,143. Cathode-ray oscillographs; frequency measurements. HADFIELD, B. M. Sept. 6, 1937, No. 24308. [Class 371 [Also in Group XL] In an arrangement. for obtaining a visual indication of the percentage make and break period and the impulsing speed of an impulsing contact such as that of an impulse relay, a dial switch, or a regenerative repeater, voltages proportional to the make and break periods respectively are applied to the deflecting plates of a cathode-ray oscillograph. This may be effected by charging a pair of condensers over resistances to an extent determined by the make and break periods respectively and applying the charges to the deflecting plates of the oscillograph so that the deflection of the beam indicates the make and break periods on a logarithmic scale. The contact to be tested is connected to the apparatus by the insertion of a plug into a jack J1, Fig. 1, closing and opening of the contact causing energization and release of relays IX, IY. The contacts Ix1, Iy1 of these relays control electromagnets DM1, DM2 which cause switches SW1, SW2 to step from one contact to the next, so as to cause condensers Cx1... Cx5 to be charged for successive " make " periods and-condensers Cy2 ... Cy5 for successive " break periods, condenser Cy1 being charged before the apparatus is set in operation. The charges on the pairs of condensers Cx1; Cy1 ; Cx2,Cy2; Cx3,Cy3; &c. are applied in succession to the deflecting plates X, Y of a cathode-ray oscillograph, the charges on each pair being applied simultaneously so that the spot takes up a position. the coordinates of which represent the " make " and " break periods. Since the charge on each condenser is approximately proportional to the logarithm of the charging time, the scale is approximately logarithmic. The charges on the condensers are finally dissipated over small resistances r1, r2, but discharging does not occur until the switch SW2 has stepped to the third contact in order to allow time for the correct charges to be registered. The complete cycle of charge, application, and dissipation for each condenser is thus effected in four steps of the switches, so that four pairs of condensers are necessary. Five pairs, however, are employed as being a submultiple of the twentyfive contacts of the common type of uni-selector switch, and the condensers associated with each switch are multipled in the bank associated with wiper 1 so that with double-ended wipers continuous operation is possible. Small condensers may be connected between the deflecting plates X, Y and the gun G to hold the spot in position during stepping of the switches from one contact to the next. In order to ensure that the switches SW1, SW2 are in step, a key KS3 is depressed, causing a relay S to hold the switch SW2 stationary until the switch SW1 reaches the correct contact. The screen of tho oscillograph is preferably of material having a long fluorescent afterglow so that a series of spots may be visually observed. The spots may alternatively be photographed with a standard target diagram superposed by uniformly illuminating the back of the screen material and placing a standard diagram on transparent material in front of and close to the screen.. Potentiometers Xs, Ys are provided for adjusting the bias on the deflecting plates X, Y so that the undeflected position of the spot may be adjusted to coincide with the corresponding point on the transparency. The gun G is biassed by 0À575 of the charge voltage with respect to the condenser charges by means of a tapping on a resistance R4, and the time constants of the make and break charging circuits are altered until the spot remains in the undeflected position. Inequalities of deflection sensitivities between the plates X, Y are compensated by throwing a key KS2 in the upward direction and thereby applying an alternating voltage between the gun G and plate Y for setting up the axes of the tube and the transparency while on throwing the key in the downward direction the voltage is applied between the gun and plates X, Y simultaneously, whereupon a diagonal line at 45 should be obtained when the input sensitivities of both axes are the same. The source of charging voltage E may be a battery or a portion of the rectified voltage that energizes the cathode-ray tube, this voltage being shunted by a neon tube or other low impedance gas discharge valve. The time constants of all the condensers Cx1 &c. with the resistance Rx and of the condensers Cy1 &c. with the resistance Ry may be adjusted to equality by inserting, small semi-variable resistors in each condenser lead. For testing, irregular groups of pulses, a subsidiary relay operated from the pulses may energize a slow releasing relay, the contacts of which cause the spot to be visible. For testing contacts giving reversed dialling pulses (i.e. 67 per cent make), a high-speed distortionless relay with small transit time may be inserted between the input jack J1 and the relays IY, TX and a break contact used instead of a make contact by means of another key. Conducting insets or photo-electric cells may be provided in the screen material in order to define the boundary of the permissible limits of operation of the contact under test. The comparison standard target diagram is traced under the control of an impulse generator arranged to deliver impulses having variable make and break percentages or a variable speed or both. The impulse generator is shown in Figs. 2 and 4, the terminals 1a, 1b, 1c, 1d, Fig. 2, being connected respectively to the terminals a, b, c, d, Fig. 4. A condenser C1, Fig. 2, is charged over a variable resistance R5, Fig. 4, and causes the anode potential of a gas discharge valve V1 to increase until, at a value determined by the setting of a potentiometer Q, the valve discharges the condenser rapidly until the voltage at the anode reaches the extinguishing voltage. The saw toothed potential thus produced is applied to the grid of a pentode or other valve V2; which is arranged to work on the trigger principle at a definite grid voltage by suitable cathode biassing, and the output is then passed through D.C. valve stages V5, V6, a relay G being located in the anode circuit of the last valve. The operation of the D.C. stages is described in Specification 505,500. The relay G generates one impulse for each charging and discharging operation of the condenser C1. The impulsing speed is determined by the value of the resistance in the charging circuit of the condenser C1, while the make percentage is dependent only on the amplitude of voltage variations applied to the valve V2 which is determined by the cathode bias. Instead of varying the bias on the valve V2, the make percentage may be varied by providing tappings on the input resistance which is fed with the exponential cyclic voltages from the time base stage via a condenser. As shown.in Fig. 3, a condenser C3 and resistance R11 plus external resistances between terminals 2c, 2d constitutes the coupling to the valve V3. Across these resistances a diode V4 is connected so as to produce a positive bias and enable percentages greater than 50 to be obtained. The arrangement for controlling the speed of impulsing and make percentage and for enabling the relay G to deliver a series of impulses of differing speeds and make percentage is shown in Fig. 4. The bank contacts of two automatic switches SW3, SW4 of the uni-selector type are connected to a series of horizontal bars 1 ... 25 arranged on one side of an insulating plate a series of vertical bars being arranged on the other side and connected to tappings on resistances R5, R6. Holes arc drilled and tapped through the horizontal and vertical bars at their points of intersection so that by the insertion of screws any contact on either of the switches SW3, SW4 may be connected to any tapping on the corresponding resistance R5, R6. The charging circuit of the condenser C1 is from earth, springs 4 of KS5, contact b1, springs 1 of key KS5, wiper and bank contact 1 of switch SW3, horizontal bar Spl, connected vertical bar and tapping on resistance R5, terminals a, Fig. 4, 1a, Fig. 2. condenser C1 and battery to earth. When the relay G operates, the contact g1 closes. This is connected by a double-ended plug to a jack J3, Fig. 4, and causes the operation of high-speed relays A, B, C. The generated impulses are made available at contact a1 over jack J4, the relay A being adjusted to repeat the impulses without distortion. The relays B, C are synchronized as regards the break contact of relay B and the make contact of relay C, and on operation cause transference of the charging circuit for condenser Cl from wiper 1 of switch SW3 to wiper 1 of SW4, and in addition the wipers of the switch SW3 are stepped on to the second contact after a period dependent on the operate lag. In order to trace out a standard target diagram, the contact a1 is connected to the jack J1, Fig. 1, by means of a double-ended plug. If the switches get out of step, the relay 1S operates when the wiper 2 of switch SW3 is on contact 1. The relay 1S has the same function as the relay S1, Fig. 1, the switch SW4 being rotated until it is on the same contact as switch SW3. Instead of using the arrangement shown in Fig. 4, the speed may be varied by connecting a variable resistance between the terminal 1a and the earthed battery terminal without affecting the make percentage, which may be altered by connecting a potentiometer between the terminals 1c, 1d, Fig. 2, or 2c, 2d, Fig. 3, and taking the terminal 1b or 2b to the sliding arm. The upper limit can then be determined by the resistance R10 and the lower by the resistance R9. In the arrangement shown in Fig. 3, the upper limit may be determined by the resistance R11. .In order to measure the speed at one setting, plugs PI, P2 are inserted at any given tapping points on the resistances R6, R5 respectively. On operating the key KS5, continu