A COMPLETE SIMPLE OSCILLOSCOPE

The oscilloscope described here is a simple one and therefore limited in its application. It has been designed, however, to include a two-speed time-base, derived from the 6.3 volts 50 cycles heater supply. This timebase will enable repetitive wave-forms at frequencies between about 150 and 10,000 c/s to be examined. Synchronisation is not possible with a simple 50 c/s time-base but it will be found quite easy to "hold" a display long enough for close examination or for photography, particularly at the lower frequencies. When the internal 50 c/s time-base is switched off, the valve used for its formation, becomes an amplifier with the X input connected to the grid. An external "linear" time-base can therefore be connected to the X terminals. Otherwise any repetitive wave-form may be connected to the X input and its amplitude controlled by the "sensitivity'' control. Simultaneously any wave-form may be connected to the Y input terminals for display in conjunction with either the time-base or with any signal applied to the X input. Provision is therefore made for Lissajous pattern work (frequency comparison as outlined earlier).

The Y shift-control (VR4) has been incorporated to provide just sufficient movement of the spot to keep the display centered. A brilliance control has been included also, but this is inoperative when the time-base is in use (VR3). In this case the brilliance level is fixed by the application of a 50 c/s voltage to the c.r.t. cathode which serves primarily to prevent the flyback trace appearing. The additional socket marked r.f. is a.c. coupled via an isolating capacitor, to one Y plate. The socket may be used for direct injection of an r.f. carrier when it is required to examine a modulation envelope, etc. This facility may be used with or without the time-base.

The response of the Y amplifier is linear between 10 and 30,000 c/s, the response of the X amplifier, when not used for the time-base, being approximately the same.
Circuit Details. The d.c. supply for both c.r.t. and valves is derived from a bridge rectifier connected across the secondary of the mains transformer. The secondary voltage a.c. should be between 300 and 350 volts although a voltage below 300 could be used with some loss of brilliance on the c.r.t.

As the circuit diagram shows, only two valves are used, one as a Y-plate amplifier (VI = EF86 or 6BW7) and the other as a combined time-base and Y-plate amplifier (V2 = 12AX7) or equivalent. This is a twin triode with its front half (V2A) switched as either time-base or X amplifier with the second half (V2B) used as a phase-splitter to feed a symmetrical voltage to the X plates.

The time-base is a rather novel one employing a 6.3 volts a.c. drive to V2A grid, so that the valve is over driven. Negative feedback helps linearise that part of a cycle used for the actual X scan. The speed of the time-base is controlled by the series resistors R7 and R8. For full operation as an X amplifier, the a.c. supply to the grid of V2A is switched off and the anode to grid negative feedback disconnected. The 50 c/s voltage for both time-base and c.r.t. flyback suppression is derived from the heater supply, one side of which must be earthed as shown in the circuit. The cathode-ray tube chosen for this simple oscilloscope is the Mullard DH-3-91 which is a 1 in. diameter tube with internal focus control. (Connections for tube and base are shown in Fig.)

N.B. The tube must be orientated so that pin 5 of the base is uppermost as shown. The X plates will then be in the correct position for horizontal scan.

Layout and Construction.

The layout of the oscilloscope is not critical since the circuits are simple. The mains transformer must, however, be clear of the c.r.t., otherwise 50 c/s trace modulation is liable to occur. The tube could, of course, be fitted with a mu-metal sleeve to obviate this, but prevention is always better than cure. In the prototype model shown in the photograph of Plate 16 the mains transformer was mounted about 6 in. from the tube and carefully orientated until no modulation of the spot or a trace was visible.

In the prototype, the valves were mounted one either side of the tube, most of the small components being mounted on tag-panels beneath the chassis, with larger components above.

THE DH3-91 CATHODE-RAY TUBE

These tubes are made by Mullard Limited and further details are obtainable from Mullard Limited, Mullard House, Torrington Place, London, W.C.2. It may not be possible to purchase them from radio-component dealers but a dealer should be able to obtain them without difficulty from his regular wholesaler.

The DH3-91 trace is green and has a medium persistence. The X plates are suitable for symmetrical operation but the Y plates are suitable only for asymmetrical operation. An internal electrostatic system is used for spot focus. The tube may be supported on the base but care should be taken to minimise the effects of shock.

It is unwise to allow full brilliance when the spot alone is displayed, as there will be a risk of burning a small dark hole in the screen. Normal operating conditions of the tube are as follows but note that deflection sensitivity will be slightly lower if the operating voltage of 500 volts maximum is exceeded.

Owing to the presence of a transparent conducting film connected to the anode and which lies between the screen of the tube and the glass, the tube may be operated with its cathode at earth potential, otherwise oscillogram distortion may occur when an earthed body is brought near the screen.

The prototype oscilloscope performed well and both sine and square waves could be displayed without distortion. The frequency limitation for square waves is, however, approximately 20 c/s to 15,000 c/s.

The frequency range of the X and Y amplifiers and the range of the 50 c/s time-base are more than adequate for checking audioamplifiers, etc. Frequency comparison by the Lissajous method can be carried out at radio frequencies by using the r.f. input socket for one source and a connection to the one of the X plates, via a 0.001 uF. mica capacitor. If work of this nature is envisaged an additional socket could be fitted and the capacitor permanently connected to an X plate, as has been arranged for the Y input.

Components.

The brilliance (VR3) and shift (VR4) controls may be standard type pre-set potentiometers such as those used in television receivers. A panel-mounting type should be used so that the small control knob protrudes through the front panel.

The X and Y gain controls VR2 and VR1 respectively are standard type carbon potentiometers and the time-base switch is a three-pole, three-way type on a single wafer (S1 S2 and S3). All other components are marked on the circuit diagram and should be readily obtainable from most radio-component dealers.