An Experimental Spot-Wobble Circuit

The following is cribbed from a pamphlet entitled "Mullard Valve Notes No.1".

The line structure of a television picture is an unpleasant feature, and is emphasized to the point at which it may become intolerable as the focus becomes sharper. The high horizontal definition achieved by good focus can be retained and at the same time the line structure removed by elongating the spot vertically. The most convenient way of doing this is by deflecting the spot vertically at a fequency which is high compared with the line frequency. This is called "spot wobble".

The best circuit arrangement for producing spot wobble depends upon the picture tube used, its associated components and the layout of the receiver. A good starting point for experiments in this field is provided by the circuit described below. It should be appreciated, however, that if the interlace is not good, or if the spot is astigmatic, or if there is appreciable deflection defocussing, spot wobble is unlikely to effect any considerable improvements. These points should therefore be looked into before attempting to apply spot wobble.

The diagram shows a simple oscillator in which the spot-wobble deflector coils form part of the oscillatory circuit. An EF80 pentode is employed, connected as a triode, and the total drain on a 180 volt H.T. line is about 12mA.

The amplitude control R1 is used to adjust the elongation of the spot to the condition in which the lines just merge. The switch SW1 permits the spot wobble to be switched off while the normal focus is being adjusted. The spot-wobble deflector coils consist of a pair of saddle windings similar to conventional deflector coils. There is, however, no yoke, and the windings are much smaller, each coil consisting of five turns of 0.018" diameter (26 S.W.G.) enamelled copper wire. These should be wound on a rectangular former 1 1/8" x 1 3/4". When removed from the former the flat winding is applied to the tube by folding the longer sides round the neck, i.e., with the 1 1/8" wide parallel to the axis of the tube. The two coils are mounted on opposite sides of the tube neck directly behind the normal deflection coils and are connected in series in such a way that their magnetic fields assist each other.

PRACTICAL HINTS

As explained overleaf, the spot-wobble coils should be mounted on the tube immediately behind the normal deflector coils. This means that part of the spot-wobble coils will have to be inside the focus and centering assembly, and the method of mounting them must permit this.

A simple temporary arrangement is to make a sliding a former from two or three turns of Empire cloth 1½ inch wide to fit the tube neck, and to bend the spot-wobble winding over this while it is on the tube, retaining the windings in position with two turns of 1½ inch wide P.V.C. adhesive tape.

The leads from the spot-wobble coils to the oscillator should not be longer that 6 inches.

CHOICE OF OSCILLATOR FREQUENCY

The spot-wobble frequency should be greater than 8 Mc/s, otherwise, with a finely focused spot, the sinusoidal pattern becomes apparent. The frequency should ideally be chosen so that neither this frequency nor its harmonics fall within the I.F. or R.F. channel of the receiver. If the whole of band 1 and all the I.F. channels that ar in use are taken in consideration, the lowest ideal frequency available for the spot-wobble circuit is 40 Mc/s. The losses at this frequency are prohibitively great, and a nominal frequency of 12.5 Mc/s has been selected. Provision has been made for adjustment of this frequency by means of C1 so that harmonics can be moved out of the receiver channels.

The original artical appeared on the page immediately following an artical entitled "Important trends in valve design" describing the EF80 valve. Was this artical therfore just Mullard trying hard to flog a few more EF80's ?

In the March 1952 edition of Practical Television magazine, Ekco announced a new 15 inch table model employing this feature but I have not seen any other manufacturer adopt this.

The following excert is from the 1960/1961 Television Servicing book

The larger the screen, the more noticeable becomes the line structure of the British 405-line picture. The 'spot wobble' system of reducing the effects of line structure is not found in any current models: not because the system is ineffective, but simply because many viewers tended to equate clearly visible lines with a good picture and were often reluctant to use the H.F. oscillator. A new method of reducing line structure is under development on the Continent, consisting of a special transparent plastics panel arranged to have the optical effect of elongating the spot so that it appears ellipsoidal with successive lines just touching".