Modification of Television Recievers
The following information comes from Mullard publication T.P.221 of an unknown date.
Before attempting any electrical conversion to the receiver a Mullard tube should be fitted in place of the original one and and the physical aspects checked.
In general, the above requirements are met, or can be met by minor mechanical variations.
Turning to the electrical aspects of the conversion, the points which require attention are as follows :
Standard Mullard picture tubes have a duodecal (B12A) base. It is usually necessary, therefore, to replace the original tube socket by a duodecal socket, the connections to which are shown below.
A supply of 6.3 volts at 0.3 ampere must be found for the tube heater.
When the cathode of a Mullard picture tube is positive with respect to a heater, the maximum permissible voltage between heater and cathode is 200 volts. In many receivers this permits the tube heater supply to be taken from the winding on the transformer which feeds the valves.
In receivers using series-connected valves it is often convenient to connect the tube heater in the earthy end of the heater chain, provided this chain carries 0.3 ampere. When this modification is made the chain current must be checked and the limiting resistor adjusted if necessary. It is a good plan to shunt the heater leads to the tube capacitively at the point where they are connected to the main chain. A capacitor of 500pF is suitable, and prevents any tendency to instability which may arise due to R.F. on the leads.
First Anode Supply
A positive potential of between 200 and 400 volts with respect to the cathode of the tube must be provided for the first anode of the tube.
In many of the latest receivers the boosted H.T. voltage in the line timebase is available to meet this need. In some of the earlier models the H.T. supplied to the timebase chassis is sufficiently high, whilst in sets using a bleeder network across the E.H.T. supply a tap can easilly be arranged on this network.
In many A.C. receivers, particularly those which employ a small mains transformer to feed the tube heater, a voltage a little in excess of 300 volts may be obtained by recifying the voltage at the 250-volt mains tapping point, as shown in Fig. 2.
Recification of the mains voltage to provide the first anode supply is, of course, only possible in A.C. sets.
Fig 2. - Method of Obtaining the First Anode Potential
by Rectifying the Mains Input Voltage
Another device for obtaining the first anode supply is to rectify one of the alternating potentials found in the timebase circuit. A suitable voltage can often be found on a blocking oscillator transformer or on a flyback winding on the line transformer. A modification of this arrangement is to use the the voltage which is developed across the line deflector coils. A piece of Metrosil and a smoothing capacitor are required and connected as shown below. This arrangement is usually satisfactory in receivers in which one side of the line coils is connected to the H.T. line.
Fig. 3 - Method of Obtaining the First Anode Potential
from the Line Deflector Coils
To use a tetrode tube in place of a triode tube it is necessary to reduce the focussing field strength.
When an electromagnet is used for focussing, all that is needed is to reduce the current in the magnet winding. A series-connected winding should be shunted by a resistor whilst a parallel-connected focus magnet will require some additional series resistance,
In a receiver using a permanent magnet the field strength across the gap may be reduced by the addition of external shunts. These shunts are quite simple to make, since they consist merely of mild steel strips about 3/8ths-in wide by 1/8th-in thick and long enough to bridge the poles of the magnet.
Fig 4. - Part of a converted chassis showing
position of Magnetic Shunts on the Focusing Magnet
At least three such shunts should be placed symmetrically around the periphery of the magnet. The exact number required must be determined by trial and error methods ; four usually are necessary. In receivers where the above suggestions cannot be applied it will be necessary to change the complete focus assembly for one having a lower focussing field.
The above article was followed by three sections :-
1) Specification for the then current Mullard 12-inch
2) Instructions for Adjusting Ion-Trap Magnets.
3) Description of actual modifications to specific television sets.
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17th September 2001