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Cover picture.
Based on the ex-Government Indicator 62 unit, careful shopping will enable this complete televisor, less loudspeaker, to be built for £9! Most experimenters have a spare loudspeaker available. This televisor will produce pictures of quite good quality, and at the same time it will provide an inexpensive experimental unit.
Below is given a shopping list with prices current at the time of going to press.
Costings. In 1951/52 the average weekly income was £7.0.0
The most costly portion of the equipment is the mains transformer, and the prices quoted are for the cheapest obtainable. It is possible to buy ex-Government transformers which will be suitable for less than the figures given above, but as the power unit is the heart of the televisor it is advisable to buy the best.
The Indicator 62 unit is built on a two-deck chassis and provides most of the components for the vision receiver, sound receiver and time base. The power unit is built on a separate chassis, as it thereby relieves the main unit of a great deal of weight, and thus makes it easier to handle.
The televisor is build around the VCR97 6 in electrostatically deflected CRT developed for RADAR use. It has a green phosphor. The valves used are:- VR65 (SP61), VR92 (EA50), VR54 (EB34 D63), 6V6, 2X2, 5U4G.
The completed Receiver
The photograph shows the position of the main items. The sound receiver is in the foreground; on the opposite side of the chassis is the time base, while the vision receiver occupies the upper deck in the background. The EHT supply is contained in the unit at the back of the vision receiver, being fed from a mains plug fitted on the back of the unit.
Stripping the Unit
The receiver with the tube removed.
When the unit is received it is advisable to check the CR tube, if possible, under normal working conditions on a friend's televisor. If this is not possible, the filament should be tested and the base of the tube examined for looseness. Most of the dealers who sell this unit will change the tube if it is faulty.
The tube can be removed by unscrewing the screw fitted at the bottom of the bracket which supports the tube holder; the bracket can then be drawn back. The potentiometer panel on the top of the chassis can then be swung back on its hinge by undoing its retaining screws, and the tube can then be withdrawn from the chassis.
After removing the valves the whole of the unit should be completely stripped with the exception of the valve-holders, the tube holder, the 0.03 μF 2.5 kV capacitor (which becomes C75), the DC restoring diode and associated resistor on the top of the chassis (these become V19 and R73), the focus and brilliance controls (which become VR9 and VR8), and the bleeder network (this becomes R76, R77, R78, and R79). Do not remove the leads from the CRT holder.
Remove everything else from the chassis including the double-sided Paxolin strip underneath the unit and the VR92 valve-holder by its side.
Fig. 1 - Layout of chassis. Top view.
The valve-holder (see Fig. 1) in V20 position is removed and is replaced with a ceramic-based type for the EHT rectifier ,valve. The valve-holder occupying the next position is removed to make room for C78. The valve-holder in V1 position is removed and is replaced with one of the EF50 type. Change the valve-holder at V10 position with that at V9. Change the valve-holder at V13 position with that at V11. Remove all the potentiometers from the top panel. Remove the front metal (double) panel.
Finally, test all the capacitors for leakage. This is important, as a leaky capacitor can cause some very puzzling faults, especially in the time base.
The First RF Stage
Fig. 2 - Circuit of the first RF stage.
The circuit diagram is given in Fig. 2. It uses an EF50 valve (V1) and provides a good signal for feeding into the sound and vision receivers. L1 and L2 are mounted under the chassis in the position shown in Fig. 1, the metalwork being drilled so that they can be trimmed from the top of the chassis. (This principle applies to all the tuning coils.) Coil-winding data is given in the table at the end of this article.
The various components can be grouped around the valve base, taking care to keep the leads short, and a metal screen 2 in by 2 in. should be erected between this stage and the first vision RF stage. It will be found convenient to erect this screen after the components have been wired. The aerial connection is made on a Pye socket fitted at the back of the unit and a piece of coaxial cable is run from this socket to L1. The sheath of the cable should be earthed at both ends.
The Vision Receiver
Fig. 3 - Circuit of the video receiver.
This consists of four RF stages using VR65 valves, which feed into the VR92 diode detector, whose output is injected into the VR65 video valve. Sound rejection is provided by L5 and L8. All coils are mounted underneath the chassis in a similar manner to L1 and L2. Screened leads to the valve grids (top caps), should not be used, but V3 and V5 valve caps should be screened. Metal screens, 2 in × 2 in, should be mounted between each stage after the components in each stage have been wired. Keep all leads as short as possible and do not mount anode components near grid components.
Anode decoupling components can be mounted on a Paxolin strip fitted on the side of the chassis underneath the valve-holders. These components are: R5, C8, R8, C10, R13, C18, R17, C22, R18, C25, R23, C31.
VR1 forms the contrast control and is fixed on the front panel.
V6 and its associated components, L9, R22, C30 and L10, are wired directly on the tag strip, no valve-holder being used.
The Time-base
Fig. 4 - Circuit of the timebase section.
All valves are VR65s with the exception of the DC restorer diode (V12) which is a VR92. (Note there are two DC restorers in this unit, one in the time-base and one in the CRT circuit.) V13 is the phase splitter, the signal for the CRT grid being taken from the cathode resistor, R43. V14, the sync separator, follows and feeds the sync pulses to frame and line time-bases.
The line time-base receives its sync pulse from the capacitor C58, which is made variable so as to obtain the best amplitude of sync pulse, for triggering the line oscillator.
Both line and frame time-bases use the Miller Integrator combined with Transitron oscillator, as sawtooth generators. V15 is the line oscillator, its frequency being varied by VR3 which forms the line hold control. Output of sufficient amplitude is obtained by para-phase amplification which employs V16. The output to the deflector plates is taken from the anodes of these two valves via C60 and C61 which are 450 Volt working.
The frame oscillator (V17), is similar in nature to V15, the only difference being in component values. VR5 is the frame hold control and V18 forms the other half of the para-phase amplifier. The frame deflector plates are fed from C68 and C74 which are both 450 Volt working.
Components marked with an asterisk in Fig. 4 are mounted on the double-sided tag, strip taken from underneath the chassis. It will be found convenient to wire up this strip before fixing it back in the chassis, leaving about 6 in long leads where interconnection between strip components and chassis components are to be connected together.
On the other side of the strip should be mounted the components for the sound receiver which are indicated in Fig. 5.
Great care should be taken not to get the leads mixed and each 6 in length should be suitably labelled.
Before remounting the strip, the valve-holder for V12 (originally removed from underneath the chassis), should be remounted towards the back end of the chassis (underneath). The strip can now he replaced and the wiring up of the timebase completed.
C54 and R44 are mounted on a small Paxolin strip fitted underneath the EHT transformer on the back end of the chassis. The strip should be fixed right at the bottom well clear of the EHT leads.
The connection between C54 and 'A', Fig. 4, should be made in coaxial cable, the outer sheath being earthed at each end. The height of the picture is controlled by varying the HT applied to the anodes of the frame time base valves. VR4 forms the control and is one of the 25 KΩ potentiometers previously removed from the top panel. it is shunted by a 1 Watt resistor R83.
The connection to the deflector plates and to the grid of the CRT can be made by utilising existing wiring.
The Sound Receiver
Fig. 5 - Circuit of the sound receiver section.
Two RF stages using VR65s (V7 and V8) are transformer coupled. V9 is a VR54 and one-half forms the detector while the second half is used for noise limiting.
The output from V9 is fed into the first AF valve, a VR65 (V10) which is RC coupled to the 6V6 output valve, V11. VR2 is the volume control. Screened lends and valve caps can be used, though they should not be found necessary.
Trimmers T1, T2 and T3 should be firmly wired and fixed so that they are easily accessible from the side. The coils are mounted in at similar manner to those in the vision receiver. When mounting these coils it will be found that the best method is to wind on the secondary, bolt the former to the chassis, and then wind on the primary.
Connection between L11 and L3 (Fig. 2) is made in coaxial cable, the outer sheath being earthed at both ends.
If the anode circuit of V11 is disconnected while the valve is working, heavy current will flow via the screen and the valve may be severely damaged. It is, therefore, wise to permanently wire the loudspeaker transformer in the circuit, detaching it from the loudspeaker if necessary, and making connection between the transformer secondary and the speech coil via a plug and socket.
EHT Supply
Fig. 6 - Circuit of the CRT power supply and network for the VCR97.
The EHT transformer is mounted at the end of the deck containing the vision receiver (see Fig. 1). it will be noted that the positive EHT is earthed. The reason for this is to keep the peak inverse voltage from the transformer windings. When the negative is earthed we have on the second half of the AC cycle (when V20 is not conducting) the potential across C78 (2.5 kV) added to the inverse voltage (2.5 kV) which appears across the windings of the transformer. This is the reason for many early breakdowns in EHT transformers.
Another benefit derived from earthing the positive is that the working voltage of the coupling capacitors to the deflecting plates of the CRT need only be that of the time base HT 450 V working capacitors provide a good safety margin.
One snag with this system is that the cathode and heater of the CRT are at EHT potential and must be carefully insulated from the earth. This feature is catered for in the layout of the 62 unit, but it is important to bear the fact in mind when handling the televisor when it is working.
All EHT wiring must be thoroughly insulated. In the prototype the wires were first covered in Systoflex sleeving of sufficient diameter to contain the wire, and then covered again with another length of systoflex of larger diameter.
Soldered terminations must be made with care, no stray ends being left to set up brush discharges.
C78 is mounted in the position shown in Fig. 1 and R75 wired directly to the top terminal of the capacitor, its remote end being supported by the insulated strip mounted on the supports of the potentiometer panel.
VR8 is already in situ and R75 is connected directly to it. R76, VR9. R77, R73, R79 will be found in situ and wired. R79 is earthed at one end and this connection is broken so that R80 can be inserted. C76 and R74 are wired across VR8, both components being supported by their own wiring. Care should be taken when fixing these two items so that they do not make contact with the chassis, or with the mu-metal screen of the CRT when it is in place.
The DC restorer V19 and associated resistor R73 are already in situ adjacent to the CRT base. The wiring can remain as it is except that the connections to the cathode and anode of the valve must be reversed, and any wiring between the cathode and heater must be removed. (The heater/cathode insulation on the VCR97 is not good and strapping the cathode to one side of the heater is normal - as in Fig. 6 - ed.)
CRT Network
Bias for the deflector plates is obtained from the time base HT supply. The coupling resistors R66, R67, R68 and R69 can he wired directly to the tube holder. R72 (2 Watts) and R70 and R71 are mounted on the potentiometer panel. VR6 and VR7 form the shift controls for centralising the raster.
It should be possible to obtain even focusing over the whole of the raster, but if this should not be the case (some tubes are temperamental). the deflector plates 11 and 12 can be taken to separate shift controls. To do this disconnect R66 and R67 at their junction and from each other: connect two more 100 kΩ controls across the biasing network in a similar manner to VR6 and VR7. Connect the centre of one potentiometer to R66 and the centre of the other to R67. (This modification was made to the prototype as a refinement, though it was not really necessary.)
Power Pack
Fig. 7 - Circuit of the power supply.
This is made on a separate chassis. It relieves the unit of a great deal of weight, though it is possible to fit it on the existing chassis, provided a transformer of suitable size can he obtained. If this is done it should be mounted underneath the Chassis at the front end, below the bleeder network. Metal rectifiers will have to be used and the whole carefully screened from the rest of the equipment. A separate chassis is recommended for the reasons given.
Time base HT is derived directly from the smoothing choke, while the sound and vision receiver supplies are separated by the voltage dropping resistors R81 and R82. These resistors should be rated at 5 Watts. It is wise to insert a fuse in the circuit as shown, because a complete breakdown of C79 would result in the loss of the 5U4G rectifier valve. V21.
The on/off switch can be incorporated with the volume control.
No details of chassis construction are given because any type which will accommodate the components will do.
Connection between the power pack and the unit is made by plug and socket.
Preliminary Adjustments
Before connecting the televisor to the mains, the wiring should be checked thoroughly. Make certain that no contacts exist between the HT line and filaments, or between HT line and earth.
Turn all controls to zero. Now switch on and wait for the unit to warm up. After about one minute advance the brilliance control until a pattern appears on the screen; now adjust the focus control until the lines forming the raster are finely focused; next centralise the raster by means of the shift controls.
Receiving Sound
Screw the iron cores until they are level with the tops of the formers. Advance the volume control to maximum position; now adjust L1 and L2 cores until the sound is heard. Bring the sound up to its full volume by adjusting T1, T2 and T3 in that order, reducing the volume control as required.
Should it not be possible to get a peak due to the stray capacitances of the wiring, add half at turn of wire to the secondary of those coils which will not peak when the trimmer is set at maximum (i.e., when the lines on the fixed and moving parts of the trimmers are together). Unscrew the iron cores of those coils where the trimmers are at at minimum without peaking, until the peak point is found.
Receiving Vision
Insert a pair of headphones in the anode circuit of V7 between R24 and R23 and disconnect the lead to the CRT grid. Set all cores level with the tops of the formers; now turn the contrast control to maximum and the vision signal should be heard. It sounds like a peculiar mixture of 50 Hz hum and motor-boating. L1, L2, L3, L4, L5, L6, L7, L8 and 9 are then adjusted for maximum signal, reducing the contrast control accordingly.
At the back of the vision signal will be heard the sound programme. Adjust L5 and L8 until the sound is no longer heard. Then readjust L4 and L7 for maximum vision signal.
It will be discovered that a peak value cannot be found on some of the coils; this is arranged purposely so that adequate bandwidth can be received. Now turn the contrast control to zero, restore the anode circuit of V7 and the lead to 'A'; reduce the brilliance control so that the raster just disappears; now by turning up the contrast control a pattern should appear on the screen. Turn the line hold control in either direction slowly and it will be found that at a certain critical setting will resolve the pattern into a picture. Adjusting the frame hold control will lock the picture in a vertical direction.
If the picture is upside down reverse the connections to 12 and 8 on the CRT. If the picture is inside out reverse the connections on 9 and 11 on the CRT.
The quality of the picture will probably be very poor. L3 L4, L6, L7 and L9 should, therefore, be adjusted to give the best quality by screwing the cores in or out. Deal with each coil in turn, noting the effect on the quality. The contrast control will have to be advanced in step with this operation.
If at all possible, final adjustments should be made on test card 'C' which is radiated from 10 am to 11 am on Saturday mornings, and at intervals between 10 am and 12 noon on other weekdays.
Details of test card 'C' were given in the June, 1951, issue of Practical Television.
When the televisor is finally set up, the coils L1 and L2 should be adjusted between the sound and vision signals. If you have plenty of signal strength in hand, the quality of the picture can he improved by adding damping to the coils. A 4.7 kΩ resistor can be connected across L1 and another across L2. 4.75 kΩ resistors can also be connected across the vision receiver coils but not across the rejector coils.
Full list of components.
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