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So near and yet so far
by David Birt

The television lab. was part of the Applications Division of Mullard Research Laboratories (MRL). Its purpose was to either create or to foresee trends in technology as far as possible. The trend to shorter display tubes with wider deflection angles: 70⁰  > 90⁰  > 110⁰  is one example. Each successive increase in deflection angle was associated with an increase in scanning power requirements. This meant that new scanning valves needed to be developed such as the PL36 for line and the PCL85 (my ‘baby’) for frame scans. New ferrite cores were needed too. Application support for set makers was provided by designs for e.g. line output transformers, together with the latest ideas in the way of circuit techniques such as flywheel sync. or noise-gated sync separators. Later, Mullard supplied components such as scan coils and line output transformers.

I think it fair to say that the Mullard TV lab. was quite “leading-edge” at the time I joined it. When I arrived in1956, a colour bar generator had been built there along with our own flying-spot film scanner etc. My first job was to design a 3 x 25kV regulated EHT unit for an otherwise-complete experimental projection receiver which used three of the established Mullard projection tubes furnished respectively with red, green and blue phosphors. As is often the way, the cabinet had been designed first, leaving a ridiculously small space for the EHT unit. There seemed only one option, and that was to build an oil-filled unit to house the RF oscillator and rectifiers which outputted 33kV. The three specially-designed 25kV shunt regulator valves were external to the oil-filled part. During its development (in a biscuit tin scrounged from the canteen), the thought occurred to point a Geiger counter at the unit. It bent the needle, and everyone fell about laughing! Despite this, I seem to have survived into my 70s. Mullards had a very interested and supportive Managing Director and so it was quite unremarkable that one of the projection receivers should be installed in his living room so that he could watch some of the experimental 405 line NTSC transmissions. By Murphy’s law, that was the one receiver in which the EHT unit developed a leak and it soaked his carpet with oil. He was very good about it, as he was when another experimental receiver incorporating these new-fangled silicon rectifiers set fire to his curtains as a result of mains surges during a thunderstorm!

After the colour projection set (which failed because thermal effects within the optics caused the registration to drift seriously) my next job was field timebases. Mullard had started to manufacture primitive, low-power germanium transistors, and in 1958 we built an experimental all-transistor black and white receiver. This used scan magnification based on magnetic quadripole lenses which surrounded the neck of the picture tube. These magnetic lenses acted upon the electron beam in much the same way that the familiar glass lens acts on a light beam. They were placed downstream of the deflection coils. The magnification was 10 times in the line (horizontal) direction and 4 times in the frame (vertical) direction, and this reduced the required scanning power dramatically. The line scan was done with an OC23 transistor (I think, but it could have been a development type N7D), and my frame scan circuit used a pair of OC72 transistors in class B push-pull. With the voice of Mr Inventor from Toytown reverberating in my mind, this was the occasion of my first patent. The flyback-pulse energy was supplied separately by a blocking oscillator, so that the class B amplifier “thought” it was feeding a resistive load. Whoever heard of a frame timebase without an output transformer: ridiculous, you may as well suggest an aeroplane without a propeller. This was also my first experience of working all night in order to ship the set up to the Radio Show at Earls Court, and once there we were to experience a dramatic manifestation of Murphy’s Law. The potential “downside” of scan magnification is an increased sensitivity to stray magnetic fields, which had not hitherto been a problem. On our private-viewing “set-makers’” stand, a splendid plinth had been prepared to display the receiver to best effect. Imagine our dismay when we switched on and saw a jumble of hum bars and distorted images. Of course, nobody could have been expected to know this, but according to Murphy’s Law, the plinth sat exactly above the umpteen kVA transformer of a basement sub-station! Fortunately, merely moving the plinth a few yards solved the problem. With 18kV EHT, the 17 inch-diagonal picture was crisp and bright, and the set ran from 12V provided by a couple of 6V motorcycle batteries, and consumed 12 Watts. As is the way of things, the Departmental Head (who had had nothing to do with the project) read a paper to the Television Society in 1959, in which he concluded that there was absolutely no future for transistors in television. Keep quiet about it, but at about the same time a BBC RD report concluded that transistors offered no advantages for broadcast equipment. Yes, I have come to know all about the “Barnes Wallace” effect during my working life! No effort must be spared in the attempt to rubbish innovative ideas!

As regards colour television, we were investigating circuit techniques and appraising various display devices. Although the RCA shadowmask tube held sway, it was difficult and expensive to manufacture. Pictures were rather dim because a significant proportion of the beam current landed on the mask rather than the phosphor dots.  The “Apple tube” which had phosphor strips, guide wires, and which used “spot-wobble” for RGB colour selection existed. So when we came to invent our own display idea, it is perhaps not surprising that it became known as the “banana tube”. Perhaps quite an appropriate name since the company skidded on the skin (metaphorically), and quietly buried the idea once it had been demonstrated at the IEE and something like £½million had been spent on its development. A project that was difficult to stop once it had started!

How did the banana tube work? It was a cylindrical tube about 2ft long and perhaps 4” in diameter. Think of it as a miniature grow-tunnel, and imagine that you are standing at the end of one, holding a hosepipe. If you tilt your wrist vertically the water lands nearer or farther away along the length of the tunnel. If you shake your wrist from side to side, you can wobble the water stream so that it lands sequentially on the rows of red, green, and blue flowers! Get the idea? There were only three phosphor strips. An electron gun was placed at the end of the tube to provide the “hosepipe” function.

Vertical scanning? All done with mirrors!

The tube was surrounded by a rotating cage of Perspex rods resembling the cutters of a cylinder lawn mower, which projected the phosphor image on the surface of a weird-shaped mirror. So this was mechanical scanning – but what’s wrong with that: we use it for taking space photographs today! The mirror was made cheaply by a company which specialised in the manufacture of distorting mirror for fairgrounds.

What you saw was a rectangular virtual image of good contrast which did not suffer from the spurious reflections of lamps or bright items in the room. That in itself was actually quite impressive. One major problem was phosphor loading. Maybe the first red London bus to appear on screen would be red, but the second tended to be a pale orange! (I exaggerate slightly.)

Yes, Minister

News of this revolutionary British invention somehow reached government circles. Accordingly the then Minister for Science and Technology (or some such title) Julian Amery announced that he would like to see a demonstration. Being a cabinet minister he couldn’t just come by car like anyone else, and had to come by helicopter. Hmmm! Where could it land? The farmer who owned the land on the other side of the railway bridge was approached, and agreed to move his cows, mow the field, and paint a big white spot there (for an undisclosed fee).

A small problem remained: there was a ditch between the landing field and the road which was council property. So along came the council workers to build a bridge over it. On the day of the demonstration the entourage of ministerial flunkies arrived early in their chauffeur-driven limousines, and parked in the road by the new bridge ready to greet the minister and ferry him all of 300 yards to the entrance to the labs. Would the minister care for tea or coffee? “Tea please –with lemon”. That request had not been predicted. The canteen manager did not have any lemons. Thus the gatekeeper was dispatched to Horley on his bicycle to buy one. Meanwhile the pilot of said helicopter, and a mere artisan, had been on the radio to base and walked (as befitted his station) those 300 yards to join the assembled company. “Sorry Minister, but the fog is closing in and we must return immediately”. Off they all went leaving a trail of half-eaten biscuits and half-empty cups having seen nothing! And then  … and then, arrived the gatekeeper: “who wants this lemon then?”

The so near and yet so far bit! We did give quite a lot of demonstrations to set makers at MRL, Salfords, and there was a good relationship with engineers from nearby Kingswood Warren who quite often attended. These were my first meetings with BBC engineers, and my desire to work for the BBC was strengthened even more by that experience, and further enhanced by the not infrequent visits with the choir I sang with to Maida Vale studio 1 for Prom rehearsals and the occasional broadcast.

I admit that what follows now is a diversion, but I make no apology for it because I am sure it will give you a laugh. The Mullard labs layout was modular, with versatile cabling which enabled anything to be connected to anything via a central apparatus room. On top of the building stood a steel tower on which were mounted various aerials e.g. one pointing at Alexandra Palace, and another at Holland so that we could receive continental 625 line transmissions. The aerial signals were routed via distribution amplifiers to each lab. As a result of a “what if..?” experiment, it was discovered that if the output of the AP aerial was amplified and fed up the spout to the “continental” aerial (with due adjustment of gain and phase) a very high-Q notch bandstop/ bandpass filter could be implemented. A novel implementation of the transversal filter! One could use this either to remove completely a specific set of frequency bars from test card C, or alternatively to cause (say) the 2.5MHz bars to appear at the left of the picture, and continue at constant amplitude right across to the right-hand edge of the picture. And of course one could completely remove the colour subcarrier! This was done on the occasion of one fairly high-powered demonstration at which the best tea service had been brought out, and the cucumber sandwiches passed round. There they all sat looking at black and white pictures! Soon there were frantic ‘phone calls to the BBC “why aren’t the scheduled colour test transmissions going out?” “Oh, but they are alright at our end.” We eventually put our colleagues out of their misery, and magically colour pictures appeared on the screens. That prank was kept as a well guarded secret amongst the cognoscenti!

Having got into anecdote mode, there is one more I must impart. Prior to the start of the 625-line UHF colour service, we at Mullards participated in signal assessment trials in a sector radiating from Crystal Palace. We owned an ex-US-Navy mobile workshop replete with towed generator, and we used to drive around looking for places to park-up this huge vehicle which was about the size of a double-decker bus. An extending ladder which supported the receiving aerial was winched up, and we made field-strength measurements and assessed picture quality before moving on to the next site. At Southborough, Tunbridge Wells, we spotted a big manor house with a long wide driveway, and this looked an ideal place to stop. This manor house turned out to be occupied and used as offices by the local department of Inland Revenue. Along the side of the driveway was a row of telegraph poles carrying a large number (~50- pairs?) of bare copper telephone wires supported on white porcelain insulators. Unfortunately in the process of winching down the aerial, we managed to “twang” one of the lower wires which promptly wrapped itself round all the other wires. With visions of red-faced tax inspectors frantically tapping the hooks on their ‘phones: hello! hello! and realising that we had put the local tax office out of action, we fired-up the 6 litre petrol engine, and  went up through the 7 gears and up to the 2000 rpm red line for the fastest get-away possible: all of 25mph!

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