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The HMV 901 Television

A 1937 HMV television displays an image on its vertical cathode ray tube. In 1984 transmissions using 405 lines came to an end but recent developments have made it much easier to operate early receivers so again it is possible to experience pre-war television.   Early cathode ray tubes (CRT) had considerable length due to their small deflection angles and large electron guns consequently set manufacturers often mounted the tube vertically in order to reduce the depth of cabinets. The picture is viewed through a mirror mounted in the lid. At this time there was only one television channel and one transmitter. This set is designed with a straight (TRF) vision receiver tuned to 45 MHz. and consists of six RF pentodes and a diode detector. These are contained in the screening cans on the left. There is no video amplifier. The detector connects directly to the CRT. To the right of the CRT are the synch separator and line and frame timebases and on the floor of the cabinet is the power supply chassis. The 5000 volt supply for the CRT final anode is generated from a step up transformer from the mains and this and the EHT rectifier are located below the timebase chassis in a metal box behind the two brightly lit HT rectifiers. Only a small part of the sound receiver is visible in this picture. It shares the first two RF stages with the vision receiver and its RF connection can be seen between the second and third cans from the top.  Click on the picture for the valve layout.     Click on the diagram below to see the schematics and layoutsb:                                                                                                                                                                                   The test card image shows a slightly darkened central area and a concentrated dot in the greyscale blocks caused by ion burn.  The CRT has no ion trap and no screen aluminising.  The EMI service notes say "A slight amount of frame crushing at the lower end of the picture is standard but in special cases only, a frame linearity control can be added provided that the Frame circuits have been thoroughly investigated." This set has no frame linearity control and shows the "standard" crushing. Another optional add-on was a picture centring coil or "push about coil" as EMI described it. This set had been fitted with one but it was found to distort the focus at the right side of the picture and adequate centring could be achieved simply by pushing the tube neck slightly to one side or another, thus shifting the screen within the rubber mask, so the coil was removed.   The 1984 Remembrance Ceremony and the "untouched photograph" from the November 1937 Wireless World which looks better than I got 47 years later.   The Queen gives her Christmas message just before the 405 line service was closed.   This photo and the previous colour Cenotaph pictures were taken whilst receiving the signal from the Kirk O' Shotts transmitter radiating on channel 3. You can just see the indoor aerial rod projecting up the back of the set. The HMV 901 is only designed to receive channel 1 so a small frequency charger was built to feed the aerial input without modifying the set.

A Real Television Picture Broadcast from 1939.   Whilst many of the Baird system broadcasts involved an intermediate film process virtually no pre-war  recordings of broadcasts from Alexandra Palace are known to exist. There are however one or two contemporary still images taken from the screens of television receivers. TThe image on the left shows the announcer Elizabeth Cowell and was broadcast from Alexandra Palace in 1939. The 1939 off screen image was copied as an MPEG and then converted using a standards converter to 405 lines and retransmitted to this pre-war television.

The Trial

Whilst EMI took a very large gamble in deciding to develop a 405 line system  based on the Zworykin Iconoscope there was really no contest when it came to the trial. The Iconoscope camera gave sufficient sensitivity to permit both studio and outside broadcasts due to its charge storage technique and cameras were portable although restricted by connecting cables. Against this Baird really had no practical camera. For studio work he was forced to employ either a spotlight scanner where the subject was scanned by a single point source of light in a darkened room whilst groups of photo cells picked up the reflection or a cine film camera that was fixed to a massive high speed development bath combined with a mechanical scanning disc. The latter had to be bolted to the floor and mounted behind a glass window to suppress its noise. The only electronic and therefor portable camera that Baird had was the Farnsworth Image Dissector which, unlike the Iconoscope, had no charge storage and relied on an electron multiplier to achieve any form of sensitivity.  The Farnsworth camera proved to be virtually unusable due to its lack of sensitivity and thus almost all Baird's transmissions were derived from mechanical scanning.

What The Wireless World said about the H.M.V. Television Receiver Model 901 in January 1937

TELEVISION reception is still something of a novelty, and in all its aspects is full of interest. Unlike the usual sound receiver, there is as yet no hint of standardisation, and most television sets are quite different from one another in nearly all their details. To the technically minded this is but an additional attraction. The H.M.V. television receiver is really two sets in one, for the vision and sound channels have only the initial circuits in common. The vision receiver is a straight set  with six RF stages feeding a diode detector. The coupling between the last RF valve and the detector is a two-circuit band-pass filter, but single-circuit couplings are used elsewhere. The eight tuned circuits thus employed are not all tuned to the same frequency, but certain stages are staggered, so that an overall response curve is obtained which is sensibly -flat over a wide range of frequencies on either side of 45.0 Mc/s. The operation of the equipment is extremely simple, and the instructions accompanying it are lucid. Since few are yet acquainted with the procedure, it may be as well to summarise here the details given in the H.M.V. operating instructions. These assume that initially none of the panel controls are correctly set. After setting the System Selector Switch for the appropriate transmission (Baird or E.M.I), the steps are as follows:— 1. Turn the Brightness control fully to the left. 2. Switch on. 3. Turn the Sensitivity control fully to the left. 4. Turn the Contrast control fully to the right. 5. Slowly turn the Brightness control until a faint illumination appears on the screen. Then turn back until this just disappears. 6. Turn the Sensitivity control to the right until traces of the picture appear on the screen. Continue turning until a light and dark pattern appears on the screen. 7. Now slowly turn the Frame Hold control until the pattern formed on the screen, which will be moving from top to bottom or bottom to top of the screen, becomes stationary. 8. Turn the Line Hold control until a steady recognisable picture appears. 9. Bring up the half-tones if necessary by manipulating the Contrast control in conjunction with the Brightness control. After this the sound is tuned in by the tuning control provided, and the volume adjusted to the required level. The receiver has been thoroughly tested for a considerable period in north-west London and used with the special resonant aerial and feeder supplied by the makers. It has given consistently good results, the picture being extremely bright and thus enabling entirely satisfactory results to be secured without any darkening of the room. Normal illumination, daylight or artificial, can be retained and all details of the picture still seen. The size of the picture is about 10in. by 8in., and the optimum viewing distance is about 6ft., the detail and contrast being extremely good. The synchronisation is practically perfect, and it is very rare indeed for a frame to slip or a few lines run out. Indeed, when this does occur it is a sign that the controls are not properly set, and this is quite possible, in spite of apparently good synchronising being secured, for the synch, hold is so strong that considerable latitude in the setting of the controls can be tolerated. Unless the settings are correct, however, interference may cause momentary faults. The testing site was close to a main road with a constant stream of traffic and also close to a bus-stop, so that the conditions were hardly ideal from the point of view of freedom from ignition interference. In spite of this, however, interference was by no means troublesome ; it had no apparent effect on the synchronising, and was only occasionally visible in the form of spots on the picture. This speaks well for the screening and the feeder system adopted. Sound reception was equally satisfactory and of a really high standard of quality. The equipment is, of course, for AC operation, and it consumes 230 watts. The. receiver is supplied complete with a special resonant aerial and feeder and is priced at 95 guineas. This includes installation by the company's engineer of both aerial and set, and maintenance of receiver and cathode-ray tube for one year. The valves, of course, carry the usual- three-months' guarantee.

When I was a kid I used to love visiting The Royal Scottish Museum in Edinburgh. It was a fascinating place crammed with wonderful technological exhibits and the one that fascinated me the most was a 1936 HMV television. The model 901 was displayed so that you could see all the works and it used really vintage technology, valve types that dated from the early 30s. For me, growing up in the 1950s, television was a modern thing but here was a television from the past. The tube had a very small deflection angle and was therefor rather long and had to be mounted vertically in the cabinet and viewed through a mirror in the lid.

Anyway, time moved on and the Science Gallery shut down for many years of renovation and my favourite exhibit was consigned to the store room. Unfortunately, when it reappeared it was pushed to the back of crowded display case with no view of its vintage works.

Here are a couple of shots of the museum's set in store.

Moving on further in time I subscribed to a newsletter for old radio enthusiasts and decided to place an advert for a pre-war television. At the start of the war about 16,000 sets had been sold in Britain but I was aware that the survival rate was very low.

Whilst some early televisions had formed parts of radios and radiograms I really wanted one that had no other purpose than as a television and ideally it should be manufactured by EMI given that it was their research and technology that had led to the birth of real television in Britain.  My ideal was either a Marconi 702 or an HMV 901.

I received two replies to my advert, one from someone else looking for a pre-war set and asking if I could pass on any replies that I wasn't interested in, and much to my delight, a reply offering an HMV 901. The only slight drawback was that I was in Edinburgh and the set was in Cornwall. As I recall the seller wanted £100 for it. This was 30 years ago and that was quite a lot of money but we set off in our Reliant Rebel Estate (which had cost less than the HMV) and needless to say I bought it. Being a fragile box of tricks it lay on a double air mattress in the back of the car and did survive the journey.

Below: Following purchase in January 1979 the HMV901 is loaded into the back of my  Reliant Rebel  for the 500 mile trip back from Cornwall to Edinburgh.

Having not been used for many years it took me quite a bit of time to get it working. The worst problem was the mains derived EHT transformer that was burnt out but fortunately my employer had a transformer shop at the time and after a couple of goes at rebuilding it I got a design that gave sufficient insulation to work and survive. You can see some of the service data and the transformer design here .

The set was only designed to receive signals from Alexandra Palace (Channel 1) so I needed to build a little frequency converter to get it working from our local Channel 3 transmitter. That worked well until the switch off of the 405 line service in the 1980s after which my set lay fairly dormant until a few years ago when I built a 625 to 405 line standards converter based on a very clever design by a German enthusiast.

Standards Conversion

The 405 line broadcasts were closed down in 1984.  In the years leading up to 1985 the broadcast authorities produced programs using the current 625 line standard and with the aid of a 6 foot high rack of equipment were able to generate 405 line versions of the same programs to maintain the old service. Technology has moved on considerably since 1984 and a number of innovative designs have been produced for standards converters in recent years. Instead of 6 feet of equipment, the converter can now be contained in a small metal box.

The converter in the picture on the left is based on the designs of Darius Mottaghian and is shown converting today's broadcast material into the 405 line signal that is being received by an early 1950s Bush TV32.

 

Darius and his first converter.

The converter uses 3 charge coupled device delay lines. Two of these form the 625 to 405 conversion and the third provides interpolation for the one line in every 3 that is discarded. Analogue samples from a 625 line are clocked into one of the CCD delay lines whilst the other is having its stored line content clocked out at a rate suitable for the longer duration lines at 405. When this is complete the roles of the two CCDs are reversed etc etc.

You can get more detailed information about Darius' standards converter here

Clips from an early television demonstration film