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Pre-War Television

Early Days

In 1934 the Marconi-EMI Television Company Ltd was formed bringing together the vision transmitter technologies of the Marconi Company with the television studio equipment and receiver technologies of the EMI Company.  It was this company that provided the technology for  Britain's television service in the years from 1936 through to the switch-off of the 405 line transmissions in the early 1980s.

The very first dedicated television receivers developed by this alliance were the HMV model 901 and the Marconi model 702. These sets were essentially identical but had differences in the veneering of the cabinet and the shape of the speaker grill so as to appeal to traditional customer loyalties for the two brands. The same design of television circuitry was also incorporated into a number of other sets carrying the HMV and Marconi names that included radios and gramophones.

This web page takes a look at one of these early receivers.

The HMV 901 Television

A 1937 HMV television displays an image on its vertical Emiscope 6/6 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 layouts:    

 

                                                                                                 


 

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. If you click on the test card image you can see that this 80 year old set is still capable of reproducing 2.5MHz bars quite well and even gives a fair impression of 3MHz.  This corresponds well with the characteristics of the Alexandra Palace transmitter of 1936 in which the overall response curve of the vision modulator was designed to be flat up to 2.5MHz and in practice was less than 0.3 dB down at 3MHz.

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. Early sets have a less compliant screen mask that has 45 degree corners. The style of control knobs also changes between early and later sets.


 

 

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 changer 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. The 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

In January 1935 the Selsdon Commitee, whose brief was to advise on the future of television, made its report. It proposed that the BBC set up a television station in London but it was unable to decide on whether the 240 line Baird system or the 405 line EMI system should be used and thus both systems should be trialled. The HMV 901 was one of the sets designed to meet the needs of this trial period. The early production units were fitted with a "System Selector Switch" and one additional valve in the timebase chassis. Blanking plates were fitted to the switch panel of sets manufactured after the decision was taken to proceed solely with the EMI system and most early sets also had their switches removed if they were subsequently refurbed by the factory.

You can compare 405 line and 240 line test cards images here.

Looking at a photograph of a 240 line Baird image it appears to compare quite favourably with the 405 line EMI images but what you are not seeing in the still photograph is the very noticeable flicker from the Baird 25 Hz frame rate. Apart from the political motives for trialling the Baird system it is really quite surprising that it was considered at all given the lack of a viable camera and the flickering picture.

The System Selector Switch has 4 poles and is marked S1 in the old timebase schematic. See Service Info

Whilst EMI took a very large gamble in deciding to develop a 405 line system  using the Emitron camera that was based on the Zworykin Iconoscope there was really no contest when it came to the trial. The Emitron 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 a combination of its distorted image and its lack of sensitivity and thus almost all Baird's transmissions were derived from mechanical scanning.

The Emitron Camera

Click on the camera for more informatio

 

For a fuller account of the start of High Definition Television in Britain see: BFI ScreenOnline

The Wireless World

 Click below to see what The Wireless World said about the H.M.V. Television Receiver Model 901 in January 1937

 

 

 

 

 

 

Production Changes

The Wireless World tested a very early model of the HMV901. In the photos below you can see the following differences with later models:
1. The speaker aperture is reduced in width and the veneering pattern altered in later models.
2. The metal screen surrounding the CRT gives better access to the scanning and centring coils in later units.
3. The large EHT smoothing condenser is moved to a position beneath the sound receiver thus giving access to the CRT base connections.
4. The valve V8 in the timebase chassis (just to the left of the TCC condenser) that is required for use with the Baird system has been removed in later models


Triptics

Whilst there don't appear to be any survivors of the early WW style HMV901 the earliest surviving sets do have the longer CRT metalwork and the components of the additional valve used to maintain regular line timebase behaviour through Baird System frame synch pulses. The set pictured below has the longer CRT metalwork and a vacant valve holder on the left of the timebase chassis whereas other sets have blanking plates.  I believe that sets without the Baird circuitry  have their plates mounted on the underside of the chassis whereas sets that do have the circuitry fall into two camps, those with and without a plate. In this case the plate is mounted on the top side of the chassis. These plates could have been fitted at a subsequent refurb but this would have required the valve holder rivets to be drilled out so perhaps these plated sets were just the last production run prior to Baird deletion.

 

A Prototype HMV901 / Marconi 702 ?

                

A few years ago a set came to light that externally looked rather similar to the HMV901 that Wireless World tested. This may very well have been a prototype for the HMV901 and its cousin the Marconi 702.  The set differed from the WW set in that it had its controls mounted on the front of the cabinet and a 9" CRT instead of the 12" but it had a very similar veneering pattern and design of speaker aperture. The owner believed that it had been displayed at Radiolympia in 1937 but this seems at odds with the reports in Wireless World. There are quite a number of reports and of course a substantial interest in television at the time and whilst the models on display were clearly referred to there is no mention of this set in either the 1937 or 1936 Radiolympia reports. If it was a prototype for the 901/702 and the other sets that shared this generation of design, then it would be at least a year older than the 1937 date quoted by the owner. It certainly bears little resemblance to the second generation of EMI televisions so it seems unlikely that it was a prototype for anything later than the first generation.

The chassis was totally different to the HMV901 / Marconi 702 and mounted at a slight angle to the horizontal and divided into a timebase on one side of the CRT and an RF amplifier on the other. It appeared to have one fewer RF stages than the 901/702. The power supply was mounted in the floor of the cabinet and the EHT parts were shielded in similar style to the 901/702. The set had no back panel and no obvious way that one was ever fitted. Nor did it carry any HMV or Marconi decal or serial plate but given its appearance and construction it seems very likely that it is a prototype for the very first commercial EMI television receivers.

Establishing the  Manufacturing Date

Most sets have a small white plastic serial plate mounted on the rear. The coding of these is not entirely obvious but the 4 digit numbers stamped onto the chassis assemblies probably indicate the time sequence of manufacture. Some data can be seen here.

 

Studio Monitors

Given that Marconi-EMI were supplying all the studio and transmitter equipment then it's no surprise that the BBC also used their domestic receivers as studio monitors. The viewing window on the right of the control room sketch overlooked the EMI studio and can also be seen in the centre of the Alexandra Palace cut-away drawing.

 

Acquiring an HMV901

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 in 1978 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
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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.

Conversion


The converter in the picture above 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.


Darius


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

 

An Alternative to Standard Conversion

Recently it has become possible to generate both 405 line and 240 line signals displaying any form of video or still images by use of a PC with a graphics card. The comparison between the two systems is interesting. The increased line rate of the 405 system is noticeable but the presence of flicker in the 240 line system is the most noticeable difference.

Some information on this technique is given here.

 

 The Aerial Plug

You may notice in the museum photo that the aerial cable and plug are substantial to say the least. Whilst my set came complete with the original cable it was missing the plug. Being a sucker for ancient technology I had been looking out for a replacement but in more than  30 years of looking I was unable to track one down. Recently I mentioned this on a vintage radio forum  and a very kind and skilful fellow forum member offered to make a replica and also the mating socket. Many thanks also to the owner of the original plug who had some years earlier created drawings and more recently made his plug available for copying. Below are photos of the original EMI plug with ruler and the artificially aged replica fitted to my aerial cable. The co-ax itself has two layers of braiding and gutta percha insulation for the inner conductor.


A short video of the fashion parade from the 1937 Television Demonstration Film

These short videos show the HMV901 displaying clips from the BBC 1937 television demonstration film. The video and sound signals are being converted for 405 line operation and modulate the original 45MHz/41.5MHz carriers which are fed into the aerial socket of the HMV901. Occasionally you will see narrow dark horizontal bands on the the picture. These are only aliasing effects from the camcorder that is capturing the scene and are not present in reality.

.. and some longer clips from the 1937 Television Demonstration Film

 

 

Viewer Reactions

In October 1936 the Wireless World reported the impressions of an observer in Welwyn who had been watching the early program material from Radiolympia and Alexandra Palace.

"The features which drew the greatest appreciation were the outdoor shots at the Alexandra Palace grounds (which never failed to excite the astonishment and wonder of those who had come with no idea of what modern television can do), the variety entertainment in the studio, with its striking use of successive shots from various angles, and, among the films, the news reels and the excert from "Show Boat."

On the other hand, in studio programmes, head-and-shoulder views and interviews soon bored the watchers. Frequent change of scene and viewpoint seems to be probably even more important in television than in the cinema. This is where the variety show scored, and although the "Show Boat" excert was simply of Paul Robeson singing "Ol' Man River" the film producer has presented this song with rugged simplicity of masses, and of light and shade, that it makes perfect television entertainment. Films with intricate small detail (particularly noticeable in captions) and films which were static, were the least effective.

The lighting of studio scenes seemed to vary considerably, but of course in this, and in the matter of make-up, the television producers will have to feel their way for some time. When the camera moved rapidly towards or away from an artist there was a frequent tendency to go out of focus, evidently because the cameraman has at present no sufficiently positive rapid-focusing device.

The most remarkable effect of television in my house was its enthusiastic adoption by children. They have never shown any great interest in sound broadcasting - even the Children's Hour. But television they watched with rapt attention, and with screams of laughter for the comic horse in the variety show; and not merely once. They insisted on seeing the programme through again and again as it was repeated daily for Radiolympia. They never tired of it, which seems to indicate that it was not the "novelty appeal" of television which was having effect, but real entertainment value."

After the official opening ceremony on 2nd November 1936 the following programs were listed...

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...and when there was nothing else, you could always sit and watch the tuning signal.

 

 

Visitors to the Radiolympia show in August 1936 could see television demonstrated on a number of different manufacturers' sets

The early BBC Tuning Signal shown in the domestic scene above is rather interesting. The Royal Scottish Museum set had a paper

copy of this placed on top of its CRT screen. Other early Tuning Signals can be seen here.

 

By 1938 receivers were still quite expensive items and a number of home build designs were available. Television & Short-Wave World announced their simple design in March 1938. In the May 1938 edition the designer, Spencer West published results photographed off the screen as received  (near Beccles, Suffolk. See map below.) 97 miles from the Alexandra Palace transmitter. Then in June 1939 a Mr W.F. Steel reported using an earlier, more complex  receiver design also by Spencer West and published in the same magazine over several issues starting in October 1936 but with an added RF stage to receive pictures in Minehead, a distance of 170 miles from the transmitter.

The aerial used by Spencer West in Beccles was a rather interesting Yagi with two directors and a reflector hung as as if from a washing line.

After the war in 1946 the BBC restarted television broadcasting using the same 405 line standard and this continued until the early 1980s.  The following film clip was used for trade test transmissions in the hours not covered by normal program transmissions. You can see the EMI control room and control gallery in operation. The video was taken from the screen of the HMV901 whose rear view is shown at the start of this web page.  The vision and sound signals are being received at 45 and 41.5MHz respectively from an Aurora standards converter.

 

Oldest Working Television

Jeffrey Borinsky holds the title for the oldest working television in Britain.

Jeffrey's set carries the Marconiphone badge and has a slightly different cabinet

design but is otherwise the same as the HMV901.