HUGO HOLDEN from Australia has done something quite amazing - taken a 1939 His Master's Voice type 904 5-inch table model television and rebuilt it to concours standard.
Here is his story ....
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INTRODUCTION The HMV 904 is a 5" television, and in addition it's a multi-band radio. It was introduced in the United Kingdom in 1939.  BACKGROUND Electronic television technology was exploding on both sides of the Atlantic in the years leading to WW 2. The BBC began transmitting in 1936 from a hill in London, the "Alexandra Palace Transmitter". The Palace housed the studio and the transmitter, and remained operational until a new transmitter was built in Birmingham in 1949.
The video carrier was 45 MHz, AM modulated, with positive modulation: i.e. synchronizing pulses reduced the carrier and white level increased it, opposite to the American system. The sound carrier was AM modulated, 41.5 MHz and nominally 7 dB down in level with respect to the video carrier. The EMI television system specified 25 frames per second, interlaced scan, 405 picture lines, and as a result a field frequency of 50 Hz and line scanning frequency of 10,125 Hz. Many observers have commented on the audible whistle of the line output stages in British 405 line Television sets. The modern British 625 line standard with the 15,625 Hz line frequency is beyond audible limits for many people.
The idea of restoring a 405 line set seemed very appealing. After comparing and contrasting the two American pre-war sets, it was really an irresistible idea to restore a British set of the time in order to experience the performance first hand.
THE HMV 904 This television set is quite remarkable as it is also a 6 tube multi-band radio of very compact design. The radio tunes over 16.5 to 50 metres (short-wave), 200 to 570 metres (medium wave) and 725 to 2000 meters (long-wave) with a very elaborate dial and chain drive vernier scale. The set cost 29 guineas new and a 7 inch crt version, the 905, was available for 35 guineas. The tubes employed in the radio frequency stages of the set and local oscillator and audio stages are shared in both the television and radio modes. This is achieved with a fairly complex arrangement of IF transformers, combined multi coil units and a very elaborate multi-wafer band-switch. The IF transformer coils in the television section have large brass tuning slugs and this technique results in a decrease of inductance of the coils they tune. The 904 does have some unique circuit features, which will be discussed. These include the frame output stage, the line output stage (without a damper diode) and the very impressive "anode bend" detector/combined video output stage. Firstly though, the 904 employs magnetic deflection for line and frame, and magnetic focus. Unlike the two American pre war sets, which employ electrostatic deflection and focusing. The tube types will not be easily recognized by most American readers, as most don't have USA counterparts, though some do. The line-up is as follows:
The set I have was sold to me in original order, less the cathode ray tube, by the Early Television Foundation in the USA. They had acquired three, restored one for their collection and sold the other two. The set had a number of fairly severe problems. Firstly the CRT - an "Emiscope 3/1" was missing. There was extensive chassis rusting, everything that was steel had rusted; mechanical parts, screws, bulb sockets small brackets etc. There was moderate corrosion of all the aluminum parts. Underneath the chassis the wiring was disintegrating, in some places the insulation had turned to powder. A reminder of just how old this set was. Every wax paper capacitor leaky, every electrolytic faulty, and badly corroded tube sockets. Some of the resistors were still ok and fortunately all the important parts such as the RF coils, IF transformers and power transformers turned out to be ok. The main dial was in good order but the vernier dial very rusty with flaky paint. The cabinet would require complete refinishing.  The task began in documentation of the chassis wiring. Due to this set being a TV/multi band radio the switching is enormously involved and the wiring and component placing very crowded. It took almost two days to accurately document the wiring in the rotary switch areas and multi winding coils to ensure accurate reassembly. The set was then stripped down completely. The chassis, brackets, multiple rusted mechanical parts, including the variable capacitor frame (from the radio section) and bulb sockets were all fine bead blasted to remove all traces of rust and electroplated with the process of "electro-less nickel" I have a preference for this because it electroplates into corners and down holes, so is excellent for complex shaped objects. It has a great satin silver metallic look to it, resembling the original plating and has excellent longevity. This can be further improved with a coat of clear lacquer. It was not practical to re-plate the hundreds of rusty screws in this set. So I obtained new ones of identical geometry and original BA threads, which were readily available. The aluminum components were polished and lacquered for protection. The tube shields were a composite of steel and alloy, were treated with rust convertor and ultimately after a lot of preparation painted with fine silver lacquer. The yoke and focus coil assembly also received the same electroplating process but were again panted with black lacquer to match their original finish. The vernier dial was repaired by scanning first, re-plating and re painting it. Then I doctored the image in Photo Studio software and printed out a replacement scale to apply to the re painted dial. The electrolytics were replaced and the wax paper capacitors re-built with new caps placed inside and the ends sealed with polyester resin. The large EHT filter cap was also re-built. The original tube sockets were badly corroded and unreliable, so I replaced them all with high quality ceramic sockets, which after a lot of hunting turned up in the UK. Finally the set was reassembled with the original under chassis layout and original tag-boards with the re-built capacitors and many new resistors too. A few of the originals were still ok. An excellent set of set of NOS tubes, including the rare X41C, were purchased from the helpful people at Colomor Electronics, in Billingshurst, West Sussex in the UK. A few challenges lay ahead.
 INTERESTING CIRCUIT FEATURES Frame Output Stage The vertical deflection yoke in this set has a relatively large number of turns and a high DC resistance, 5K. The output tube's anode load is a 10 K carbon power resistor. The yoke is coupled to the anode by an 8uF electrolytic capacitor from the anode of V12 and returned to the cathode of V12. The load, unlike modern magnetic deflection circuits, is dominantly resistive, not inductive reactive. The anode voltage waveform in this set is nearly perfectly saw-tooth in character to produce a saw-tooth scanning current. (When the load is partially reactive the correct drive waveform is trapezoidal, ie, a combination of a saw-tooth and a rectangular wave to result in saw-tooth scanning current). The plate resistor they have used is very inefficient however it does provide a satisfactory degree of damping and doesn't occupy much space and obviously an inexpensive option. Line Output Stage This is an interesting stage based on V13 and V14. The blocking oscillator is configured in the screen grid circuit of V13 and the output derived from the plate to drive V14. Feedback from the output transformer to the oscillator transformer via C85 appears to assist rapid flyback. The output transformer core can just run satisfactorily at 10,125 Hz. If this circuit is set to run faster the linearity suffers badly with compression of the left side of the raster. The line yoke coils have a very low DC resistance around 11 ohms, and represent a very inductive load. There is no damper diode, and the damping is merely resistive. This damping and to a degree the linearity, is adjusted by a control labelled "Form". Despite this, the linearity on the correct scanning frequency is quite acceptable.
It appears that the first person to postulate the use of the damper diode, in 1936 in the UK, was Alan Dower Blumlein, the "inventor" of stereo audio. He patented "binaural audio recording" in 1931. Tragically he was killed in a plane crash in 1942 while testing radar. Damper diode function was very well examined by RCA laboratories, (post war period), in an article by Otto. H. Schade. ref.(1) In this article reference is made to Blumlien's original patent for a non-linear deflection circuit with diode, 1936. Over the years "efficiency diode" or "booster diode" became synonymous with damper diode. In these early years it became obvious that magnetic deflection circuits really only need be energy control/management systems. In deflecting a beam about center no overall energy would be required, only enough to overcome losses. This is analogous to a swinging pendulum, requiring small amounts of additional energy per cycle to keep in going. Despite the early work by Blumlein in the UK, the damper diode concept had not found its way into the HMV 904. Detector/video output stage This is based on the MS4B,V9, a metalised glass tetrode, biased to be an anode bend power detector. This is the first time I have encountered this in a television. It is a very good idea. The anode is direct coupled via L29 C60 and R65 to the CRT's cathode. In effect the tube is biased as a class AB amplifier. The no signal plate current is very low compared to its class A counterpart used in most television sets. This saves power loss in the anode load resistor. The grid is driven directly with the video carrier, and the positive half cycles of the carrier are preferentially amplified due to the bias conditions being set for that mode. The carrier is filtered off by L29 and the associated capacity of the components and cathode circuit of the CRT. Oscilloscope analysis of the detected and amplified video shows it to be excellent, and 25 to 30 volts peak to peak video without any difficulties.  ELECTRICAL ALIGNMENT Following the manufactures advice I set up the RF, oscillator and IF stages, first the radio section and then television. Due to the sound and vision IF being common there is interaction between the two and when one is adjusted the other must also be re set. After completing the alignment I swept the IF in the usual way, and much to my astonishment found that the intended video IF bandwidth was only 1.4 MHz. Despite this the screen image was acceptable. With a few minor adjustments, and the use of the sweep generator I was able, without any modifications, to get the bandwidth to 2.4 MHz. This substantially improved the picture detail. SUBSTITUTE CRT A replacement emiscope 3/1 CRT could not be found. One fellow in the UK told me he had been looking for one since the late 1950's and had no luck. Steve McVoy of the Early Television Foundation suggested a 5FP4. This is a magnetically deflected, magnetically focused tube that was designed post war by RCA for the viewfinder on the TK30 camera. The radar 5FP7 version dates from 1942. I located some 5FP4's and started testing. The neck on the 5FP4 is a little larger than the 3/1, but removal of a small amount of cardboard from the center of the yoke allowed it to just slip over the neck of a 5FP4. The 5FP4 makes an excellent substitute, see the un-retouched screen image photo. The tube specs suggest a minimum EHT voltage of 4Kv. I have had no difficulty running it on the 2.4Kv in the 904. It also became obvious right away that the magnetically focused 5FP4 is superior to both the 5BP4 and 5AP4 in the Meissner and Andrea (US-made TVs) respectively. The latter tubes lose focus as the beam intensity increases or is varied. This is due to the influence of the grid voltage on the beam and changing relative potential with respect to the focus electrode. The 5FP4 on the other hand maintains excellent focus at all beam intensities, however as the set warms up with time the focus coil current changes and requires re adjustment with the front panel focus knob from time to time. I don't think constant current sources were on people's minds back then. 405-LINE SOURCE Vintage television collecting is becoming quite popular in the UK. A few talented people have turned their hands to making standards convertors. These receive a 625 line video source, basically digitize it and download it to memory and then read it out at the lower 10,125 Hz line rate. I acquired a standards convertor as a set of two boards and small motherboard from David Grant (an exceptional Electronics Design Engineer in the UK). I designed and built my own modulators modifying some existing Aztec units and providing appropriate clamping and polarity inversion for the video. In addition a mixer amplifier and mini circuits, an RF attenuator was used to control the RF levels. This unit effectively recreates the signals generated by the original Alexandra Palace transmitter. The unit can provide up to 14 mV rms RF out into 75 ohms, but in practice 3mV is a suitable level for the 904.   CONCLUSION The overall performance of the 904 is very good. The radio section is an excellent performer too. The CRT image is quite acceptable despite the relatively low bandwidth, primarily because on a 5" CRT the lower resolution is not as noticeable. The benefits of magnetic focus is obvious, so despite the poorer IF bandwidth compared to the Andrea and Meissner, the overall picture is comparable over a range of contrast settings on the three sets. The sound on the 904 is very impressive. These prewar TV sets have a relatively wide bandwidth in the sound channel compared to standard AM transmissions on medium or short-wave. The audio quality to me is indistinguishable from FM sound in modern sets. The effect is enhanced by the usual class A audio output stage and good sized wooden cabinet with a permanent magnet 6 inch speaker in the 904. I have no doubt that the deflection coil and focus coil assembly and the line output transformer in the 904 would have been more expensive to produce than using electrostatic deflection. This may have been compensated for a little with the simpler magnetically deflected CRT. On the other hand the 5AP4 and 5BP4 CRT's with their more elaborate gun structures probably cost more than the emiscope 3/1 or the 5FP4 to produce, but probably not by a great deal. Ultimately magnetic deflection was to win out over electrostatic. Primarily this was due to the difficulties of acquiring high enough linear saw-tooth voltages as the EHT was increased for larger tubes. Linear amplifiers producing them would have to run off very high HT voltages. In electrostatic deflection the amount of deflection is inversely proportional to the EHT (final anode voltage). So if you double the EHT you need double the deflection voltage for the same width/height. In magnetic deflection the amount of deflection is inversely proportional to the square root of the EHT voltage. So if you double the EHT voltage then you only require a x 1.414 increase in the deflection current for the same width/height. Finally one cannot be unimpressed by the level that television technology had reached by 1939. Viewing programs on these sets is an experience not a great deal different from observing them on any black and white television manufactured decades later. Hugo Holden References
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