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  COMPLETE RESTORATION OF A PRE-WAR MARCONIPHONE 702 TELEVISION SET
               Page 1


The content requires some technical knowledge in order to follow fully....

PHILOSOPHY OF THIS RESTORATION
This lovely old television set is now over 70 years old and is also very rare. This will be a 'sensitive' restoration - the project was forced on me by circumstances. However the opportunity has now arisen to restore this set to a higher standard of truth to originality than was previously the case. Evidence of post-war servicing will be removed or hidden. Cabinet damage will disappear. My idea will be to 'wind back the clock' and achieve an end-result where this television does not appear 'restored' but instead, closely resembles the state it was in when new, or perhaps had been carefully stored since new and barely used. This television set represents the very birth of a technology which has since transformed the world. In the future, it will be seen as an incredibly rare and historically significant artifact. It now falls to our generation to preserve and enhance this heritage for the centuries to come.

This very original mirror lid set had been in my possession since 1983, when it was rescued from a tip in south-east London and sold on to me for the princely sum of £50. I then repaired it to working order, using a spare cathode ray tube which was (literally) kicking around in the BBC redundant plant stores at Wood Norton...

Unfortunately in early 2005 the shower outlet pipe in my bathroom sprung a leak. This pipe was embedded in the ceiling directly above where this set was standing !

 

Quite a bit of water damage was caused to the cabinet. In addition, the picture had fizzled out in late 2004, while a house guest and I had been watching a long series of 1930s Mickey Mouse cartoons. This was rather appropriate really, since Mickey Mouse had also been the last programme being viewed when the service closed down at the outbeak of war in 1939...

Sides of the cabinet are pulling away and the top finish has become ripply and rough. So a full cabinet and electronic restoration was now called for - to 'Radiocraft' standards of course...;-).

The blanking panel where the Baird-EMI 240/405 changeover switch once resided dates this set to early 1937 or after.. Underneath the spring-loaded flap we see the 'occasional' controls: form, width, focus and height. The 'form' is line-linearity.
 And just look at this heavy duty television aerial coaxial plug and cable for the outlandishly high frequency of 45 megacycles !  I believe there's a proper name for these connectors. Anyone know what it is ?

First to get it to the workshop. It needed two men and a trolley - it weighs approximately 13 stone, that's 182 lbs or 82 Kilos.

 So let's get started... The lid stay screw is removed and the lid carefully folded back. The mirror on this set is rear-silvered and is removed for safe storage. It looks original. There is some minor fading and light damage to the silvering. It would be nice to have this either replaced or re-silvered. The backing glass is very thin: 2.2mm - or less than 1/10 inch in thickness.

  The lid is removed, then it's time for a peek in the back.  Here we see the seven-pot TRF vision strip down the left side. The timebase unit (known as the 'synch' unit in the 1930s) lies to the right, with the very heavy power supplies along the base. Two U12 HT rectifier valves are used in parallel here to handle the load. The EHT mains transformer, its rectifier and the smoothing condensers sit toward the front of the cabinet.

In these very early televisions, the cathode ray tube sits vertically, facing upwards. This was necessary because at that time only narrow angle cathode ray tubes could be manufactured safely. Therefore to get to even a 12-inch screen size meant a long tube. The image on the screen was reflected forward by the large mirror on the inside surface of the lid, shown earlier.

 Even so, safety was still a big concern... If the tube did ever implode, the 'bang' would have been considerable. This thick glass was supposed to afford some protection to viewers. It is 6.6mm thick - over ¼ inch.
  And this is the view looking down onto the tube face. Only round tubes were available then, at least in Great Britain.

  The whole tube assembly with its screening cradle lifts out as one unit. There's a fair bit of weight being supported by the glass tube at this point so it's a good idea to treat it with respect...!

I experience some difficulty in removing the cradle for the tube; there are foam rubber pads interposed and these have stuck !

  In the end I ease this off by sliding a flexible plastic table knife up in between the surfaces. Even so, as you can see some of the foam rubber is left stuck to the tube glass.

This tube has been blown from Pyrex and the markings 'BK90' and 'R' become visible.

 This tube is a 12-inch Emiscope 6/6. I stop to look up at it and admire it... To me, this is highly reminiscent of that famous view of the Alexandra Palace mast !

The scan coils look rather heavy in their yoke. This won't be a good thing when the tube has to be later manhandled - it bugs me...

  So the heavy laminations are removed, leaving just the encapsulated coils themselves, which show no sign of wanting to be disturbed. I'll probably be leaving these in situ...
  Time to remove some more cabinet panels. This is the port side, seen-from-the-front. The side of the timebase complex is here presented, for easy servicing...

And the starboard... Here we can see the vision RF pots arrayed vertically. On their right sides they contain MSP4 valves; on the left their associated coils are hidden.

On the left of the picture (at the front of the instrument) we glimpse the sound unit. On the Marconiphone 702, this is a superheterodyne receiver with an intermediate frequency of only 1500 kilocycles. This relatively low I.F. means the local oscillator must run at high frequency. Even slight variations in this frequency can cause noticeable variations in the sound gain. That's presumably why EMI provided a manual 'sound tuning' control for the benefit of the user.

I always found it necessary to readjust this after about ten minutes from switch-on...

  Here's the cover removed from the EHT section on the power unit. The EHT ('extremely high tension: the voltage used to pull the electrons down the tube and illuminate the phosphor coating on the screen is 5000 volts (5Kv). Touching this when it's working is like doing pull-ups from electricity grid conductors, or volunteering to test out an electric chair. You won't be able to let go and it will definitely kill you.

To the right we see the replacement EHT transformer which I had made for this set in the 1980s.

And on the left is the U16 EHT rectifier valve. But wait a moment, there's something wrong isn't there ... ?

When I remove this valve, it falls apart in my hand. Not only is there the white deposit of 'gas', but there has been a catastrophic failure of the glass envelope, leaving a chunk missing and the electrodes unstuck.

This valve is marked 'BBC Valve Section O.K.' with two dates: '5 Sep 49' and '21 Feb 49'.  Well, it's certainly not 'O.K' now...

Hopefully this was the reason why the picture 'fizzled out' - meaning the really important bits in this set are probably still fine. Good news ?

Well er... actually not such good news. Somebody saw this picture here and got in touch to tell me a diode has obviously been wired into this valve. Now I look again, this is clearly true. Yet I hadn't noticed it before !  I now have a red face.

So maybe there is a deeper fault still present... :-(

The story continues... CLICK HERE

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