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...
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 outbreak 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...;-).
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.
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.
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
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.
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
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.
has been blown from Pyrex and the markings 'BK90' and 'R'
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
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...
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
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.
found it necessary to readjust this after about ten minutes
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.
is marked 'BBC Valve Section O.K.' with two dates: '5 Sep
49' and '21 Feb 49'. Well, it's certainly not 'O.K'
this was the reason why the picture 'fizzled out' - meaning
the really important bits in this set are probably still
fine. Good news ?
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
So maybe there
is a deeper fault still present... :-(