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Next
I must join the extension wiring to the control panels at the
other ends.
These
panels will eventually be mounted at the side and back of the
cabinet, so the leads have to be the right length.
The
new video conversion and switching functions will be added later
after I've got the basic TV set working satisfactorily in its
new 'extended' form.
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At
this time I repair the damage to the screen bearing section, starting
off with plastic wood.
The
rest of the repair goes to plan and the end result proves satisfactory.
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Now
the control extensions are finished, I can hitch all the units
up for testing on the workshop floor.
The
black wire seen here, leading to a piece of foil tucked under
a tube strap, is the new 'tube earth'. This part is easily overlooked
but performs the vital function of earth return for the EHT smoothing.
It is touching the aquadag tube coating.
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Eventually
I get all the units and their new control panels hitched up.
Next,
to switch on.
First
I run it up slowly on the workshop variac. I monitor the heater
voltage across the tube for an indication of how hard the heater
chain is being driven. The set is now being fed from the new mains
isolating transformer, remember...
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The
picture appears but to start with is dim. It proves necessary
to use the +15v 'boost' tap provided on the isolating transformer
to restore correct mains voltage.
The
picture is then back as it was. This is best described as 'early
50s mild' rather than 'late 50s brilliant'. I can think of one
of my old customers who wouldn't be impressed! A better tube might
improve matters, but there isn't one to hand.
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Since
this is to be a 'luxury' set, the sound will be as important as
the vision.
It's
time to try the separate valve amplifier I'm providing for the
sound. This has much better bass response than the set's own audio
output stage and as expected, 50Hz hum is now noticeable.
This
is greatly reduced by bootstrapping the black capacitor shown
onto the set's HT rail. The total capacitance now becomes 650uF
!
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The
final wiring work is now done. This is to connect up the multiway
wafer switch that will control the various input functions. Multiple
screened leads converge here.
I shall
have a choice between three types of input to feed this set, enabling
it to work with a wide variety of sources.
I have
obtained medication for my Parkinson's at last. This means I am
now going like a bomb and the work is soon completed.
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At
this stage here's what the various gubbins looks like, taking
up a big chunk of workshop floor. All this will later fit neatly
inside the cabinet - I hope!
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I switch
on the set, along with all the new switchery. It works, but straight
away I hit trouble.
The
signal from the Domino converter is leaking through the switching
and interfering with the view from the Aurora in both sound and
vision.
Not
only this, but their carrier frequencies are obviously slightly
different, revealed by the presence of a new audio tone on the
sound channel.
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After
working through various remedies, here's what I finally come up
with. The section of the switch that handled the sound is now
pressed into service to control the power into the Domino. This
is shown in the blue block on the diagram.
'CNV'
and 'MOD' relate to the conversion and modulator sections of the
Domino standards converter. This old unit, which pre-dates the
Aurora, will live inside the set where it will have the job of
making a 405-line Channel 1 signal out of any 625-line baseband
source that gets plugged in.
The
sound (Aud in) is now connected directly, missing the switch.
This doesn't matter since the bottom switch section already ensures
no signal from the Domino, including the sound, can get through
when the rig is operating in standard 'television aerial' mode.
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The
remedy does the trick. I can now try the rig on the output from
my ancient VHS video recorder. Amazingly, after years of inactivity
and a brief winding session on the tape, it works.
I can
now start delving into my collection of vintage videos, recorded
on both 625-lines and 405-lines in the 1980s. Now, at the flick
of a switch on the back of this set, I can view all of them again.
On
the screen appears the opening sequence from the film "Radio
Parade of 1935", with singing telephone switchboard girls!
The
picture is shown reflected in a mirror. The movement in the picture
means it hasn't been very well captured here.
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A top
Radiocraft customer who has since become an old friend now comes
to the rescue by donating a valuable and rare Mazda CRM123. This
is a most generous gesture.
This
tube has been rebuilt at some expense by RACS in France. It should
physically fit the set because it's actually very similar to the
Baird's original tube. So the Bush/Baird mix now becomes even
greater.
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There
are some electrical differences between the (existing) Mullard
and (to be fitted) Mazda tube, which will necessitate some preparation
work to the chassis.
First
the Mullard is a tetrode and the Mazda is a triode. There could
be 'issues' with getting the Mazda to focus using the magnet intended
for the Mullard. I just don't know yet how it will turn out...
Here
is the base on the Mullard - a cut-down "British 12-pin"
arrangement.
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And
here's the end of the Mazda with the base I have rustled up for
it.
This
is a Mazda Octal (MO). This was a British version of the International
Octal (IO) type and it was not intended to be compatible. It has
a larger central boss and spigot than the IO.
This
base was first seen on British valves in about 1938. The earliest
version of the tube I am about to fit first appeared in 1939.
This is truly vintage technology.
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This
may be a CRM123, but being a RACS rebuilt tube, it doesn't have
the original 2-volt heater. Instead, it is now 6.3 volt...
I don't
know how much current it will draw and whether it will be suitable
for hooking straight into the existing 0.3 amp heater chain on
the set.
The
only thing is to try it on a bench power supply. To my surprise,
at 6.3 volts it takes as much as 0.6 amps. This is a heavier drain
than I expected. Am I overloading it?
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No,
the colour looks about right at this current so 0.6 amps it will
have to be! It's invisible at the (expected) 0.3 amps.
This
will mean it won't hook into the existing heater chain and special
powering arrangements will need to be made.
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