The scan coils
on the replacement tube will also need to be hitched up.
I shall use the original interconnection lead with its British
7-pin chassis plug for this. However, now the scan coil
wiring has been disturbed, it turns out that the rubber
insulation has lengths that are perished.
So new rubber
wiring is inserted into the original fabric shroud and connected
to the plug.
Both of the
scan coils attached to the tube are about twice the impedance
of the 907's originals, so it will be interesting to see
how they perform and what modifications to the circuitry
will be required.
base connector for the original 3/3 tube does not fit the
6/5 tube that's now going in. Instead, the 6/5 uses two
special side-fitting connectors. There will be no room for
these inside the box extension to the cabinet back and in
any case I haven't got any - so instead I've decided to
use modified, car-type 'bullit' female connectors. These
will push straight onto the tube base pins and fit within
the room available. They've been sleeved in black rubber
so they won't look 'new'.
extra wires have been added to the harness going to the
tube. These feed the accelerator and the focus anodes.
are now complete. Next, to switch the set on and see what
of course, all the various units have to be hitched up. At
this point, I give myself a little warning lecture about SAFETY.
There will soon be 3,500 volts DC swilling around here, and
I don't want to come into contact with it, because if I do
- I won't ever get another chance. At this point, the workshop
also becomes out of bounds to cats - and other people.
thing that happens as I power up the chassis on the Variac
is a 'spak' - the sound of a large spark! This is
a one-off event and so I carry on advancing the Variac.
The spark is coming from somewhere the main chassis. It's
likely caused by arcing over of the HT somewhere, as it
surges before the set warms up. I'll have to look into this
later. Such sparks have the unpleasant habit of going away
again when you go looking for them, only to return later...
something forms on the screen. But it isn't a picture -
it's what I call a 'zizz'. Something is 'taking off' (oscillating)
somewhere. I have an old note about one of the IF valves
possibly causing this. The KTZ41 is duly replaced.
and after doing this, and correcting some incorrect component
connections in the sync separator, and some jiggery-pokery
on the controls, the British Heritage Television test card
am unsure as yet whether the instability is really cured.
Only time will tell.
tube seems to be well suited to the job. By sheer chance
I've even got the scan coils connected round the right way
too. The picture, seen from the rear, of course shows a
mirror image view.
The new tube
is more sensitive to vision modulation than the original.
After trying various schemes to reduce this sensitivity
at the vision output, I eventually decide to put things
back to original. This gives the best picture.
is bright enough to be easily viewed in a well-lit room,
showing that pre-war televisions didn't have a dim picture
when new. A small ion burn has rapidly developed though.
It's mild and not really noticeable.
Next, I run
over the vision IF alignment. For a while, some suspicion
falls on the condition of inductor L25, so I strip it out.
Here you can see its brass slug emerging. All is well here,
so I put it all back.
after the profile of the upper vision sideband. The
original alignment spec on this set only provides about
1.7 MHz of vision signal bandwidth, which will give a decidedly
'woolly' picture. Since the upper sideband points away from
the sound carrier, this presents the possibility of an almost
'free lunch' in extending this bandwidth and thus also the
picture sharpness. Therefore I tweak it out a bit more.
This trick will only work with a double sideband signal
such as that coming from the Aurora. It wouldn't give any
advantage with a vestigial upper sideband signal, as might
be radiated to London in future by British Heritage Television.
out the alignment I made myself a special chart, based on
a digital photo, showing the positions of most of the adjustables.
Here's the basic photo I started with. It gives a good general
view of the main chassis.
has revealed another fault, the synchronisation becomes
degraded after 5 minutes from switch on, and the test card
'cogs'. This sort of 'warm up' effect is likely to be caused
by a valve. I've tried changing the MSP4 sync separator
valve with no improvement however. Maybe it's time to get
the 'scope out - and see what circuit conditions are actually
changing... let's hope the presence of a x10 scope probe
won't stop this happening!
control is crackling at the start of its track and is not
really acceptable. This is a shame because I had already
replaced it with a new (vintage) one. You can see that the
mains switch is separate from the potentiometer and actuated
by a rotating peg. Another vintage-style one was duly found
and fitted and the crackling is no more.
The sync separator
action was improved by careful setting of the synchronising
potentiometer, which controls HT on the sync separator stage.
As the valve warms up, the sync separator cut-off action
drifts. The trick is to set the potentiometer so it covers
for this drift. The situation isn't helped in that I'm now
operating the set at a reduced contrast setting, to compensate
for the increased modulation sensitivity of this tube.
set is quite sensitive. It seems happiest with a 6dB attenuator
in the aerial lead from the Aurora.
has never returned. I like to think that this is because
of some re-routing of a wire that I did. The instability
too has gone. Replacement of the KTZ41 seems to have cured
alignment of the radio section is now checked. A very standard
type of radio receiver is included in this set. There is
no separate RF amplifier; the aerial goes straight into
the mixer stage via some tuned circuits.
All goes well
until I have to set up the front end on the short waves.
First I check the oscillator section is calibrated correctly
and the oscillator level stays adequate over the whole band.
The front end alignment then involves inserting an insulated
'wand' with a nick in its side into the appropriate coil.
This has a loop of wire that is manipulated with the wand
to fine tune the inductance. The wand moves the loop OK,
but the range of adjustment proves insufficient...
make a plunger out of ferrite and insert it into the coil
from the top of the chassis. Its circumference is padded
out with a boot to ensure it stays where it is put. The
short wave adjustment can now be correctly made.
I try the television on programme materials rather than
test card, to ensure the sync separator remains happy. It
does! The picture is bright and the quality is lovely.
now beckons. First I apply another thin coat of lacquer
to the top of the cabinet. I've had to use thin coats to
prevent the central transfer being attacked. Also, heavier
coats applied after a delay can later cause the finish to
craze or crack...
It now looks
a lot better than how it was when I first took charge
of this cabinet - with lifting veneers and the base plywood
coming apart underneath. .
to preserve the original 'deep brass' colour of the speaker
grille, originally created in the factory by overspraying
the brass with toner (lacquer).
out the debris and dust with compressed air, the surface
is thoroughly cleaned, then dressed with a rub-on coating
normally used for vinyl dashboards! It now 'glows' and looks
original identifier tab is remounted on the lower rear of
the cabinet. It was crooked originally and is put back in
its original position! The serial number of this set is
radio dial frame and glass is cleaned and goes back; the
little central 'television' red indicator lens too...
screen glass is removed and this and the rubber mask carefully
cleaned. The glass turns out to be nearly 7mm thick and
I see the area of the mask around the tube has been painted
originally with deep brown paint.
I mount the
tube into the cabinet. Only... I'm now going to have to
do the same job again when I return to it on Tuesday, because
I've just noticed there are a few small specks of dust on
the tube face! There are also some bubbles in the glass,
but I can't do anything about those...
You can see
how the replacement tube juts out at the back. Adapting
the back for this will be more 'fun' in store!
now goes back in. By the way, the flash from the photography
is here making the interior black surfaces look much more
'chalky' than they really are.
it all still works when I switch it on!
Here are the
knobs, prior to cleaning. Unfortunately three of them have
chips in their edges where someone has tried to lever them
off in the past and this will have to be repaired. Another
epoxy putty is used as the repair filler. This can be deftly
shaped with water. When set, the result is finished with
wet 'n dry paper and dark brown paint dabbed on and later
Here are all
the knobs, including the repaired ones.
... and they
are fitted back on to the set. Some of these knobs fix to
forked ¼-inch shafts in time-honoured Marconi/HMV
fashion, using special drive screws. Believe me, any old
self-tapper won't work! Fortunately I have enough of these
original-type screws in stock.
Now it's time
to think about the cabinet back. The replacement tube is
longer than the original and now juts out quite a way. After
some rumination, I decide to make a new protection/extension
box in the same pattern of the original - but deeper.
Here we see
the original box in the top left corner, after removal from
the back. The new box will be made of hardboard, which is
being marked out on its smooth side. Using the rough side
of the hardboard as the exterior surface of the new box
and, then sanding and painting it, will give a good approximation
to the original stippled finish.
The new box
is then painted. Here is the new larger box alongside the
lead had been replaced before. Only the cable used had been
too fat and the live wire had become crushed where it entered
the connector - a possible safety hazard.
A more slender
fabric covered cable is now fitted. The connector mounts
into the back, ensuring the mains is disconnected whenever
the back comes off. This feature must have saved the lives
of inquisitive owners, since when the back comes off there's
3,500 volts about!
feature meant that re-assembling the connector became a
right fiddle. The internal parts of the connector would
fall out as the housing was assembled. In the end, as a
temporary measure, I held the thing together using extra-long
screws and tightened them in stages as assembly proceeded.
Once everything was clamped up, the original screws could
back with its big new protrusion. The fitting of this didn't
go perfectly to plan. The back hinges at the left side and
swings in (and out). In its original position, the aperture
and the box would foul one side of the tube base as it swung
in. So I have moved the box over to the right by half an
inch and everything now fits as it should.
the set is now finished, though I haven't yet switched it
on. When this set came to me, it had already had "attention".
Nevertheless all the original parts that I've replaced have
been retained, to assist historical reference.
a peek at the picture. This is much as how it must have
looked in 1939. Sharpness isn't its strong point and there
is a small ion burn, but it's bright enough to be viewed
under most conditions. The tube may further improve with
I have had
to add a centring magnet behind the scan coils to centre
the picture - the electron guns on these tubes are seldom
But one other
thing... that pesky surge 'spark' has appeared again occasionally
just after switch on. It only is present after a rest of
several days - otherwise it doesn't occur.
I have taken the chassis out again, re-connected it to the
rest of the set, and will switch it on every few days to
try to locate the source of this brief event. The first
trial will be tonight, but I'll have to wait 'till it's
the spark then went away! At least, until I had put everything
back together in the cabinet again - then it came back!
time I saw roughly where it was coming from - the region
of the height and width potentiometers in one corner of
the chassis. Things are rather crunched in here and high
tension is present on tags near to the metalwork.
I took this
section apart and reassembled it, making sure there were
adequate spaces between the tags. I also added rubber sheet
insulation in crucial areas. No more spark!
I did was to re-route the cathode lead running to the tube
separately, away from the other tube lead. The cathode lead
conveys video to the tube. This has reduced stray capacitance
and has made the picture sharper than seen in the photo
further up and removed a slight tendency to waviness or
'cogging' on the test card.
restoration story ends here. However the story of the set
itself will continue, as now it returns to its owner - to
be used and enjoyed.
75 year-old television set has been re-born and once again
can occupy pride of place in the home.
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