is now connected up. I want to get the set going, even though
it still lacks the EHT transformer. The picture tube won't
be connected up yet either.
the radio section can be got going, and aligned.
problem. This set dates from long before the 'Belling-Lee'
coaxial aerial plugs that we know today, even though these
go back more than 60 years!
are two flat slots, one above the other, to connect to the
two poles of a television aerial feeder. You can see these
over to the left - the round holes on the right are for
the radio aerial and a gram pick-up.
arrangement looks like it might be a balanced aerial input,
in fact it is not. One side is grounded, just like a modern
domestic TV aerial. Once I've got everything else working,
I'll measure the impedance of this - it'll be interesting
to see if it comes to anywhere near the modern figure of
But how to
connect a television aerial to this very ancient socket?
Well, I was
able to adapt a pre-war American mains plug. This meant
changing the spacing and profile of its pins and shortening
them, but the plug now fits well and could even pass as
an original HMV television aerial plug!
will be connected into this plug and the other end will
go to a modern coaxial plug, suitable for connection to
an Aurora 405-line converter.
section was then switched on and tried. After a connection
to the aerial circuit was fixed it worked first time. A
very good radio is included with this set! Performance on
all the bands appears to be excellent.
One of the
tasks of the EHT transformer is to supply the heaters of
the television section's valves, so the television section
cannot yet be tried. I shall now await delivery of this
transformer. Until then nothing much more can be done...
except to re-mount the dial and band pointer, and give the
radio section a complete IF and RF alignment. This proved
to be slightly out.
On this set,
the dial pointer is moved by a chain drive. The chain is
quite fragile, and one of the links broke, necessitating
a tiny repair involving a little solder.
this point I receive a letter from a member of the Vintage
Radio Restoration Board called line sync. He tells
me that In the past he restored an original 907, and
this still had the original electrolytic block atop its chassis.
He includes some photos. So I now can create a copy of the
original Dubilier (not Hunts) unit that was fitted. Thank
mains EHT transformer, which also supplies some of the valve
heaters - has arrived. Today I fit it.
has done a really fantastic job here. This transformer not
only will perform exactly as the 1938 original - it looks
exactly the same too!
supply chassis is now complete and ready for action.
is now connected up and run up for the first time - on the
variac. After some attention to the brightness and contrast
potentiometers, and replacement of the volume control (the
faulty one was not original), operation - of a sort - is obtained
from the television.
is much brighter than reported by the last owner, probably
thanks to the tube rejuvenation process, but it is still
rather insipid, with an obvious ion burn (as expected).
haven't even started to explore all the various adjustment
options possible; nor have I looked at the IF and RF alignment
TV section alignment followed. This had to be carried out
over two sessions, since two of the coils requiring adjustment
were underneath the chassis and couldn't be reached when
the chassis was upright on the bench. There seemed to be
lots of wires to connect and disconnect when manhandling
the heavy chassis units, and of course at all times I was
very careful never to go anywhere near the mains
minor problem that appears to be originating from Valve
V3, early on in the Vision IF strip, whereby this takes
off (oscillates) as the set is first switched on and then
'pops' into a stable state after a few minutes. I'm
hopeful that valve replacement, or even just adding a top
cap to the screening can, will fix this.
I am extremely
impressed with the quality of Mike Barker's work
on the EHT transformer, which not only looks correct, but
is providing exactly the right readings under load. This
is not the first time I have used him for demanding transformer
work. In my opinion, the value of his contribution to the
cause of keeping vintage televisions working ranks alongside
RACS, to whom the tube will now have to be sent...
Yes, I'm afraid
the effect of the rejuvenation didn't last, and the tube
has now returned to almost its previous dimness. However,
following alignment the underlying picture is now much better
and I expect the set will deliver good results once the
tube has been attended to.
The two restored
plinths (skirts at the bottom of the cabinet) are now re-attached.
A final touch
before this set is put to one side to await the tube rebuild...
a new label has been made to replace the torn and incomplete
was kindly provided by Brian Cuff about the label on his
Marconiphone variant of this set (a 709). From this,
an HMV equivalent label was worked up in Paint Shop Pro
and then printed onto yellowed paper. An attempt at ageing
the label was then made by lightly spraying it with clear
new label was lightly attached to the cabinet using spray-on
Scotch paper adhesive. The original damaged label remains
accessible beneath, for the benefit of future researchers.
The old label
was slightly crooked - as is this one!
been approaching two years... but the story now continues.
The Emiscope 3/3 picture tube was duly despatched to RACS
in France. I later followed up with a personal visit. They
had encountered a problem when taking the vacuum down to
air: cracks appeared in the pyrex around the EHT connector.
The original tube was in separate parts and it's now impractible
(and pointless) for it to be reassembled and returned.
The only solution
now is to use a different tube. What can be done?
It just so
happens I have an even earlier 9-inch pre-war tube in stock!
This is an Emiscope 6/5, as used in the first generation
of pre-war EMI television sets. This tube was used in some
of their 1936-7 models whereas the 3/3 was used in the 1938-9
range. This 6/5 is in its original crate and believed to
be new and unused. Its good condition has now been confirmed
on the tube tester.
The 6/5 is
a few inches longer than the 3/3 and has the earlier base
but I've found it fits the HMV 907's rubber screen mask
very well. Since the original tube now won't be coming back
from RACS, this would appear to be a very acceptable substitute
for a 9-inch EMI pre-war set.
tube substitution will actually be easier than putting in
a post-war 9-inch tube, since a later tube would have a
wider deflection angle and there could be problems with
providing enough scan power. Also, there could be complications
regarding the area on the replacement tube over which the
scanning effort (and focus field) should be applied. This
might not be matched by the existing scanning & focussing
The 6/5 uses
electrostatic focus rather than electromagnetic, so there
will be no need now for the focus coil. Fortunately, this
'new' 6/5 comes with its own set of scan coils, which I
shall be using instead of those from the 907. I believe
the 907's timebases should electrically match reasonably
well into these, though there may need to be some minor
mods. As it happens, I would have had to do this anyway,
since the 907's original focus/scan assembly wouldn't have
gone over the side-pin base on the 6/5 !
set will remain "all-EMI" and "all-pre-war". I also believe
that doing it this way will ensure the set's market value
will be maintained.
Time to trial-fit
the replacement tube. The original mounting spars prove
fine for the job. The only change required is the omission
of a 'crinkle' type spacer around the neck within the rear
bracket, and its replacement with a thick ring of felt.
A long sponge pad is also added to the forward bracket where
it passes under the tube.
The rear end
of this tube is now 4½ inches proud of the back.
A larger protection box will have to be fitted to the 907's
back to cover this. It will have to be done so it looks
original. I may have such a box amongst the other old television
backs in stock...
looks fine at the front and suitably 'vintage' in appearance...
a post-war tube would not have looked as correct.
tried irradiating the screen phosphor with an ultra-violet
torch. This confirms it is white, and there appear to be
no central burns either...
have to make some minor changes to the set, to adapt it
to use electrostatic focus. This replaces the former electromagnetic
focus, which involved a focus coil.
original 3Kohm focus potentiometer must be replaced by one
of a much higher value. The 'new' pot is here seen in the
centre, before wiring up. It's a high voltage shrouded type
of around 250K and is of a vintage design.
needs to be a lead run between the main and power chassis
units so that the focus control can reach the tube bleeder
resistor network. This entails fitting connectors at the
junction. I've used a pair of Bulgins, which should be up
to the job.
A high voltage
decoupling capacitor is now also added to the focus feedpoint
on the bleeder chain.
the focus connection, which goes to the 1st anode, this
older tube also requires a voltage for an 'accelerator anode'
(shown as 'acc' here). To start things off, I'll connect
this to the HT rail via a 100K resistor. This is what was
done in the set from which it came.
In the HMV
907, vision is fed to the tube cathode and 'brightness'
bias to the grid. This is the other way round to the set
from which it came.
tube heater winding is locked to deck via a centre-tap potentiometer,
there will be some voltage stress between heater and cathode.
The only difference to before is that this stress will vary
with the vision signal rather than be static and determined
by the brightness control setting. I believe this will be