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Vintage Radio
By Associate Professor Graham Parslow
Restoring a 1946 HMV
Model 456A mantel
Above: the fully-restored HMV Model 456A. The
cabinet in particular required a lot of work.
Built into an attractive up-market timber
cabinet, the HMV 456A is a mid-price large
mantel set from 1946. The circuit uses just
four valves and covers the AM broadcast
band only.
Francis Barraud’s
painting of the
Jack Russell
terrier Nipper,
titled “His
Master’s Voice”.
The image was
acquired by the
newly-formed
Gramophone
Company in
1899 and the
design was used
on its record
labels from 1909.
Photo: HMV/EMI
Archives.
90 Silicon Chip
S
OME SETS ARE a real challenge
to restore and this 1940s HMV
456A mantel set was no exception.
The set in my possession was a gift but
it brought with it “obligations” that a
bargain purchase would not. The radio
was passed onto me by Peter, a fellow
member of the Mustang Car Owners’
Club, who knew that I collected radios.
Basically, Peter has a mate who was
going to throw the radio away but Peter
saved it, thinking that it was part of our
radio heritage and as such, should go
to a caring collector. It was transferred
between our car boots on a dark night
but in the cold light of day, it looked to
be more a liability than an asset – not
that I wasn’t appreciative to have had
it given to me.
Unfortunately, this set had had a
hard life. The dial glass was missing and timber cabinet was water
damaged. The wood veneer was also
cracked and had de-laminated in several places, while the righthand end
panel of the cabinet was missing.
If I had paid a few dollars for it, then
it would have gone straight onto my
spare parts shelf to be cannibalised.
However, because it had been given to
me and because it’s potentially such
a nice set, I felt that I had some moral
obligation to fully restore it.
Identifying the set
Confirming the identity of this set
was easy because the top of the case
and the speaker were both branded
HMV and the chassis proclaimed the
radio to be a model 456. If the dial
glass had been present, it would have
been even easier.
Searching through the “Australian Official Radio Service Manuals”
locked in the year of manufacture as
1946. This was the year that domestic
radio production resumed after ceasing in 1941 for the remainder of WW2.
During the war, HMV was occupied
with developing and manufacturing
radar. However, despite the many
advances made during this time, the
components and circuits used for rasiliconchip.com.au
These photos of the cabinet and the chassis shows the very poor condition of the set prior to restoration. The cabinet
had been badly water damaged, the righthand end panel was missing and two of the knobs were also missing.
dios manufactured immediately after
the war were little changed from the
early 1940s designs. Part of the reason
for this was that old stock had been
stored and it was prudent to use it.
In the HMV model 456, the valve
line-up is quite conventional, as is
the electrodynamic speaker with its
1500Ω coil that serves as both an
electromagnet and a filter choke for
the high-tension rail. In fact, the model
456 circuit is almost identical to the
1939 HMV model 449.
The relatively simple 4-valve configuration made the set more attractive
for me to restore because my collection
already included the more expensive
5-valve 2-band HMV model 886. The
886 is contemporary with the model
456 and used the same style of timber
cabinet which is also a pre-war design.
Because my model 886 was in pristine
condition, I could see what the model
456 should look like once it had been
restored.
Australia from 1936 at 2-6 Parramatta
Road, Homebush, NSW. The 1936 advertisements for HMV proclaimed that
the radios were for “discriminating
buyers who demand the best”.
Circuit details
Fig.1 shows the circuit details of the
HMV Model 456. It’s a conventional
4-valve superheterodyne set with
a 6A8G mixer-oscillator, a 6G8G IF
amplifier/detector/first-audio stage,
an EL33A (or EL3NG) audio output
stage and a 5Y3G rectifier.
The set has a 457.5kHz IF stage, a
conventional power transformer and
an electrodynamic loudspeaker. It also
has AGC and this is derived from the
second diode in the 6G8G and applied
to the first and second stages from the
junction of R10 & R11.
Restoration
As with all such restorations, the
first step was to remove and clean the
chassis. For metal work in fair condition, I use turpentine and a brush to
clean away the grime but this one
required sterner treatment.
First, the valves were removed and
the power transformer covered in
plastic to make it waterproof. That
done, degreaser was liberally sprayed
over the top surfaces which were then
brushed. The chassis was then hosed
with water to remove the degreaser and
The famous HMV logo
His Master’s Voice is the title of a
painting by Francis Barraud which
featured the dog Nipper listening to a
cylinder phonograph. The trademark
image was later acquired from the
artist in 1899 by the newly-formed
Gramophone Company (London) and
subsequently used for gramophones
and records.
In 1931, The Gramophone Company
and The Columbia Company merged
as Electric and Musical Industries
(EMI) and began manufacturing radios.
The company made HMV radios in
siliconchip.com.au
Also taken prior to restoration, this inside view shows the poor condition of the
2-core power lead (it had been joined at some stage in the set’s past). The chassis
and the valves were also dirty and the speaker cone had been punctured.
August 2013 91
Fig.1: the circuit uses just four valves in a fairly conventional superhet arrangement, with AGC applied to the first two
stages. The valve line-up includes a 6A8G mixer/oscillator, a 6G8G IF amplifier/detector/first audio stage, an EL33A/
EL3NG audio output stage and a 5Y3G rectifier.
then blasted with compressed air to
evaporate all visible remaining water.
I have learnt the hard way not to
power a set up immediately after this
sort of treatment, so the set was left for
several days to completely dry. The
electrolytics showed no signs of leakage so I didn’t bother replacing them.
With the valves still out of circuit, I
next connected my bench HT supply to
the cathode pin of the 5Y3G rectifier.
By the time I ramped up to 100V, the
set was drawing 15mA, a clear sign that
the electrolytics needed to reform. As
a result, I backed off to give 10mA and
then slowly increased the voltage in
small steps to ensure that the current
never exceeded 10mA.
After about three hours, the 450V
electrolytics were happy to accept
330V at 2mA. The working radio subsequently settled at a HT of 340V, so I
was not too far off by stopping at 330V.
My next step was to replace the
dodgy 2-core power lead with a modern 3-core lead, so that the chassis
could be earthed.
Before reinstalling the valves, I did a
high-voltage leakage test on the power
transformer to ensure it had survived
the chassis-washing process. It proved
to be OK, so I then checked the two
dial lights. These both had darkened
glass envelopes, a classic symptom
produced by the tungsten filaments
slowly evaporating with long service.
Both dial lamps proved to be open
circuit, a logical consequence of the
filaments evaporating. As a result,
the old globes were discarded and
two new 6.3V 300mA (2W) globes
installed.
Next came the anxious moment of
first switch-on. I was expecting to see
two brightly-lit dial lamps and about
5-6W showing on my power meter
but that didn’t happen. Instead, both
lamps were off and the power meter
was only showing 1W. At least this
indicated that the power transformer
had no shorted turns.
The problem with the dial lamps
was easily diagnosed. The connecting
lugs in the bases were insulated by an
Another two views of the set prior to restoration.
The dial glass was missing and the end panel was
completely missing on the right
hand side of the
cabinet, which meant that a new piece had to be
made and fitted.
92 Silicon Chip
siliconchip.com.au
oxide layer that prevented good contact with the globe terminals. Cleaning
them fixed that particular problem
At this stage, everything was looking good. The power transformer was
obviously OK and the electrolytics
were accepting the applied HT without
any signs of stress or undue leakage.
As a result, the four octal valves and
the speaker were plugged in and the
set optimistically switched on.
The power consumption rose to a
reasonable 44W but my optimism was
misplaced because the only noise that
came from the set was due to arcing
inside the EL33A pentode valve. This
was attended by a fine visual display
of purple plasma in the envelope.
A classic symptom of a gassy tube
is oxidation of the silvery magnesium “getter” that’s deposited inside
the envelope to absorb residual gas.
However, this EL33A was externally
painted in red near its base, so there
was no visible clue.
It’s interesting to note that some
gassy tubes can still be functional (see
www.thetubestore.com/Blue-Glow).
I specifically recall a PA amplifier at
my local townhall in the 1950s that
remained working while the output
valve glowed purple.
In this case, the old HMV’s EL33A
was headed for the bin of bad valves.
With the EL33A removed, a signal
tracer was connected to the slider of
the volume pot and the radio switched
on again. Result – nothing, absolutely
nothing! There was no hiss or any
hint of even the faintest reception.
The power consumption was 28W, an
aerial was in place and no components
were overheating or showing any other
signs of failure.
Above: this view shows the chassis
wiring of the HMV Model 456A.
Very few of the original parts in the
set required replacement.
Troubleshooting the problem
Feeling somewhat disillusioned by
this, I decided that the front-end fault
just wasn’t worth my time and so I
left it, not intending to come back to
it. However, after several weeks, it got
the better of me and I decided to see if
I could solve it.
Because the circuit is relatively simple, every component was tested and
everything checked out fine. Admittedly, some of the resistors had gone
high by up to 20% but this should
not have resulted in a complete loss
of function.
With no obvious faults present, I
powered the set up again but as expected, it still didn’t work. It was then left
siliconchip.com.au
A new dial-scale was made by photographing the dial-scale of another HMV
set, then printing it out (after processing) and attaching it to the backplate.
August 2013 93
The partially-restored chassis is shown
at left, complete with its electromagnetic
loudspeaker. The view at right shows the
fully-restored chassis inside the cabinet,
with the replacement audio output valve
(a 6F6) fitted and a replacement speaker
transformer.
on while I prodded each component
in turn with an insulated rod to see if
an audible crackle might reveal a dry
joint or some other intermittent fault.
This drew a blank so I then decided
to try wriggling the valves. As soon as
I nudged the 6A8 mixer-oscillator, the
set produced some hiss and a faint station could be heard through the signal
tracer. I then adjusted the tuning gang
Making a new end-panel and other
repairs to the cabinet were a timeconsuming part of the restoration.
94 Silicon Chip
over its full travel and the radio tuned
in stations over almost the complete
broadcast band. Judiciously bending
one plate in the tuning gang restored
its clearance and the set then covered
the full MW range.
Unfortunately, the “cure” was shortlived because the radio soon fell back
into complete silence and no amount
of wriggling the 6AE8 or any of the
other valves would restore normal
operation.
Hours of frustration then followed
with the set occasionally springing to
life but mostly failing to function. As a
result, I began progressively replacing
likely suspect components, including
the 6A8, in an attempt to identify what
seemed to be an intermittent. Even
the electrolytic filter capacitors were
replaced, despite the fact that they
had been successfully reformed and
appeared to be functioning normally.
In the end, the problem was solved
by methodically measuring the voltages on the pins of the 6A8 mixer-oscillator. These measurements revealed
that 113V was present on pin 4 when
the set was functioning but only 10V
when it wasn’t.
Pin 4 is the screen adjacent to the
6A8’s anode and the two components
attached to it (C10 and R6) had been
replaced without fixing the problem.
In addition, the valve socket and the
6A8’s pins had been cleaned and the
valve inserted and removed a number
of times to clean the contacts. But it
hadn’t been enough – pin 4 of the
socket required extra attention to ensure a reliable contact.
And that was it – the radio then functioned reliably. I didn’t have an EL33A
output pentode in my valve drawer, so
a 6F6 with comparable specifications
was substituted. Unfortunately, the
radio only worked for a few minutes
with the replacement pentode in place
before again going completely silent.
A few quick voltage checks revealed
that there was no HT on the pentode’s
plate, indicating an open-circuit speaker transformer. This was subsequently
confirmed by a resistance measurement and a replacement transformer
restored operation.
Restoring the cabinet
The old HMV 456 had lost both
its dial glass and the pointer. A new
pointer was made from white solidcore hook-up wire, so that problem
was easy to solve.
The original dial was on the back
of the glass, so that it sat in front of
the pointer. Not having a replacement
dial-glass, I decided that a printed
dial-scale attached to the backplate
would look reasonably authentic.
This dial-scale was created by first
photographing my model 886’s dial
and then modifying the image using
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Arcing inside the
EL33A pentode valve
was accompanied by
a fine visual display of
purple plasma in the
envelope, indicating
that it had gone
“gassy”. The valve
was replaced with a
6F6 which is a close
equivalent.
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photographic editing software (I use a
freeware program called “The GIMP”).
The editing involved extending the
base-lines of the dial image and modifications to the contrast and brightness
to turn a clear dial-scale into one with
a black background. A sheet of clear
glass was later fitted to the set in place
of the original dial-glass.
The timber case was perhaps the
most demanding part of the restoration
because of its poor starting condition.
The biggest job was making and fitting
a new end piece. The remains of the
original finish on the cabinet were
then removed using paint stripper,
after which it was sanded back and
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the veneer restored. It was then given
the paint and poly
urethane treatment
so that it looked almost new.
A large hole in the original speaker
grille cloth meant that this too had to
be replaced. Whatever had penetrated
this cloth had also punched a hole in
the speaker cone which sits immediately behind it. This damage was
repaired by cutting a patch from the
cone of a non-functioning speaker and
attaching it using PVA glue.
As shown in the lead photo, this all
worked out quite well and the completed set looks quite authentic. The
end product in now a worthy part of
SC
our radio heritage.
H SILICON CHIP logo printed in
gold-coloured lettering on spine
& cover
Silicon Chip Publications
PO Box 139
Collaroy Beach 2097
Order online from www.
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August 2013 95
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