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VINTAGE RADIO
By JOHN HILL
A fault with a difference
One intriguing aspect of vintage radio
repairs is the number of obscure faults that
one has to deal with from time to time. Few
repairs are straightforward and there is often
a hidden and challenging problem to solve.
It’s not really surprising that vintage
radio receivers can produce obscure
faults. No doubt the age and poor con
dition of some of these old receivers
has some bearing on the matter.
A classic example of this was a
fault in an old 1932 model Precedent,
an early 5-valve superhet with 2.5V
valves and an 8-inch (200mm) elec
trodynamic loudspeaker.
The restoration had been without
incident and the set worked quite well
for an old timer. However, there was
an annoying problem that resulted
in noticeably distorted sound when
the set was hot. Turn the set off and
then on again a few hours later and
it would be OK.
It took quite some time to establish
a connection between the fault and
the length of time the receiver was
in operation. The problem never be
came apparent until the set had been
operating for several hours. From then
on, the distortion would gradu
ally
creep in. After several unsuccessful
attempts at locating the fault, the
problem was finally traced to the
loudspeaker.
To cut a long story short, gradual
heat build up in the field coil cre
ated physical distortion between
the speaker frame and the field coil
housing. Twisting the housing with
one hand would increase or decrease
the audible distortion, depending on
which way it was twisted.
This heat induced warp eventually
caused the voice coil to foul the centre
pole of the electromagnet. Once that
happened, the sound deteriorated
until the unit was switched off and
the speaker cooled.
It was noticed that although the
voice coil did not rub on the magnet
when the speaker was cold, it was
very close on one side. As there was
no central “spider” on the cone, I de
cided to reposition the pole piece to
see if that would solve the problem.
One favourable aspect of early elec
trodynamic loudspeakers is that they
are held together with good old-fash
ioned nuts and bolts. This allows
them to be taken apart for repairs. In
later years, speakers were riveted or
spot welded together, which made
dismantling extremely difficult, if not
impossible.
Several nasties
The large bolt head on this Precedent loudspeaker at top holds the central pole
piece in place, making it easy to remove for repairs.
74 Silicon Chip
The Precedent speaker was of the
nuts and bolts variety and it required
little effort to remove the field coil and
the central pole piece of the electro
magnet. This revealed several nasties.
First, despite being plated, the pole
piece was now very rusty. This had
the effect of enlarging its diameter,
thus reducing the clearance between
it and the voice coil.
Second, the field coil was wound
on a cardboard spool which was a
loose floating fit on the pole piece.
Vintage Radio
This pole piece shows
obvious signs of
damage. Note the scuff
marks at the end where
the voice coil has been
rubbing against it.
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The speaker cone on this unit is still attached to the frame. Note the “spider” in
the centre of this model, which is used to position the cone.
Shrinkage over the years had caused
this problem.
The cardboard spool is perhaps the
cause of most faults that develop in
electrodynamic loudspeakers. The
spool absorbs moisture from the at
mosphere when the speaker is not in
use. When the field is operative, the
heat generated within the coil drives
the moisture out of the cardboard and
it condenses on the cold pole piece.
Even though the iron was originally
electroplated, 60 plus years of heating
and cooling can eventually result in
severe rusting and a fouled voice coil.
Moisture in the field coil spool
can also cause electroly
sis which
was possibly the main cause of field
winding failure. If moulded bakelite
had been used instead of cardboard,
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March 1998 75
This back view shows the speaker frame and voice coil. The coil is wound on a
thin cardboard former which often goes out of shape over the years, especially if
the cone has sagged.
A typical field coil housing. Unlike the Precedent loudspeaker, this unit has the
pole piece riveted in place which makes repairs more difficult.
electro
dyna
mic speaker fields may
have been more reliable.
Proposed repair
The proposed repair for the Pre
cedent loudspeaker was to clean
up the pole piece and reposition it
centrally in the voice coil. However,
the job would not really be complete
unless something was also done about
the moisture absorbent cardboard. So,
after leaving the spool out in the Sun
to dry for a few hours, it was given
76 Silicon Chip
several coats of clear lacquer to seal it.
A couple of thin cardboard wash
ers were also made, so that the field
coil would be held firmly in place
and these washers were sealed from
moisture too.
All that remained was to reassemble
the speaker and the distortion prob
lem would be solved. Unfortunately,
one cannot really expect to solve dif
ficult problems as easy as that.
It would appear that electrody
namic loudspeakers were originally
assembled from rear to front. In other
words, the cone was glued into posi
tion last. Putting a speaker together
with the cone already in position is
not so easy. No matter how the bolts
that hold the frame and field housing
together were jiggled and twiddled,
the voice coil always rubbed on the
pole piece.
After going through this routine
often enough to realise that it wasn’t
going to work, the situation called
for drastic measures. The pole piece
was set up in a lathe and a quarter of
a millimetre was turned off its diam
eter at the front where it fits into the
voice coil.
After that, no further problems were
experienced with the speaker’s assem
bly. The cone moved freely without a
hint of interference.
Now one would expect that increas
ing the clearance between the pole
piece and the voice coil would reduce
the speaker’s sensitivity. If that’s the
case, it was by no means noticeable.
What’s more, if the same problem
arises again with other speakers, I will
have no hesitation in taking a skim
off the pole piece in order to give it
adequate clearance.
Because the voice coil is wound on
a thin cardboard former, it is asking a
lot to expect it to be perfectly round
50 or 60 years after it was made.
Reducing the pole piece diameter is
one way of compensating for an outof-shape voice coil. A cone that sags
and loses its form is one reason that
voice coils go out of shape.
While on the subject of voice coils,
remember that they are only glued
to the speaker cone and a touch of
lacquer to reinforce the area of at
tachment is highly recommended. If
you have ever encountered a speaker
with a loose voice coil or voice coil
winding, you will appreciate the need
to pay attention to that part of the
speaker while it is accessible.
Other methods
There are other ways of clearing
fouled voice coils without having to
resort to the drastic methods previ
ously described.
One way is to move the cone in
and out while applying side
wards
pressure to the cone. In other words:
try to loosen the dust, grit and barna
cles by forcing the inside of the voice
coil to rub gently on the pole piece.
After that treatment, lay the working
was used. Unfortunately,
this speaker is of slightly
different construction,
which just goes to show
that some may be easier
to repair than others
(note the “spider” shown
in the photograph of this
model).
One pr o b lem t h a t
would arise if doing a
similar repair on this
second speaker is the
fact that the pole piece is
riveted in place instead
of being held by a bolt,
as was the case with the
Precedent speaker.
No money
In conclusion, I see
little point in spending
large sums of money on
old radios, especially
The hole in this backing plate is a neat fit around the voice coil. In some instances, this hole
when the price of some
may require enlarging so as to clear a coil that has gone out of shape.
repairs exceeds the re
ceiver’s value. This par
speaker face down on the workbench Once again, give the speaker a run ticularly applies to replacement field
for a while so that any rubbish can in the face down position so that the
windings and speaker cones where
work its way out of the gap.
gritty granules can find their way out. one can easily spend $100 on an old
If that fails to do the trick, slide a
In my case, the repaired Precedent
speaker restoration.
thin piece of shim brass between the speaker was reinstalled in its cabinet
Why pay to have these things done
voice coil and the pole piece, work before I realised it had the potential when they can often be restored by
ing it all the way around if possible. for a Vintage Radio story. So for the either a straightforward repair or by
This procedure is likely to be more purpose of supplying photographs, combining various good components
SC
effective than the previous method. another electrodynamic loudspeaker
into one working unit?
This 2.5kΩ field coil is wound on a cardboard former.
Sealing the cardboard with lacquer will help to
moisture-proof it.
Most electrodynamic loudspeakers employed a
hum-bucking coil which was used to neutralise hum
induced by the field coil.
March 1998 77
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