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 condition 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) electrodynamic loudspeaker.
The restoration had been without
incident and the set worked quite well
,
11e
.
41411
-',
The large bolt head on this Precedent loudspeaker at top holds the central pole
piece in place, making it easy to remove for repairs.
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 became
apparent until the set had been operating for several hours. From then on,
the distortion would gradually 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 created
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 decided to reposition the pole piece to
see if that would solve the problem.
One favourable aspect of early electrodynamic loudspeakers is that they
are held together with good old-fashioned 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 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 electromagnet. 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
74
SILICON CHIP
This pole piece shows
obvious signs of
damage. Note the scuff
marks at the end where
the voice coil has been
rubbing against it.
aglifirs
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.
loose floating fit on the pole piece.
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 atmosphere 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 electrolysis which was possibly the main cause of field winding
failure. If moulded bakelite had been
Vi000,V,
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.
used instead of cardboard, electrodynamic speaker fields may have been
more reliable.
to dry for a few hours, it was given
several coats of clear lacquer to seal it.
A couple of thin cardboard washers
were also made, so that the field coil
Proposed repair
The proposed repair for the Precedent 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
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 problem would be solved. Unfortunately,
one cannot really expect to solve difficult problems as easy as that.
It would appear that electrodynamic
loudspeakers were originally assembled from rear to front. In other words,
the cone was glued into position 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 diameter
at the front where it fits into the voice
coil.
After that, no further problems were
experienced with the speaker's assembly. The cone moved freely without a
hint of interference.
Now one would expect that increasing 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 ad-
equate 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. Re-
ducing the pole piece diameter is one
way of compensating for an out-ofshape 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 attachment 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 sidewards pres-
sure to the cone. In other words: try to
loosen the dust, grit and barnacles by
forcing the inside of the voice coil to
rub gently on the pole piece. After
that treatment, lay the working speaker
76
SILICON CHIP
The hole in this backing plate is a neat fit around the voice coil. In some instances, this hole
may require enlarging so as to clear a coil that has gone out of shape.
face down on the workbench for a
while so that any rubbish can work its
way out of the gap.
If that fails to do the trick, slide a
thin piece of shim brass between the
voice coil and the pole piece, working
it all the way around if possible. This
procedure is likely to be more effective than the previous method. Once
again, give the speaker a run in the
face down position so that the gritty
granules can find their way out.
In my case, the repaired Precedent
speaker was reinstalled in its cabinet
before I realised it had the potential
for a Vintage Radio story. So for the
purpose of supplying photographs, another electrodynamic loudspeaker 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 problem that
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 when the
price of some repairs exceeds the receiver's value.
This particularly applies
to replacement field windings and
speaker cones where one can easily
spend $100 on an old speaker restoration.
Why pay to have these things done
when they can often be restored by
either a straightforward repair or by
combining various good components
into one working unit?
SC
This 2.5kS2 field coil is wound on a cardboard former.
Sealing the cardboard with lacquer will help to moistureproof it.
Most electrodynamic loudspeakers employed a humbucking coil which was used to neutralise hum induced
by the field coil.
MARCH
1998
77
Loading...