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VINTAGE RADIO
By RODNEY CHAMPNESS, VK3UG
Safety with vintage radios
If you don’t know what you are doing or
become complacent, a vintage radio receiver
can be a very dangerous device. Here’s some
advice to ensure your personal safety.
Certainly, we now hear a lot about
safety in our communities. One example is road safety and this includes
the condition of the roads, road signs,
the weather, the amount of traffic, the
time of day and traffic speed. We also
hear about how cars are being made
safer, with ABS brakes, airbags, seat
belts, better lights, better tyres and
so on.
What has this to do with vintage
radio you might ask. The only thing it
has directly to do with vintage radio
is that you and your newly-acquired
treasure are more likely to arrive home
safely. However, once you’re home,
you should follow a few strict procedures when working on a vintage
radio set to make sure you stay safe.
No warnings
Have you noticed all the safety
warning signs on your vintage radios? No? – well that’s not surprising
as there usually aren’t any. Can you
pick up and turn over the set you are
restoring while it is running and not
run the risk of putting your fingers on
some point that has a lethal voltage
onto it? The answer is probably no.
Obviously safety was not of much
concern to the designers and builders
of what are now vintage radios. Manufacturers would not get away with
this careless attitude today.
With this in mind, it was decided
that the subject of safety in vintage
radio restoration should be revisited. Some vintage radio buffs like to
build replica sets and safety should
be one of the prime considerations in
the mechanical construction of these
sets too.
The older the set that is being restored, the more likely it is to have
exposed mains and high tension
leads and terminals. Some European
sets are very bad in this respect, with
exposed mains and HT terminals in
very easy to touch locations. Take a
careful look at the set that is about to
be restored and see where the danger
points are. Where will your hands go
when you are moving the set around?
Will your hands touch any of these
danger points? It is very easy when
concentrating on the job in hand to
forget the danger points.
Minimising the danger
A typical core balance detector. These devices are cheap life insurance at $20-30
so there is no excuse for not having one.
86 Silicon Chip
How do you minimise the danger?
You could get someone else who is
more experienced than you to restore
the set! However, most restorers like
to do the lot; to be able to proudly say
“I did it all myself”.
Having observed where the danger
points are, it may be possible to put a
cardboard or plastic sheet cover over
them, or even to physically shift the
danger point to a spot where it can
not easily be touched. Where I haven’t
been able to do any of the preceding
things, I have put glue over the exposed terminal. This may not be the
most foolproof method of approaching
safety but with several layers of glue,
your chances of escaping an electric
shock or electrocution are much better
if you touch something you shouldn’t
in a moment of carelessness.
Power transformer
In many early mains-operated sets,
there is the very real possibility that
the power transformer insulation
may not be as good as it had been in
years gone by. It is possible that the
mains winding or the wires to the
transformer are defective and so a
short or partial short to the frame of
the transformer and hence the chassis
may occur. And a live chassis is a very
dangerous item indeed.
An easy test here is to use the various ohms ranges on a multimeter to
test for shorts or leakage between the
transformer’s mains winding and the
frame or chassis. Any indication other
than a momentary kick on the meter is
to be treated as a possible dangerous
circumstance.
If the set has been stored in a damp
environment it would be worthwhile
putting the set chassis or the transformer alone into the oven in the
kitchen. Heat the set for several hours
at about 60°C to dry the transformer
out and hopefully get rid of the leakage. If leakage is still evident, things
are not looking good and a test with a
high voltage insulation tester such as
John Hill has described or as appeared
in SILICON CHIP (May 1996) would be
desirable.
Note that an ohmmeter will only
detect shorts and leakages that are
not voltage dependent. An insulation
tester, on the other hand, checks the
transformer under stresses similar to
when it is operating.
Before applying any power to the
set, it is important to check the insulation of all the various wires to
ensure that no short circuits exist.
That’s because insulation can deteriorate over the years – some types
more than others. Any insulated wire
that has badly deteriorated insulation
must be replaced or the wire sleeved
with insulation tubing. The mains
cord must be replaced without further
thought if it has cracked insulation.
Up until the time that valve sets
were being replaced by transistor sets,
most sets used twin-core power lead.
Ideally, the twin-core lead should
be replaced by three-core lead, par
ticularly where the chassis or other
metal can be touched when the set is
back in its cabinet (I am only referring
to mains-operated sets here).
For the sake of authenticity if the
set had a fabric covered mains lead it
would be nice to replace it with one
that looks the same. Burton Cables
do produce an unfilled brown fabric
3-core mains cable that looks much
the same as the cable being replaced.
The 3-pin power plug should also
be inspected. The Bakelite on old
power plugs may be chipped and a
strand of wire could extend beyond
the side of the plug – and this could
be the Active mains wire! Yes, it is
nice to keep the set looking as authentic as possible but safety must
be considered and it may mean you
have to fit a modern plastic plug. So
be it – at least you will be around to
enjoy seeing and hearing your set.
Core balance detectors
These devices go under a variety of
trade names and a typical one is shown
in the accompanying photograph.
What they basically do is detect when
there is more current passing through
the Active 240V lead than is returning
via the Neutral lead. How can this
be? If there is leakage or a short from
the Active lead to Earth, some of the
current will return via the Earth lead
– or you, with possible deadly results,
if there is no Earth lead.
When the current on the Neutral
lead is 30 milliamps (30mA) less than
through the Active lead, a sensing
circuit detects this and trips a small
circuit breaker to remove the power.
Fig.1 shows a simple diagram of
the sensing circuit of a core balance
detector. The Active and Neutral leads
go through the centre of a toroid ring
core. These act as 1-turn primaries of
a transformer, while a third winding
consists of many turns of wire to act
as a step-up transformer.
Because the currents through the
two single-turn primaries flow in
opposite directions and are normally equal, the two magnetic fields
cancel each other out and no voltage
is developed in the third winding.
However, when there is leakage and
not as much current flows through
the neutral lead, the magnetic fields
are not cancelled and so a voltage is
developed in the third winding. This
is detected in the device which trips
the circuit breaker and removes the
240V AC from the lead.
The response time for core balance detectors is very fast and your
chance of being electrocuted is low,
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May 1998 87
Fig.1: the basic sensing circuit of a core balance detector.
Fig.2: the tone control circuit shown at (a) can be made safer
by wiring it as shown at (b). The capacitor voltage rating may
need to be increased though.
should there be a problem in the set
with mains leakage to chassis. These
devices are cheap life insurance at
$20-30 so there is no excuse for not
having one. They also have four power
outlets with an overload trip as well.
I’ve mounted mine on the back wall of
the workbench and plug any devices
that I am working on into it.
Under some circumstances they
will trip when no fault exists, mostly
at switch on or switch off of the device
being run on the detector. This possibly occurs because the interwinding
to frame capacity of the power transformer is being charged or discharged,
causing a momentary unequal current
to flow through the sensing core.
As wonderful as these core balance
detectors are, they will not protect
you against the effect of voltages after the power transformer. These are
every bit as dangerous as the mains,
so be vigilant.
It is particularly important to be extremely cautious when endeavouring
to restore a set that has been “butchered” by someone in its past. You
may have picked the set up because
it couldn’t be made to work or worse
88 Silicon Chip
was even known to be dangerous!
Some time ago, I had the task of
restoring a commercially-made late
1920s TRF set. It had been considerably “got at” and required a complete
redesign to make the set a goer. In the
process, I nearly fell for one of the
traps mentioned earlier in this article
and could have been electrocuted.
Now we all tend to believe that the
shafts of volume and tone controls are
earthed to the chassis, even when the
moving arm is physically attached to
the control shaft as did occur with
some early controls. In this set, they
weren’t earthed, having barely visible
fibre washers insulating them from the
chassis. One was about 50 volts plus
above the chassis, while the other was
at a whopping 400 volts!
I broke out into a cold sweat when
I realised how close I’d come to departing this world. Fig.2(a) shows the
circuit that the tone control was wired
into, while Fig.2(b) shows how it
could have been wired and made quite
safe. The capacitor voltage rating may
have needed to be increased to cope
with the DC plus the audio voltages
across it. This is a small price to pay
for a life possibly saved.
One statement that is often made
is to work with one hand and have
the other in your pocket. It’s often
not very practical but the thought is
there to minimise your contact with
lethal voltages. I often go part way
there by clipping a short lead (with
small alligator clips) between the set
chassis and the negative lead of the
multimeter. The positive lead is the
only one then that is being used, so
one hand in the pocket is possible.
When changing components in
a set, always turn the set off at the
mains socket and remove the mains
plug. You should also short the HT
line to the chassis using an insulated
lead in case any charge is left in the
electrolytics.
It is advisable to show someone else
in the home where the main power
switch is so that it can be switched off
should something untoward happen.
And it isn’t a bad idea to have a rubber mat on the floor where you stand
to work on your radios, as this will
minimise the chance of electric shock.
AC/DC sets
There are some other rather frightening sets that you may come across
from time to time. They are the rare
AC/DC sets and the even rarer pure
DC sets designed for 240 volts DC.
Some of these sets even have one side
of the mains lead (Active or Neutral)
connected to the chassis. WOW!
With care, these sets can be quite
safely serviced once you make sure
that the NEUTRAL lead is attached to
the chassis, not the Active. It is easy
enough to change the wiring of the
plug over so the chassis is connected
to the Neutral, which is also at the
same potential as earth. However, this
does not mean that all power points
are wired correctly, so never assume
that the chassis will be neutral when
plugged into just any power point.
These sets are usually quite well insulated so that you cannot touch the
chassis when it is in the case.
Even into the 1950s, a number of
portable AC/DC sets were made like
this. They had a 2-pin non-polarised
socket that could be put onto the
set plug either way so that a “cold”
chassis could be obtained. Also
when you opened up the set case,
the power socket was automatically
disconnected. The ones that I mostly
serviced in those days were Astors
Summary
(1). Before applying power to any
receiver, check that the transformer
insulation is in good condition, visually and by instruments.
(2). Check other wiring to make sure
no shorts exist in the wiring.
(3). If the power cord is old and the
insulation is at all suspect, replace it.
The same goes for the plug if it is old
and unsafe.
(4). If the set is an AC/DC type, make
sure that the Neutral is connected to
the chassis or the negative bus. Also,
do not take it for granted that the power point is wired correctly. The use of
an isolation transformer is strongly
recommended.
(5). Use a core balance detector
(CBD) on the mains.
With all the above completed, remove the rectifier valve and try the
set on power. Check for voltage on the
chassis that should not be there, then
connect an earth wire to it without
touching it yourself. If there are no
sparks and the core balance detector
does not throw itself out, the chassis
will be safe to touch.
Run the set for some time like this
while you are there to make sure
nothing untoward happens to the
transformer. Don’t leave the set during
this time – it wouldn’t be much fun
to come back a few hours later only
to find that the set had started a fire.
Finally, check and re-check
everything at all times to prevent
being electrocuted. And never rely
on units like a CBD, even though you
may have one in use – to do so is to
become lazy and complacent.
In conclusion, think safety (yours in
particular) and you will be restoring
SC
sets for many years to come.
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and Healings. Ideally, if you want
to service any of these AC/DC sets,
you should use a 240VAC isolation
transformer.
Straight 240 volt DC sets are a real
problem to service as there are probably no locations where 240 volts DC
is even available now. Sets like this
should be just set aside to admire,
unless you care to make a 240 volt DC
power supply capable of supplying
up to 300mA.
My advice is to leave AC/DC and
pure DC sets strictly alone, unless you
know exactly what you are doing.
(Editorial note: we think that they’re
death traps).
May 1998 89
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