- PLEASE READ THIS NOTICE
pointers are provided in good faith but no responsibility can be accepted for
accidents or injury. The best safety measure is common sense and a careful,
considered plan of work. Respect
electricity - work safely.
powered radios fall into a number of groups but all require the presence of
potentially lethal high voltages. Transformer-equipped sets can offer a greater
degree of protection but whether the set is designed for AC mains only and
possesses an isolating transformer, or is designed for AC/DC operation and is
linked directly to the mains supply, there will be points within the chassis
assembly that carry ‘raw’ mains. It is therefore essential for the restorer
to bear these points in mind whenever working on an exposed chassis. It is worth
keeping in mind that a radio that once might have been safe could now be
compromised by time* or by inexpert modification.
a timely reminder that safety must be uppermost in the restorer's mind. This is
an excerpt from a letter I received recently which goes to show that even the
experienced and knowledgeable radio enthusiast (which this gentleman certainly
is) can make incorrect and potentially dangerous assumptions...
night, I was using my version of your power/control box** to get (the radio)
going and doing rather well, when without thinking I reached out to the volume
control shaft. The (shock) I got was a hefty one. The short of it is, I was
fooled by the printing on the back (cover) of the set and the fact that there
was a pretty decent sized transformer on the set... can you please use this
experience to press home the point of how easily I might have been killed."
is the test lamp set-up on the 'PROJECTS' page (Click on 'RESTORATION' and
follow the link)
use of the test lamp limited the severity of the shock he suffered but the
moral is: never take things for granted and ENSURE, regardless of type of set,
that the chassis is never live to full mains potential, only neutral. This can
be done easily by the use of a mains neon screwdriver/tester. Touching the
chassis with the tester probe when the power is connected should NOT result in
the neon lighting. Also, ensure that mains connections are the correct way
around and that where chassis should be earthed, as in FULLY ISOLATED
transformer-fed power supplies, that it actually IS. NEVER earth the chassis of
transformerless AC/DC types and ALWAYS check them for neutral connection. NEVER
leave exposed grub-screw heads in the knobs of live-chassis sets. Where such
knobs are standard for the set (many AC/DC types use spring-grip push-on knobs,
to avoid the use of grub screws) the screw should be sealed with hard wax melted
into the hole. The oddity is the fact that the chassis was live in the case
quoted. It may be that the transformer primary has shorted to its laminations,
not unheard of with old equipment. It
would have been quite possible to produce AC/DC sets without making the chassis live, but
few if any makers bothered to do so.
radios suffer from the ravages of time in a number of ways, even those
that have been well stored. All can have dangerously crumbling
insulation on power and other internal leads, capacitors will leak (pass
current) and resistors may overheat or cause problems through value
changes. Valves themselves are remarkably robust and long-term storage
often has little effect except perhaps to soften vacuum with some very
all valve-type mains radios with great respect. Ensure that your work, when
completed, cannot pose a danger to any subsequent user.
are some important general points to note:
working area should have good illumination.
building a series-type test lamp.
floor should be made of or covered with well-insulated, dry material such as
thick linoleum or carpet. On concrete, a wooden ‘duck board’ is a good idea.
The workbench should be of wooden construction, with a wooden, plastic, thick
linoleum or cork surface. Definitely NOT metal.
use of a mains isolating transformer (1:1) gives a good measure of personal
protection to the restorer.
A residual current circuit breaker, fitted to
your mains outlet and supplying ALL your bench i.e. the set under test plus any
test equipment in use, gives protection BUT MUST NOT BE USED TOGETHER WITH
AN ISOLATION TRANSFORMER or
its safety function will be compromised.
NEVER put both hands into or on
the chassis of a working set.
Work with one hand only.
ALWAYS check the polarity of any
fitted mains plug. Do NOT assume that an already-fitted plug is sure to be
correctly wired. In fact, it is safest to assume the opposite.
Do NOT rely on the set's own
on-off switch for isolation.
Turn off at the mains before attempting any work.
a mains transformer has exposed connections, tape over them, if only
temporarily, to avoid inadvertent contact. Be especially careful if you wear a
metal ring, bracelet or ‘dangly’ necklace.
A trap for the unwary: Valves, especially output and
rectifier types, can become extremely hot in use, as can power resistors.
Avoid contact with
electrolytic capacitors as these can hold a charge long after switch-off.
Although unlikely to be fatal, such a charge can give you a nasty jolt and
could cause injury from the reaction. Such capacitors may be safely
discharged through a high value resistor (connected from + terminal to the
- terminal or the metal can)
safety guidelines (if you read
nothing else, read these)
connect to the mains until the mains input and HT lines have been checked
for condition and for short-circuits. Power the set up using either a test lamp (see 'things
to make') or a variac and watch for signs of trouble. My personal preference
is always for a test lamp.
completely before taking resistance measurements or touching exposed parts
of the chassis
Always work with
only one hand inside the set, never by holding the chassis with one hand
when working on the set with the other
safety (properly connected and secure mains cable, secure rear panel,
correct knob types) before allowing others to use the set
conditions. If set has been stored in a damp area, allow ample time for
moisture to evaporate before testing
insulating material such as dry carpet
data whenever possible
Use an RCD (residual current detector) plug