These designs are offered
in good faith. It is the responsibility of the constructor to ensure his
personal safety, and that of others, when building and using the devices.
Bear in mind that mains-powered items must be
constructed to a high standard of safety. If you are in doubt about your knowledge and ability to construct
and use safely, you are advised not to attempt the work.
This simple design is extremely useful to the restorer.
It allows power to be
fed to an 'unknown' radio in order to test for potential problems with mains
transformers, smoothing capacitors or rectifier short-circuits. The device works
The power is applied to the set under test in series with a 60W
lamp*. Once satisfied that no danger of burn-out or catastrophic failure is
likely, the switch is thrown to short out the lamp and apply power directly.
It's that simple - and that useful. If there is no problem with the set being
powered the lamp will light quite brightly but very quickly dim
down to a feeble glow, indicating low current. No lamp light at
all tells you that the input to the set is open-circuit. If the
lamp remains brightly lit there is a short or severe
leakage across the power input of the set.
It should be remembered that
some receivers, particularly superhets, may refuse to work when
powered through the test lamp because the HT level will be lower
than in normal use. Even so, the lamp can still be used as an
indicator of an open-circuit or short-circuit, as described
NOT attempt to build this device unless you are sure of your ability to do
apply power through lamp limiter or direct until you have ensured no
obvious short-circuit or other power supply fault exists. The lamp
must be a conventional filament (old type) lamp, not one of the
energy-saving devices. Such lamps can often be found sold as
'rugged duty' or 'heavy duty' lamps.
DUMMY AERIAL for signal generator
This simple little device is built into a felt-tip pen
and uses a short length of plain brass rod as a probe. It is suitable for use in
the alignment of most valve radio sets. It is not suitable for alignment of
short waveband RF stages.
dummy aerial? Well, radio sets are intended to work with an aerial of some kind:
valve portables usually have frame aerials, which are, in effect, the aerial
coils would onto a large former. This, by the way, is the reason why the cases
of portable receivers often need
turning, as best reception occurs when the axis of the frame aerial is in line
with the transmitting station. The same sort of process gives the principle
behind radio direction-finding. Portables should always, therefore, have their
frame aerials in place when alignment is carried out. Such sets need the
generator to be linked to the aerial by means of a inductively-coupled coil when RF stages are being aligned. Inductively means, in practice, that no electrical
connection is made - ideally, a radiating coil, shielded and constructed to the
Radio Manufacturers Association standard, should be used, connected to the
generator AF output by means of a shielded cable and placed about a foot from
the radio set's frame aerial. However, here's an alternative
for battery portables. Construct a loop consisting of about four turns of -
preferably - stiff insulated wire on a former about the size of the set's frame
aerial. Connect the loop ends to the generator output and set the aerial about 2
feet from the receiver. Initial setting-up might require the generator leads to
be placed near the frequency-changer grid for inductive coupling, to roughly
align, before proceeding with the loop.
Construction is simple. The components
should be mounted on veroboard. The cable should be fed through a hole
drilled in the base of an empty thick felt pen case and soldered
into place on the board. A plastic strap should be fitted to prevent strain on
the cable. This should fit inside the end of the assembled case. A hole to suit
the brass rod should be drilled in the nozzle (cap) end and the rod bonded with
Araldite resin adhesive so that about an inch sticks into the case body when the
cap is replaced. The rod should be notched with a file to help the Araldite
resin glue hold it firmly in place. If the cap is too loose, a turn of adhesive
tape around the body top should help. The probe may be insulated by sliding some
plastic sleeving over it, leaving just a small tip exposed. The sleeving can be
obtained by stripping heavy duty electric wiring cable.