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Disclaimer

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.

TEST LAMP

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 like this:

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 above.

Do NOT attempt to build this device unless you are sure of your ability to do so safely.

*Never 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.

Why a 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.

 

 

     

 

 

LIST OF COMPONENTS AND MATERIALS FOR TEST LAMP

1 X 60W mains lamp. 1 X batten holder to suit above lamp. 1 X one-way mains-type surface mounting light switch and pattress box, plastic. 1 X surface mounting switched 13A socket and pattress box, plastic. 1 X choc-bloc, at least 3 way @ 13A, for facilitating lamp wiring. Quantity of timber. Original uses a 9mm ply top panel with 15mm MDF side rails. Woodscrews, fixing clips for cables, PVA adhesive, Danish Oil or varnish.

 

LIST OF COMPONENTS AND MATERIALS FOR DUMMY AERIAL

One felt-tip marker pen, about 0.75" diameter by 4" overall length.

One brass rod, about 3mm dia. by 3" in length

One 300pF capacitor

One 390 ohm resistor (ideally a non-inductive type)

One length of flexible screened or co-ax cable

One plug to suit RF output socket of signal generator

One short section of matrix board or veroboard

 

VINTAGE RADIO world: SIXTEEN YEARS OF WEB PRESENCE