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VINTAGE RADIO WORLD - SAFETY POINTS AND TESTING HINTS FOR PORTABLES

SAFETY POINTS

Obviously, a battery-only receiver does not, under normal circumstances, pose a problem of electrical safety to the user. Batteries have far too high an internal resistance to allow lethal currents to be developed. It follows that makers tended to disregard safety aspects. For example, access to the interior of such sets, far from being made consciously fiddly, is often very simple and quick – essential when changing batteries. Knobs are usually attached with grub screws. Cabinets may have metal external parts which are not internally earthed, such as speaker grille.

None of these points make battery sets dangerous when used as the makers intended. It is only when such sets are coupled in some way to the mains supply, perhaps by fitting some kind of power supply, that safety has to be considered. Providing that any mains or high voltage points are well insulated and cannot come into contact with fingers, there should be no real problem.

It is essential that any mains-driven power supply should use a fully isolated double wound transformer, properly earthed.

Signal generators produce radio-frequency signals to assist in the alignment of IF transformers and RF stages (tuning stages). Correct alignment means highest gain, most sensitivity and can be critical to a set's performance

An 'Advance' signal generator

One effective way to prevent inadvertent shock is to make access to the innards more difficult - perhaps by fitting extras back screws.

Ensure knobs are well seated with no exposed metal control spindle accessible. Grub screw holes can be sealed with melted wax crayon of a suitable colour, using a soldering iron.

A practical battery eliminator design is shown in the VRW eBook ‘VINTAGE RADIOS’

ALIGNMENT

Portable valve equipment must be set up with accuracy if good results are to be obtained. There is little room for error. If there is no sign of IF transformers or trimming capacitors having been tampered with, they are best left well alone until such time as testing points firmly to faulty alignment of either the IFTs, the RF stages or perhaps both.

A reliable and accurate signal generator is essential for accurate alignment. A multi-test meter can be used as an output meter, but do not rely upon your ears to tell you when maximum output is achieved, because the human ear cannot accurately distinguish between quite widely varying levels of sound.

Where receivers are normally operated from internal frame aerials re-alignment must be carried out with these aerials in circuit. For R.F. alignment the signal should preferably be injected via a standard shielded coil.

The connecting leads should also be screened. For preliminary adjustments, it may prove necessary to loosely couple the signal-generator output lead to the grid of the frequency changer valve by laying the lead near to this point. The receiver should always be aligned with the batteries and loudspeaker in the same position relative to the frame aerial as would be the case under normal operating conditions, otherwise the inductance of the frame aerial may be affected.

TESTING HINTS

	Remember that voltages are considerably less with battery circuits than is the case with mains receivers.
	The filament voltage of all-dry valves is 1.4V – don’t test for continuity with a 6V battery!
	All components are rated for lower voltage and current. Handle all components with care, especially the capacitors.
	Output transformers are lightweight and the windings can easily be ‘blown’ if a short is placed in series with them. From personal experience, the primaries of such transformers are especially prone to this. Take care.
	Avoid creating sparks – these can flash over to the valves, transformers and other sensitive components and cause failure. Be particularly careful when taking voltage measurements around valve bases – it is easy to slip with the test prod and short filament to anode, blowing the filament instantly. (And sometimes more than one - as the filaments are usually wired in parallel, the lot can blow!)
	A good way to fault-find is to use a simple test oscillator/signal tracer. Couple to the set using a good quality AF type capacitor, say, 0.01µF. Never connect the signal output directly as there may be a DC potential present at the oscillator output that could do damage.
	When connecting or disconnecting batteries, be sure to turn the set off at its on-off switch. This prevents inadvertent high potentials being applied to low-tension inputs.
	Do not unplug or fit valves when the set is switched on. It is all too easy to bend pins or force the valve into the socket incorrectly. Much damage can occur this way. Take your time, switch off and be sure you replace valves in the correct order when refitting.
	Chassis in general are inclined to be less robust than mains set chassis are, for reasons of lighter weight and lower manufacturing cost. Handle with care when chassis are out of cabinets. Accessibility is also often something of a compromise, so extra patience is needed, along with good dexterity and tools such as long-nose pliers and tweezers.

GENERAL TIPS

	Many restorers recycle spares from scrap radios to restorable mains powered ones. Purchasing a battery radio for this purpose can prove to be a problem, for the following reasons:
	Battery sets generally use capacitors of a lower working voltage rating than mains-powered sets require.
	Resistors found in battery sets are always of a modest current rating: values of a quarter-watt and less are common. Again, these may be unsuitable for use in the higher powered mains set.
	Output transformers are of a lighter construction and are made neither to match nor withstand the power requirements of a mains output valve.
	Loudspeakers, on the other hand, tend to be very sensitive types and these may well prove useful at times, providing the mains set is of a modest power rating, for example a TRF or small table type.
	Of course, all such components are in general suitable for restoring other battery-driven radios, though it is my belief that capacitors are better replaced with new or new, old stock (NOS) types as the reliability of old capacitors is often suspect. Electrolytic capacitors are usually best replaced by new ones.
	Battery portables were designed for sensitivity and low running costs, not for hi-fi reproduction, so do not expect the reproduction to be of the quality of a good mains powered receiver.

A good design for a combined square-wave generator/tracer is to be found in the 'Vintage Radios' eBook

VINTAGE RADIO WORLD