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 Because there was a very large market for portables, lots of them are still available. Many sets, especially the later 'all-dry' miniature valve designs, are potentially simple restorations. They often lack the complexity of mains-powered radios and because of the limitations of battery power, rarely ran 'hot' or, due to the high cost of batteries, for long periods at a time. Some points made on these pages apply to these later, B7G based all-glass miniature-valved sets, although much is common to the earlier wet-cell 2V LT radios. Ever-Ready 'Sky Queen' |
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Mains/battery models
With
these sets, great care must be taken to avoid electric shocks when working on the live
receiver. Many are ‘universal’, i.e. they are designed to work from any
power source – AC mains, DC mains or batteries. Some (table models, rather
than portables) even allow the choice of a single 2 volt cell accumulator or a
1.5 volt dry battery for the filament supplies. Because
of the DC mains requirement these sets do not use an isolating mains
transformer, which in turn means that the mains power is directly connected and
under certain circumstances the metalwork within the set (chassis, loudspeaker
frame etc) may be ‘live’. This of itself is not a problem
but the potential risk of a severe
electric shock makes it essential
that
a check is made of the polarity of
the mains connection and to work in a safe manner at all times.
Other
sets may be ‘battery/AC mains’ and these usually have an isolating
transformer. Intrinsically rather safer, they should still be handled with great
care. Battery/mains sets tended to use metal oxide rectifiers within the power supply arrangements and these are not and never were the most efficient devices. It is logical when replacing these to use a suitable silicon diode with a low-value limiter resistor in series as a surge preventing device. The filaments of many of these sets will be found to be wired in series, therefore with a dead set all filament continuity should be the first check as one failed filament will prevent the set from functioning. When purchasing, watch for signs of misuse - loose wires where there should be a battery connector, for example. This can indicate that someone has tried to power the set wrongly and may mean that the valve filaments have been blown or the sound output transformer destroyed. Items such as carrying handles, Rexine case covering materials and items such as metal case latches and carrying handles can sometimes be replaced - but loudspeakers and output transformers are less easy to source. Another potential problem can occur if an exhausted battery has been left in the set. Sometimes they can leak, the acid compound damaging chassis members and other delicate parts. Severe rust effects found in an otherwise clean set may indicate this trouble. Acid must be neutralised or it will continue to eat into the metal parts. One easy and reasonably effective way to do this is by scraping off as much as possible then 'scrubbing' with methylated spirit on a stiff-bristled brush such as a discarded toothbrush. POWER SUPPLIES
One
of the major problems associated with the restoration and use of valve portable
radios is the unfortunate fact that suitable batteries are no longer
made. |
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These
are your options:
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 photo courtesy of Jim Exley |
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A
design for an inverter for HT power from a low-voltage battery is shown on Alan
Lord’s web site. This design does not provide low tension, which can be
obtained from a suitable arrangement of single torch cells in parallel or, in the case of certain receivers where
heaters are in series, a low voltage battery, possibly with a series resistor as
a limiter. It is always worth while checking vintage radio web sites for other designs which might appear.
SAFETY Remember that battery-only radio sets were never designed with mains powered operation in mind. For example, control knobs will probably be held by exposed grub screws. Obviously, there's little danger of a fatal electric shock from a high-tension battery* but it is a different story if a badly designed mains unit is fitted to convert the set. Great care should be taken when making power supply units and precautions should be taken when handling a set converted by someone else - at least until you've checked things out and are satisfied that things are as they should be. Remember too that vintage eliminators may not be safe and should be checked before use for insulation by a qualified person. * Which should not be taken to mean the HT batteries are perfectly safe. They are not, if mistreated and even if a shock isn't fatal, it is an unpleasant experience. Also, shorting such high potentials can cause excessive heating which can lead to fire.
Safety precautions are not simply for the sake of the radio enthusiast, but also for anyone who may come in contact with the set when in use: children, partners, relatives, subsequent purchasers.
All rights reserved. © VRW 2006/12 |
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