Vacuum Tube (Valve) Radio and Audio Repair Guide: A few words about safety precautions

Before we start on radio repairs a few words have to be said about safety, because mains powered receivers have the capability of delivering very nasty, if not fatal, electric shocks. Fortunately danger may be avoided by taking some fairly simple precautions which soon become second nature.

As mentioned earlier, one side of the electricity supply mains (the neutral) is connected to earth and the other (the live) is at 230 V above earth. The amount of electric shock sustained by anyone coming into contact with the live main will depend to a large extent on whether he is standing on a surface which provides a good or bad conductive path to earth. It is quite possible for someone working in an upstairs room on a dry wooden floor to touch a live object and feel no more than a slight tingle, yet if he were standing on a garden path he would most certainly receive a bad shock. Thus the first precaution is to make sure that the floor of a workshop, if not of wood, is insulated in some way such as laying down boards and rubber or plastic mats.

The next thing to remember is that electric shocks are normally far more serious when they travel through the body across the heart, such as would happen if someone were to touch a live object with one hand whilst the other was in contact with an earthed object. The golden rule here is never to use both hands at a time when working on a radio set: always keep one hand firmly in a pocket out of the way of danger.

The danger points

Most UK AC/DC receivers and a high proportion of part AC and AC/DC/battery sets have one side of the mains connected to the chassis, so it is absolutely essential that this should be the neutral main. Don’t assume that if the mains lead is fitted with a three-pin plug all will be well if the neutral wire apparently is connected to the neutral pin, for if someone has replaced the on/off switch in the set at some time it is conceivable that the wires may have been crossed and that the neutral lead is now live and vice versa. In any case, don’t trust the colour marking of wires because they may be non-standard, the standard in this case being the old one in use during the tube (valve) era of red = live and black = neutral. You may find two leads of the same colour, or one red and green, or almost any other combination, so to be sure check the resistance between the chassis and each lead with the on/off switch in the on position; there should be a very low resistance in one case and a fairly high one, 500 C or more, in the other. It is the first one that should go the neutral pin and the second that should go to the live.

Resistive mains cords

A resistive mains cord is unmistakable with its one lead made of flexible asbestos and covered with ‘wool’ of the same material. This lead invariably should be connected to the live pin of the plug.

More than two leads

Some resistive cords have two ordinary conductors plus the resistive one; in such cases the latter should be connected to the live pin along with the lead having the higher resistance of the other two (or no apparent connection) to the chassis. This lead will be the one used to feed AC to the anode of the rectifier.

Spare conductors

In a very few cases you may find a third plain conductor in the mains lead which does not appear to have been connected previously. Leave it alone because it may be (a) an unused spare conductor associated with a resistive mains cord or (b) an ‘aerial’, especially in the case of a midget receiver.

Nothing certain yet

Even with the neutral mains lead definitely connected to the neutral pin on the mains plug do not assume that all must be well, especially if you are not working on home ground. Be particularly careful if extension or amateur wiring is involved. It is by no means unknown for the wiring to a power socket to be reversed, making the neutral pin live and vice versa. The most dangerous aspect of this is that any appliance remains live even though ostensibly switched off. The present writer has encountered a number of such cases including one in which all the wiring in the house was ‘back to front’ due to the electricity company having connected up its meter incorrectly.

The safety routine

The safest way to proceed is first to check the mains plug on the set as above, then, with the back off the set and its switch in the ‘on’ position, plug into the mains and switch on at the socket. Now use a neon screwdriver to check if the chassis is neutral: the lamp should not glow. Since these devices do not ‘fail safe’, then make sure that it will glow on a live surface by touching it on some part of the set that ought to be carrying AC, such as the mains dropper or a fuse if fitted. Only after all this is it safe to assume that the chassis is indeed safe to touch. In fact, the checks take far less time to carry out than to be described and experienced engineers do them by habit.

Don’t relax your guard

What you must remember now is that although the chassis itself is dead with respect to earth various items on it, including those just mentioned and the on/off switch, still carry live AC and must be avoided.

Other points in the set, such as the smoothing capacitors, the output transformer and sometimes again the mains dropper, will carry the HT + voltage from the rectifier and these will be up to 250V DC with respect both to the chassis and to earth. So long as you touch these only with the insulated test prod of a multimeter you will be safe.

Genuinely AC-only receivers

These have double-wound mains transformers that isolate the mains from the set completely, and their chassis will be safe to touch whichever way round the mains leads are connected. That is the theory and in the great majority of cases it is realised in practice but things can go wrong and precautions should be taken before working on any unfamiliar set. It is not unknown for electrical leakage to occur between the primary of a mains transformer and the core, nor for either wear and tear on the mains lead or careless handling to bring about a break in the insulation where it passes through the chassis. Be suspicious of any anti-modulation hum capacitors connected from primary to chassis and also of ‘mains aerial’ capacitors. Always measure the resistance of each conductor in the mains cord with respect to chassis; it should be close to infinity unless, of course, a three-core lead with one of them as an earth connection is being used.

As in AC/DC sets, all the various AC and DC voltages before and after the rectifier will be live to chassis and contact must be avoided except with a meter prod.

The isolating transformer danger

Yes, this is a deliberately provocative heading designed to draw attention to the fact that safety devices don’t always guarantee immunity from shock. The writer has seen it asserted in some circles that an isolation transformer affords complete protection against electric shock to anyone engaged on repairing mains radio receivers but this, unfortunately, is nonsense.

An isolation transformer is a double-wound device with a 1:1 ratio, so that when 230V AC is fed to the primary from the mains the same voltage appears on the secondary. The latter is completely isolated from earth so it is possible to touch either output terminal separately without receiving a shock. When an AC/DC radio is powered from such a transformer its chassis will not be live with respect to earth whichever way round the plug may be inserted.

What the isolation transformer doesn’t do is to alter in any way the position regarding all the points on the set that are live to chassis when the set is powered directly from the mains. In other words, exactly the same precautions have to be taken as before.

Another point: if two or more AC/DC sets are connected to an isolating transformer, unless great care is taken it is quite possible that their individual chassis might be at mains voltage with respect to each other.

To sum up: by all means use an isolating transformer for its ability to make one chassis at a time safe to touch, so long as you don’t trust it to do anything else.

Other hazards

Electric shock is not the only potential hazard for anyone engaged in radio repairs. If you have lived long enough to read this guide you presumably will already have learned to treat ordinary tools with respect, such as always keeping sharp ones pointing away from yourself, but you may not previously have done much soldering. Always try to arrange what you are doing so that the iron and the joint are above the surface of the bench and not above your person. Molten solder dripped onto trousers or socks soaks irretrievably into the material and can causes very painful burns. Even worse is any action that results in hot solder being flicked into your face: you have only one pair of eyes (see next paragraph). Incidentally, despite the near automatic reaction of trying to catch any falling object, never try this when a hot soldering iron falls off the bench.

Fumes from solder

The flux agents used in some cored solders produce very unpleasant fumes when they are being melted by the soldering iron. By definition, most of the time you will be working close to a soldering job to make sure of a good joint, thus placing yourself in a good position to breathe in all those fumes. Whether or not they actually are harmful the writer does not know, but what is certain is that in the confines of a small workshop they can cause sore eyes and throats (this is likely to be far worse for non-smokers whose lungs are unused to being assaulted by fumes). If you are vulnerable a full-face respirator (not a dust mask) will be a good investment — and it also keeps hot solder out of your eyes!

Chemical warfare

Some of the multifarious cleaning fluids and aerosols also can be unpleasant, if not actively hazardous in a small workshop. It is not always the more arcane substances that are the worst in this respect: many years ago the writer left an employee working on a television set in a fairly large workshop. This chap had decided to clean the screen of the set with ordinary methylated spirit and by the time the writer returned the fumes had overcome him and he was lying slumped over the bench. Fortunately he soon revived when brought out into fresh air, which brings us to another very important point...

On your own

It is more than likely that when you are working on radio receivers you will be alone in a workshop which may be in an outbuilding well away from other people. Can you be certain that anyone would hear you call for help should an accident occur? This is meant to be a sobering thought which hopefully will bring home to you the necessity of taking care.