If you are thinking of repairing or restoring a vintage wireless (radio) set then you need to be aware of the hazards involved and understand how to avoid accidents. Hopefully the information on this page will assist you in this and allow you to enjoy these rewarding pursuits safely. However, nothing is certain in this world (apart from taxation and death!) and so, whilst this information is provided in good faith and is widely considered to be good practice, I can accept no responsibility for its use or misuse. If you are in any doubt about your abilities to use and work with electrical equipment safely, you should get expert local help.
The information given is primarily directed at working with valve (tube) radios.
Within the European Community the mains voltage is currently 230V +10/-6% (50Hz) between the LIVE and the NEUTRAL terminals, together with a separate protective EARTH terminal. When this high voltage is developed across the human body it could gives rise to a fatal electric shock. Therefore you MUST NOT under any circumstances simultaneously touch both the LIVE and the NEUTRAL terminals or you are very likely to die. A further risk arises because the NEUTRAL wire is earthed back at the substation. Therefore you MUST NOT under any circumstances simultaneously touch the LIVE and EARTH terminals (or anything else that is earthed, e.g. water pipe, other electrical appliance etc.). This latter risk can be reduced by fitting an RCD (Residual Current Device). These devices monitor the balance of current flowing in the LIVE and NEUTRAL wires. If an imbalance of more than (typically) 30mA is detected then the RCD will shut off the power. However 30mA is still quite a high current and would give you a nasty shock and might even kill a person with say heart problems.
In the rest of the world various supply schemes are employed, ranging in voltage from about 100V to around 250V.
The system used in the USA is worth a special mention. In the USA Power distribution around each state is 3 phase (delta or Y configuration) at the several kilovolts level. Locally a single phase transformer across two of these 3 phases, provides a 240 volts center tapped output. This center tap is grounded at the transformer and called the neutral wire , then the three wires are run into each house along the street. The two live (hot) wires each separately provide 120 volts relative to the neutral wire and go to wall outlets to run low power devices (lights, TVs etc.). Power hungry devices like electric stoves and water heaters which require 240 volts are connected across the two live or hot wires. Each house also has a good local ground such as an 8 ft ground rod or a cold water pipe which also goes to each power outlet as the ground pin. Risks associated with the US system are much the same, therefore, as the European system.
Mains power is now universally AC but many years ago electricity companies supplied DC mains power instead. Therefore, many vintage radio sets you may encounter will not have a mains transformer fitted and as a result will have all of their internal circuitry directly or indirectly connected to the live mains supply. This will include control shafts, grub screws, chassis fixing screws etc.
AC ONLY MAINS SETS: i.e. those that use a mains transformer to supply power to the circuits. Voltage hazards include:
Mains voltage at the input plug, the switch, the fuse, the mains tapping arrangement and the transformer itself.
AC voltage at the transformer output and the rectifier input circuit.
Up to 500V DC HT (high tension) at the rectifier output and on the HT line(s). This voltage will be with respect to the chassis and with respect to earth where the chassis is earthed.
AC/DC MAINS SETS: i.e. those that don't use a mains transformer. Voltage hazards include:
Mains voltage at the input plug, the switch, the fuse and the mains tapping arrangement.
Up to 270V DC HT (high tension) at the rectifier output and on the HT line(s). This voltage will be with respect to the chassis. Remember that control shafts, grub screws and chassis fixing screws etc. are all likely to be connected electrically to the chassis.
BATTERY POWERED SETS: i.e. those that use exclusively battery power.
These are generally much less of a threat. The HT supply was traditionally provided by a battery consisting of many primary cells (zinc-carbon) connected in series and tapped at useful voltages. Voltages range from about 60V to around 120V DC. This HT can still give you a nasty shock however, so beware.
MAINS/BATTERY SETS: i.e. those that can be switched between battery and mains power.
Hazards are a combination of 1. or 2., and 3. above.
HIGH VOLTAGE CAPACITORS
Radios use high capacity high voltage condensers (capacitors). These can carry a large amount of energy that could give you a really nasty shock. Sometimes manufacturers connect a bleed resistor across such capacitors to discharge them when the set is switched off, but that's not always the case.
It is not unknown for electrolytic capacitors to fail in a quite spectacular fashion. When they do this the innards get ejected explosively from the end to which the connections are made. Beware.
METAL WATCH-STRAPS & JEWELRY
These can cause you to inadvertently come into contact with live circuits.
Stand on an insulated surface when working on live sets. A dry wooden floor or rubber mat should be OK.
Don't wear metal watch-straps or jewelry.
Keep children and pets away from the work area.
Where your mains supply has a NEUTRAL wire, use an RCD and ensure that you connect the chassis of your set to the NEUTRAL wire.
Work with one hand kept out of the way (say in your pocket). That way you are unlikely to get a shock across the chest.
Use an AC voltmeter to check that the chassis of your set is at a low voltage with respect to earth (it should be at no more than a few volts).
Remember that old test equipment e.g. signal generators etc. sometimes have fairly large capacitors connected between their live mains terminal and the chassis and sometimes between the outer of the coaxial output and the chassis. So avoid touching both the chassis of your radio and the connections to the test equipment or you could get a shock via these capacitors.
Ideally you should use a mains isolating transformer to supply mains to your set while you are working on it. This will prevent the chassis of an AC/DC set being live with respect to earth. But take care! Firstly make sure that it is indeed an ISOLATING transformer; an auto-transformer will offer no protection - BEWARE. Secondly ensure that the isolating transformer is in good condition (with an AC voltmeter connected between earth and any one of the secondary terminals you should get virtually no voltage reading when mains is connected to the primary). Thirdly, remember that the voltage between the input plug, switch etc. and chassis will still shock you. Finally, only ever connect ONE radio to the secondary of the transformer at any one time. If you connect two radios to the same secondary you could end up with one chassis live with respect to the other.
Check that high value capacitors are discharged before working on your set. To be on the safe side, check the voltage using a DC voltmeter - it should be less than say 12V. If necessary discharge the capacitor using a small mains light bulb (10-15W) connected to insulated probes (it will flash briefly on a well charged capacitor). Remember that these capacitors can take some time to discharge to a safe level, so leave the bulb connected for about a minute or so.
Never get in the firing line of a large electrolytic condenser. When these things blow (and they occasionally do, quite spectacularly) you would be wise to be out of the line of fire! Keep the 'connections end' pointing away from you.
It's not just high voltage circuits that can be hazardous. A high current LT circuit (including of course LT accumulators) can generate considerable heating in a metal watch strap, bracelet, screwdriver etc. Even the flash from an LT short can take you by surprise and cause involuntary reactions that could lead to other injuries.
Take care and enjoy your radios.
NOTE: this is a serious attempt to increase people's awareness of the hazards involved with working on vintage radios and suggest how accidents may be avoided. I welcome any comments or corrections that could improve this page. Please email me using the link below.
Copyright Ó 2000, 2001 Lorne Clark