Antique Radio Maintenance

THE type of current and voltage of the electric mains supply vary according to the district. Apart from certain variations in the number of volts, the mains supply may be either Alternating Current or Direct Current. Wireless receivers are designed to suit one or other (or both) of these variations in current and voltage. Although several versions of wireless receivers are made available to meet these contingencies it is essential to specify the current and voltage of the mains supply when ordering a receiving set. This will not, of course, apply to battery receiving sets which are operated independently of the mains supply. However, when it is proposed to purchase an all-mains set, the details of current and voltage may invariably be obtained from the specification plate secured to the house meter. Once the dealer has been supplied with this information, with his advice the selection of an appropriate model will be a more simple matter.

To get best possible reproduc- tion, correct positioning is necessary. Probably the best place is across the corner of the room, against an outside wall, bearing in mind, of course, that if it is an all-mains set, a handy supply point will be needed. The main thing is to select a position where the earth lead will require a minimum length of wire.

If the set is a table model, a firm table should be provided capable of carrying the set comfortably, and large enough to allow the set to be turned around for inspection and cleaning. To allow for all factors the set may have to be placed parallel with one of the walls, but it must not be hard up against the wall, or resonance may occur and spoil reproduction. There should be about 8 to 12 in. between the back of the set and the wall.

When the position has finally-been decided upon, and the power plug made available, the connection of the earth and of the aerial completes the installation. It should be noted that when an outdoor aerial is used it is worth while to install a switch outside, to connect the aerial direct to earth in the event of a thunderstorm, and at the same time to isolate the set from the aerial. Such a switch can be obtained from a radio shop.

When the aerial and earth wires have been installed, all that remains is the actual connection to the set. Plugs are usually provided with the set. The set is then ready for use.

If it is desired to have radio available in two rooms, this can be managed with the aid of an extension loudspeaker. In every installation it is advisable to allow for this at the outset. Choice of the extension speaker will depend on the radio manufacturer’s recommendation in the instruction book supplied with the set. All modern sets are provided with sockets to take an extension loudspeaker, which gives the most satisfactory reproduction if fixed in a corner of the room, fairly high up on the wall, say at the picture**********-rail level, with the speaker pointing down at a slight angle.

Lightly insulated wire of small section may be used for connecting the extension speaker, bell wire being quite suitable. If the speaker is likely to be wanted in a number of rooms, sockets can be permanently wired so that the speaker may be plugged-in wherever required. With these sockets, it is advisable to run the wires direct under the floorboards, to keep them as short as possible. Connections should be checked for tightness, for plug pins that fit loosely in their sockets give rise to crackling noises in the speaker.

The majority of modern sets are of the all-mains type, and with these the location of defects and the nature of running repairs are somewhat more complex than those relating to the battery set. But the latter, if an old model, will certainly need regular attention. If reception is poor, and results are generally unsatisfactory, fault-finding should commence with a careful checking of all external connections, and not until this is done should resort be had to dismantling the set.

Battery sets have a considerable number of plug-in connections, up to 4 leads and 3 or 4 grid bias leads and 2 accumulator leads. Each of these is a possible source of trouble, but much can be done by keeping all connections tight and clean, and by bearing in mind the golden rule, ‘Before disconnecting anything attach a label to it so that it will be refitted correctly’.

Keep the inside of the set scrupulously clean, and occasionally check aerial and earth terminals, to ensure tightness. Make certain that the batteries are fully charged and replaced at regular intervals, and that the accumulator is also kept charged. The L.T. Leads to the accumulator are usually of the spade type, and these must be kept free of acid corrosion, a smear of vaseline being applied after they are cleaned.

Should the battery receiver fail to give any signal, the following tests should be made with the aid of a voltmeter and battery,. (1) Make certain the aerial and earth leads are connected up. (2) Check loudspeaker leads for continuity and sec they are securely connected to the terminals. (3) Test battery for voltage drop, and accumulator terminals for corrosion. (4) Check all battery leads for a possible break inside the insulation. (5) Test valves for continuity of filaments and see they are properly in their holders. (6) Test for voltage on the anode of the valves. (7) Test transformers and loudspeaker windings for continuity. (8) Test- any condensers across the supply for breakdown to earth. (9) Inspect on/off switch to see if contact is good. (10) Test all resistances for con tinuity.

The all-mains set requires considerably more skill to discover faults and rectify them. Indeed, it may be advisable to return the set to the dealer when the defect is persistent. The procedure to follow when the set ceases to function or is giving poor reproduction is to check all external connections before dealing with the set itself. Checks on similar lines to the battery set tests can be made under the following headings. (1) Is the power supply ‘alive’ at the plug-point? (2) Are both aerial and earth connections in tightly and the connection intact? (3) Are the loudspeaker plugs in firmly? (4) Is the set switched on? (This may sound non- it is a point to sensical, but observe!) (5) Test the mains leads for con tinuity. (6) Is the wave change switch in the correct position? This should be tried in all positions for the test. (7) If the set is for D.C. Only, try the mains plug the reverse way in the socket. (8) If it is an AC/DC set, check the resistance lead which is incorporated in the mains lead, to see if the connection at the plug end has been disconnected.

If all the foregoing points prove to be in order, the fault probably lies in the set. When working on an all-mains set, in no circumstances should the back be removed until the supply is switched off and mains plug removed from its socket. First free the interior of dust; an excellent way is to blow it out with a cycle pump or some form of light air spray.

Remove the valves, noting their positions carefully, and test the filaments in the manner indicated in the battery-set tests, remembering to pull the valve out by its base and not by the glass. If the fila- merits are in order, a further check can be made by replacing the valves and switching on. With some valves it is easy to see if the filaments are alight, but with the metalized type it is not easy. To test these, switch off and place a hand on the valve; it will have become quite warm if the filament is in order.

If a filament is thought to be broken, the valve must be replaced with a new one. When putting a valve back into the set, make sure it fits firmly in position, and that valves with top connec tions are not touching any metal screens.

If the set still fails to function, it may be necessary to take the chassis out of the cabinet to test all the compo- nents. Testing of components may have to be restricted to a continuity test if the only available equipment is a D.C. Voltmeter and a 2-volt battery. It should be noted that if condensers are being tested and continuity is indicated on the voltmeter this will show that the condenser plates are shorting, which will be one of the causes of failure.

Considerable annoyance may be caused by interference due to the operation of electrical machinery in the immediate neighbourhood, such as electric signs, fans, battery-charging sets, or any electrical machine which has contacts that continually make-and-break and hence cause a spark. Even switches in the house may be troublesome, setting up a crackling in the speaker each time they are used.

It has first to be ascertained whether disturbing noises are reaching the set via the aerial or the mains. This is done by removing the aerial plug from the set while the noise is still being received. If this disconnection stops the noise, it may be assumed that the interference is coming in by way of the aerial, and to effect a complete cure the actual source of the crackling must be discovered and dealt with. If a switch in the house is responsible, and this can be discovered only by careful trial-and-error, its mechanism may or may not be amenable to treatment. If investigation of the switch action (after the supply has been turned off) reveals serious wear or breakage, a new switch should be installed. Noise may be transmitted to the set via the mains and still be apparent with the aerial disconnected. A remedy for this is to insert a ELF. Choke in each of the mains leads with a o.I mfd. 500-volt condenser across them.

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