Methods of Improving Reception

Radio Record, Volume II, Issue 28, 25 January 1929, Page 26

Methods of Improving Reception

~|T' is intended this week to run over the whole radio installation, from aerial to loudspeaker, giving a summary of all likely sources of trouble and hints for their remedy. Unless a really efficient receiver is used, distance work is out of the question this summer, but a great deal of improvement, both in regard to the sensitivity and tonal qualities of the set, can be obtained by an occasional overhaul. It is difficult to write an article that will apply to every individual set, so likely faults that can occur in the more popular circuits will be enumerated. Even if the reader has an efficient commercial receiver, the accessories he uses, batteries, valves, ete., need occasional inspection, and an instrument will be described for checking the batteries and valves irrespective of the receiver. The Aerial and Earth Systems. (COMMENCING with the aerial and earth system. The strength of the broadcast reception depends, in the first place, entirely on the amount of energy fed into the receiver from the aerial. This is quite evident. Unless a really powerful receiver is used, good reception cannot be expected from a poor aerial. Length of aerial, except.in the ease of a erystal set, has very little bearing on aerial efficiency. A longer aerial certainly does collect more energy than a short one, but other factors are brought into play, and these "other factors" counterbalance any improvement gained with a long aerial. In Qugland the greatest length of aerial and: lead-in permissible is 100ft., and in practice an aerial of this length proves most efficient. This is speaking of the conditions most likely to be met in the average home installation. tion. The greatest point to note regarding aerial effiicency is the effective height. This is not always height from the

ground, but distance from nearby conductors, such as trees, buildings, etc. It is said that to increase the effective height of the aerial by 12ft. is to add the efficiency of another valve, | Many listeners, not satisfied with their distant reception, buy or construct extra H.F. boosters. Besides being a rather expensive items in the first place, the extra valve needs feeding, and the majority of such boosters add another tuning dial to manipulate.

If no tuning dial exists, then the circuit employed is such that much amplification from this extra accessory cannot be expected. Before speculating in more sensitive apparatus overhaul the aerial. If prauticable raise it to +40, 50, or even 69 feet. The writer spent most of last summer travelling around the East Coast. In several cases set owners were advised to raise their aerials, As a result, stations that were inaudible previously came in on the speaker when the set was connected to the higher aerial. Examine and clean the insulators. In districts near the sea especially, 4s the salt is carried with the mists that blow in from the sea and collects on the insulators. During the evening. moisture will collect with the salt and form a conductor through which the minute high frequency currents can leak away to earth. Give the earth, too, all the attention it deserves. Remember that orda —

inary iron piping will soon rust when buried, and iron oxide is a bad conductor of electricity. In preference to a solution of salt for the earth, pour a solution of bluestone, copper sulphate, around the pipe, as this tends to plate the exposed iron with metallic copper. Besides being illegal, gas pipes niake a poor earth connection, as the joints are seldom electrically good. Red lead and tow are not good conductors. .

The Arrester. ‘THE insurance companies demand the use of a lightning arrester. Whatever make is purchased, always examine carefully before installing. At the back is usually found a cavity that has been filled in with wax. See that the wax is quite intact, and is not loose in the hole. The arrester is usually screwed to the wall outside the house. Between the component and the wall is a good trap for moisture, and if the inside of the arrester is damp woe betide the chance for D.X. results. If the wax is loose or cracked put the whole arrester in-the oven, unless it is made of ebonite or bakalite, and beat up sufficiently for the wax to melt and drive off any moisture inside. Inspect the lead-in tube and clean the joint between lead-in terminal and aerial. A touch of solder will remedy any trouble here for-ever. Many cases have been brought to the writer’s notice of set owners using twisted flex for connecting between the aerial and earth of the set to lead-in.and water pipe. There is no doubt that this is still done in various homes, This is a deadly practice, even though it is much neater. Aerial and earth wires must be kept separate if good results are to be expected. Inspecting the Receiver. OW let us turn our attention to the receiver itself. In the commercial sets very little can be done as they are usually built of components with very little latitude for adjustment. If the

panel and baseboard can be readily taken out of the cabinet, peep inside and inspect all the soldered joints. Some fluxes used for.soldering tend to corrode the copper after a few months’ exposure, The only remedy here is to thoroughly clean the joint and resolder with fresh wire if necessary. Certain kinds of insulated sleeving used ons commercial receivers is inclined to absorb moisture and grow a fungus. If this is very bad the whole set needs removing, and is best put into theg hands of a practical dealer. There are small adjustments, however, that can be done by anyone. Clean the variable condensers and solder a pigtail flexible connection between the moving plates and the frame. This will remedy most of the crackling noises experienced when tuning in. Because a 2 meg. grid leak is universally used is no indication that this suits all valves. For distance work a higher resistance 3 to 7 megohms will usually be found more suitable. If stability is not obtained by neutralising the high-frequency waves, test all grid resistances with a pair of phones and a 44 volt battery. If any are found to be defective consult the local dealer as difficulty may be experienced in.obtaining this resistance already wound. Several commercial machines use this method of damping, and defective resistances are a common source of trouble. The symptoms are instability and absolute lack of sensitivity. Testing Ganged Condensers. I the receiver employs gauged condensers, each of the stages should be tested to see if they are all in resonance. To do this accurately needs special apparatus, but a right indication can be obtained by the following procedure.Make sure that each condenser on the common shaft is readily accessible. Tune in a distant station to maximum strength, and insert a piece of mica about 1-16in. thick between the fixed and moving vanes of each condenser separately. Mica has a higher dielectric value than air and the effect of a mica separation between the plates is to slightly increase the capacity of that condenser. No metal parts must be touched with the hands and the mica should preferably be mounted on the end of a short length of ebonite rod, similar to a fish slice. | If the signal strength is increased when the mica is between the plates of any one condenser, carefully bend the end vane, whether fixed or moving, slightly towards its neighbouring vane. — ee —

ST "THE hot, dry summer, with its bad radio atmosphere, is with us, and, unless the receiver is in first-class condition, the owner will lose much of the enjoyment derived from radio. In this article "‘ Pentode" describes how the amateur who knows little or nothing about his receiver, may set about tuning his set to maximum efficiency. AUIUALOUASUUOUQUDUSGSU4S0000ULOULNALOOSDOSNSOLUASUOSO CRSA AAA

Of course if small compensating con- . densers are used in the set, this is not necessary, as. any. discrepancy is made up by-final tuning on the midget condensers,. Methods of Coupling. ASSING to the audio-frequency of side, there are two or three methods of coupling to be considered. Transformer coupling is most popular; resistance and impedance coupling can be considered together, as they are subject to similar. failings. It is on the audio side that the tonal qualities are mostly developed, and it is this. end that so many receivers fall short of perfection. It is possible to get almost perfect esults from either of the above ethods of intervalve coupling, and it "is a mistaken idea to think that better quality can be obtained using resistance than transformer coupling. Especially is this a fact when a study is made of the: different methods adopted by the average constructor when using a resistance coupled amplifier. In the first place all high frequency currents must be rigorously excluded from the low frequency amplifier. After the detector valve must be a suitable by-pass condenser from the plate to the earth. If it is possible to insert a RI choke in the plate lead between the plate or reaction coil and the audio-transformer or resistance the tonal qualities will be greatly improved. Transformers.

WORD on transformers. During recent years manufacturers have been experimenting with and im‘proving their instruments until they have obtained a uniform amplification over a very wide range of frequencies. This has been brought about merely in the design and use of special forms of iron compound. Cheap transformers and ones that are several years old were not made on any scientific principle and in use give distorted and unfaithful reproduction. No casual test can be applied to see if the transformers are efficient, and the. owners of any set that utilises cheap transformers is well advised to substitute for components made by a firm of standing. If replacements are undertaken, carefully tag each wire directly it is unfastened from the old transformer, so that no mistake can be made when the new one is substituted. Resistance-Coupled Stages. Receivers employing resistance, or Dr coupling are liable to one or two serious breakdowns. In ,the anode of each valve is either a resistane or a choke. If either are at all defective, the anode current cannot get to the valve which refuses to operate. The coupling condensers also are the cause of 75 per cent. of the trouble experienced with these forms of amplifiers. All coupling condensers have to be of good quality and free from even the slightest leak. Only reliable mica condensers should: be used, and even these checked over with a 90 volt battery and a pair of ’phones. After the first loud clickeno more should be heard /uhtil the condenser is discharged. +4 The grid leaks. too, sometimes ‘use rouble. Usually the valves are one meg. 4-meg, and = jmegohms for first, second, and third stages respectively, and ‘these "ean bo tested roughly with a pair. of puones und a 224 volt battery. Lf a coustant subdued’ whistle is vreserit

~~ throughout the broadcast, and it is © found to be due to slight oscillation on the audio frequency side, this can generally be remedied by connecting a }-meg. grid leak in series with the grid leak of the first audio frequency valve. This subdued oscillation often occurs when using two transformers of the same make and type. Having gone over the set bit by bit, all that remains to be examined are the valves, batteries, and speaker, Let us try the batteries first. When testing the B battery with a high resistance voltmeter, good class instruments are usually of this type, always take thie

reading after the set has been in use for an hour or so. Dry batteries quickly recuperate when not in use, and a high resistance instrument gives a reading in excess to that of the actual voltage, as applied to the set. The surest way to test an accumulator is by the use of a hydrometer. Two volts per cell is maintained right to the bitter end, but a hydrometer indicates the state of the change at any time. Speaking to the owners of small amplifiers, merely 45 volts B voltage cannot give good quality reproduction. For an average-sized room even 90 volts is not enough. It is said. with good reasons, that real quality cannot be expected with voltages lower than 300 volts. Some readers may take exception to this, saying that their reception is perfect with only 90 volts. The reply is simple. It is well-known that a moving coil speaker reproduces all orderable frequencies with most fidelity. Try to operate a dynamic speaker with merely 90 volts, using but a single valve in the last stage. Radio assumes a totally different complexion when really good quality is produced, and it pays the listener to keep his batteries right up to scratch, substituting new ones for those in which the voltage has dropped more than two-thirds the rating. C. batteries are inclined to be neg: lected. The public are told that these batteries last from 12 to 18 months, and leave it at that. Poor insulation of transformers, damp battery cable. ( «+, Will often reduce the life of a © battery to merely a few months, and these should be tested with a. voltmeter. A run-down © battery can aeccount for distortion, paralysed valves, and quickly run down B batteries. The Valves. NEXT come the valves. After two years’ service it cun be assumed that e vaive has done its duty, and, al-

— though the filaments may be intact, these veterans can be put aside, to be called on in case of emergency. A diagram is attached for a simple method of checking over the valves, and those that have lost their emission through old age. The grid bias battery in dotted lines can be inserted to check over the characteristics to see if they conform to those given by the makers. Pay special attention to the last valve. If good quality for an averagesized room is required, this must be of the power of semi-power type. Loudspeaker. FUUNALLY, there is the loudspeaker that demands a little attention. The bell of a horn type seems to be 2 magnet for little pieces of paper and chips of wood, especially in the hands of the youngfolk. Take offthe horn and shake out any dirt that is resting on the top of the diaphragm. Thisisa tip to the owners of the short horn type of speakers. Obtain a length of brass or lead tubing of the same dia-

are as tom -_ meter as the small end of the horn, and about 12 to 15in. long. Unscrew the horn from the unit, and insert this length of tubing between the two, fastening with small lengths of rubber tubing slipped over. This increases the length of the horn, and prevents a great deal of the rattle and ‘tinny effect so often noticed in a horn speaker. Unscrew the unit, and ingpect the magnets. If these have lost their magnetism, it is well worth the trouble and expense of getting them remagnetised. If the speaker ever has the misfortune to burn out during the cirly evening, quite good results can be obtained if the burnt-out: coil is located and short circuited, running temporarily on one ‘coil only. It is very seldom that both the coils go simultaneously. A small hole about 1-16in. diameter, drilled in the centre of the diaphram of a horn type speaker, will often make quite a difference to sensitivity and tone. With the latest cone types of speakers very little adjustment can be —

-- a done to the unit, Ifa cone of the free edge type is used, it is well worth the time and trouble taken to make a baffle. A hole cut in a piece of wood, four-ply is quite suitable, the size is the outside of the cone, and the speaker fastened with either side of the cone on opposite side of the baffle. The tone is at once noticed to be richer, and the individual instruments of an orchestra come out in their true colour.