Questions and Answers

Radio Record, Volume II, Issue 37, 28 March 1929, Page 10

Questions and Answers

A Defective Condenser. A difficulty is encountered by "H.J.C." {Wellington}, whe has a three-valve regenerative receiver constructed from a kit set. His signals are very weak, although his serial and earth systems give him splendid results when worked from a erystal. He has tested the set by the *phones and battery method, and all the components and connections gave a loud e¢lick, except the grid condenser. which gave a very faint click. ANSWER: From the test all appears in order, so that it would be difficult to suggest a fault without examining the set, check over the wiring. Does the set oscillate? If not, try more tickler turns. Could you recommend a good sensitive make of loudspeaker which would be suitable to the set? ANSWER: This is really a question for a dealer to decide, as he can discuss the merits of particular makes relative to the set used. In general, if the signals are not going to be very strong a small cone speaker of a good make should give excellent results. Distortion with Volume. H. M. (Matumutu). finds that his set © is distorting when volume has to be handled. He is using the following valves: A442. 415 as detector, and as first and second audio 409. ANSWER: It appears that the last valve is unable to handle the volume. Try a larger capacity valve, say 405 or 406. Use of Leclanche Cells. "J M.H." (Raetihi) asks the following ¢ questions relative to the use of Leclanche cells as "B" batteries: 4. Are these cells entirely suitable? ANSWER: Yes, but they are rather messy unless carefully watched, and to get high voltage a large number, requiring a great deal of space, would have to be used. 2. Would they be suitable for either six 201A or five of these and one 171A? ANSWER: Yes, it would be quite K. 3. How long would they deliver continuous current to the set, which seems to be heavy on "B" batteries? "ANSWER: It all depends on the size of each cell as to the length of service that can be expected, but on the set in use the correspondent should get two or three months’ use from’ each set of zines, using a 171 valve. Dip all the jar tops in wax to ‘prevent creeping, as this is the most common source of lost energy, through leakage. "Megohm’s" "B" Accumulator. I HAVE just completed "Megohm’s" . "B" accumulator, as described in the "Radio Record" of April 13, 1928, writes "Mack" (Johnsonville). It has not been a complete success. I charged up with a chemical rectifier for the correct number of hours, and it took ue first charge seven minutes to disLE 4 fran tee O

charge through a 60-watt lamp. The ‘voltage by this time had dropped to 85 volts. I have charged up for three nights now, and in the mornings readings are OK, but in the evening I find voltage has dropped to about 10 volts. The vaseline has run somewhat with the heat of the room and has formed a film on the acid in some of the tubes. These I find impossible to keep clean. I have been advised to use the correct paste, as described by "Megohm." ANSWER: Try mineral oil floated on the top of the acid and see that there is no sediment in the bottom of the tube. Correct lithage is advisable and should be obtainable from Kempthorne and Prosser. Changing Valves. ; a J.1." (Otago), wishing to change * his valves in his American fac-tory-built set, asks what valve or valves we would recommend in the radio stages. ANSWER: It is difficult to say offhand what valve should be used, but if the set is factory-made the valve should be replaced as they are now. Care should be exercised not to change the valve of an American factory-made set to any other type than those used, as the chances are that the balance will be upset. Generally speaking, 201A valyes are used in the radio stages of American receivers. 2. Would a screen grid valve be suitable in this cireuit?

ANSWER: No, very many aiterations would have to be, made. 3. What would be a suitable detector? ANSWER: 201A, or the special detector 200A. 4. What valves would you recommend for the audio stages, giving their positions? . ANSWER: This all depends on the voltage available. 112A is a good valve, requiring 135 volts, but probably a 201A in the first audio stage and a 171 as power valve (providing adequate current, 185 volts, is available) would be the better valve for the circuit. 5. Would a pentode be suitable for use? ANSWER: In this set, no. There would be a tendency to overload. Oscillation Complications. "MUGGINS’ (Gisborne), finds that after adjusting the reaction condenser to stop the set oscillating, it is necessary to turn it back some 40 degrees or more before music becomes audible. He wants to know how this can be rectified. ANSWER: The trouble may be in one of three places. ‘The grid leak may_ be defective, or may be of the wrong value. Especially with a shortwave set should the value of this be varied. Then again, EN NEN

the radio frequency choke may be of the wrong inductance, and the valve is quite frequently of the wrong type. Almost every shortwave set has its own particular characteristic, and it is usually some time before an operator can find the most suitable valve. Interference from a ‘Telephone. A CORRESPONDENT writes complaining that he is getting interference from the telephone. When this. rings, he gets a certain amount of erackle from the receiver, and just recently he has noticed that at a certain point of the dial there was a blank spot at the edge of which he could get a definite whistle, and on cutting this out, heard the conversation over the phone. ‘This set is 20ft. from the phone, though in the same room, The earths are separate. He asked how he could get rid of the interference, and how he could do away with this peculiar method of eavesdropping. ANSWER: The blank spot effect is probably being caused through the set becoming tuned to the same frequency as the current in the telephone. Actually, the microphone of the telephone is acting as @ miniature transmitter. Try a small series condenser in the aerial lead, such as .0001 to .0005 may help to lessen the effect. A counterpoise earth or a series condenser in the earth lead are other suggestions which may help.

Difficulty to Get Lower Wavelengths. "NTANY-WAVER" (Havelock North), who uses an all-wave receiver, cannot tune in the shortwave stations. His main trouble is oscillation, which goes into flat spots on the short waves, but on the longer waves is quite easy to control. On the short waves, there is a noise sounding very much like the throbbing of a motor-boat, accompanied by a whistle which changes in pitch.

ANSWER: The following suggestions should help the correspondent: Try varying the values of the grid leak, making it higher, and of the radio frequency choke. Vary the B_tappings and try different valves as detectors. Strange as it may seem, cheap transformers, or those with a high ratio, are better for shortwave work than the low ratio type. In other words, the primary should have a low impedence. The grid condenser ‘need only be .0001 for shortwave work. Shortwave tuning is very tricky and the chances are that’ the correspondent has missed dozens of stations. It is possible to tune in about ten stations on a single degree on a 180. degree dial.

Home-made Accumulators. "(NX R.B." (Takapau) asks the following * questions relative to the accumulator made from old B battery plates, deseribed in the "Radio Record" recently. Specifically the questions are :- 1. Can I use positive car plates for the negative plates in the B battery? ANSWER: No, they are unsuitable. 2. Would I have to treat them with anything? ANSWER: No, treatment would have no effect whatsoever. 8. What value of fixed condenser would I have to use to change the value of 4 .0005 condenser to a .00025. or a .00035? How is this computed? ANSWER: By the use of a .001 condenser in a series, a .0003' will be the equivalent. Tables of combinations were worked out by Megohm and are republished in the "Beginner Corner" this week. To. Get the Americans. " A G." (Wanganui) wishes to "get the eAmericans." He has a 5-valve fac-tory-made set, a good aerial and earth. He ean get all New Zealand and Australian stations with good volume, but he is unable to pick up the Americans or Japanese, . :

ANSWER: Probably the correspondent is not looking for them at the correct time, as American stations are rarely heard after sunset, while the Japanese are usually very late at night. To get the Americans, the aerial should be directional to America, that is, it should run in a north-west direction. The correspondent could increase the power of his set by using more B battery current correctly applied. Short-Wave Adapter. "W T.S." (Wellington), asks for @ ¢ diagram of a shortwave adapter for a 4-valve Browning-Drake. He has learned that adapters of this description. do not give satisfaction with the B.-D. ANSWHER: Actually, the shortwave adapter is not being used with the Brown-ing-Drake, but with its amplifier, which is really no different from the amplifier of any other set. The characteristics of the B.-D. is contained on the radio-fre-quency side, and as a shortwave adapter plugged into the detector socket, the Browning-Drake part of the receiver is: eliminated. An adapter will work wei" with any good amplifier. In our "Listener’s Guide," a shortwave adapter using the screen-grid radio frequency valve has been described.

A Puzzling Phenomenon. "PUZZLED" (Takaka), writes: "If I remove the leads.from the B_ battery terminal, and then replace it, I hear a distinct click in the speaker, and if shielded from the light, I can see a spark when the wire touches the terminal. Could you explain?" ANSWER: The correspondent has not said if the filaments of his valves are turned on. If they were, there is nothing

unusual about this, for the distinct click Merely indicates that the set is in working condition, and ready to receive signals. The small spark is caused through ’ the circuit of the B battery being completed through the loudspeaker, and valves. If, however, the filaments are turned off, it would indicate that the B battery is being short-circuited, and that insulation has somewhere broken down usually the bypass condenser, Batteries Unsatisfactory. HE same writer adds, "Being unable to procure a battery for some weeks, I have been running my set one block short. Now that I have procured one, I found that on connecting it along with the others, the volume has considerably decreased. If I disconnect it again, the volume leaps up. ANSWER: The question is a little vague, but one would take it that the valves, particularly the detector and the lio-frequency, are being overloaded h B voltage, and this would cause a marked drop in_ quality as well as in volume. If the radio-fre-quency and detector Valves have 135 volts on them, this: decrease of volume would be expected. The detector rarély needs more than 45, so that the correspondent would do well to see to his connections, so that each valve is getting no.more than its correct amount of current. Various Points. " D." (Whaka) asks the following * questions :-- 1. Why does my set still work with no reduced volume when the grid leak is removed? ; ANSWER: Probably the frequency valves are detecting. 2. On removing the "B" plus detector wire from the batteries I can still get the New Zealand stations. . ANSWER: This seems to more than ever that the R.F. valves are ‘detecting, in which-case little can be done ‘\ beyond increasing B voltage on RF ' yalves. 3. On setting the dials to any wavelength between 300 and 350 metres, and bringing the set into oscillation, a roar will start and increase in volume till it shakes the windows. ANSWER: Caused evidently through a microphone detector valve, Try another detector.

~K 4, I wish to put a ‘phone jack Detweel the first and second stage. How is this wired ? ANSWER: As the set is a factory-made one and has a metal panel our candid advice is "Don’t." It is not advisable to try these additions to factory-made sets. Single Dial Control. I HAVE a four-valve set. which I would like to convert to a single dial control, but at present: the two variable condensers do not read the same for all giations. The aerial tuning condenser jeeds more capacity (greater dial readang) than the grid condenser. What would. you .advise?--"M.L.’ -ANSWHR:: -See "Beginner Corner." ; _Péntode’s 3-in-One. "VALVE" (Christchurch) asks the following questions relative to the *3-in-one"’ : ; 1. Could honeycomb coils be used in this set? If so, could the 18th turn and 50th turn coils be used in conjunction with a two-way coil holder? oo ANSWER: A honeycomb coil can be used, but its: capacity: would be slightly different from that of a plain solenoid coil, The grid coil would need to be much less than 50; in fact, 30 would almost cover the broadcast band, that is, provided it was tuned with a .0005 variable condenser. 2, Could the Browning-Drake type of eoil with the moveable tickler and an extra winding of 20 turns for the aerial be used? _ , SNSWER: Yes, it would be quite all

8. Should there be a wire between the bottom of the grid coil and the earth? ANSWER: "his is contrary to the. regulations of the P. and I’. Department, but the battery can be earthed, 4, Would 24 guage D.O.C. wire be suitable for these coils? ANSWER: If honeycomb-yes. If solenoid, put on a few turns less. 5. Please advise the specifications of coils for wave lengths between 80 and 200 metres. ANSWER: Twenty-two turns secondary and 7 turns tickler on a 3-inch former. with 18 tinned copper wire, will tune from 65 to 110 metres, while, say, 30 turns secondary and 10 turns _ tickler would probably cover the remainder of the band. However, this would have to be decided by experiment. Improving a 4-Valve Set. Wit regard to a 4-valve set, ""Phasatrol" (Paeroa) asks: 1. Is a clarostat in conjunction with a .00025 condenser correct for a phasatrol : ? ANSWER: A phasatrol used in the position suggested by the correspondent would not be advisable, as it is going to weaken volume considerably. The best method of neutralising is on the neutrodyne principle as described in the Beginner Corner last week. 2. Could you suggest any alteration or improvement on an ordinary four-valve ANSWER: Yes. The following should improve tone. 1. The incorporation. of a by-pass condenser and radio frequency choke — between the tickler and the first audio transformer, such as was described in the "Radio Record" recently. Connect a fixed condenser of a capacity ranging from .001 to .0005 from the transformer side of the tickler to "A-.’ Between the point where the wire "to the condenser leaves the lead from the tickler insert a radio frequency choke to econnect to the audio transformer. 2. If an output filter such as described in the "Radio Record" of February 8 were added an improvement should he noticed. The condenser from the radio frequency choke should be at least .5 to 1 mfd., but the .001 condenser put directly across the speaker is not necessary.

_ Pentode’s Dynamic Cone. if making the moving coil loudspeaker as described in the "Radio Record," writes "Fan" (Dunedin), and would like verification on the following points :- i. If a pot made entirely of ordinary east iron, ineluding the ends and the core, were used, what thickness of walls and general dimensions would you advise. the gap being 3-32nds. instead of 5-32nds? ANSWER: There is no need to make any alterations in the specifications; try to obtain an end plate of: dynamo steel. 2. Could I have a high resistance movying coil so as to avoid the necessity for a step-down transformer? ANSWER: It would be difficult to construet such a coil with a gap of 38-32nds, as 1000 turns of 44 gauge wire would have to be wound on. 8. If the pot-magnet coil is wound for 230 yolts, ean it be plugged into the lighting mains without any intervening units? ANSWER: No, the A.C, current would have to be rectified. Rectifiers have been fully dealt with in the "QListeners’ Guide." 4, Is the 230-volt winding as effective as the 6-volt accumulator winding, and could the latter be energised by a Tungar charger of 6v. 2 amps. output? ANSWER: To the first part of the question, "Yes’; if the winding was to be energised by a charger, it would be as to connect an accumulator in parallel with the speaker. In other words, to float an accumulator across the speaker windings. 5. I presume that an ordinary 6-volt accumulator, which is already feeding 5valves would not be able to stand the strain of a moving coil loudspeaker as well as the set? Am I right?

ANSWER: Yes. The strain would be rather much. Connect the Tungar charger to the accumulator, and float this across the speaker windings. Position of Transformers. I AM building a set, using two transformers of the ratios of 6-1 and 4-1. Which transformer should be the first? H.J.F.-P." (Henderson.) ANSWER: It is claimed by recent investigators that better results accrue if the smaller ratio is used in the first stage. A Very Long Aerial. . A.K." (Mangamahu), writes, "J *have erected by the way of experiment, an aerial approximately T50ft. long and between 100 and 130ft. high, using 12-guage copper wire. I find that I eannot tune to the lower wavelengths. although I have tried different capacities of condensers in series with the aerial. How might I tune in the lower wavelengths." ANSWER: An aerial of this dimension has a very great self-inductance, so that less inductance is necessary in the aerial coil. Try reducing this. It will be found that volume will be slightly decreased. and this is the general problem of aerials, but if the correspondent works along the lines suggested, he should do some interesting experimental work. Try a neutralising condenser in series with the aerial lead. The Heat of Filaments. "G I." (Hastings) writes: "In last * week's ‘Record’ I read that audic valyes should be burned full on, but my set works best with two audios controlled by one rheostat, and turned about

three-quarters on. The R.F. and the detector are both controlled by a rheostat, which works best when turned down. If turned on too far the set bursts into oscillation. If the audios are turned on fully, there is no noticeable increase in signal strength, while static is ‘far louder and comes in with. sharp cracks instead of the usual crackling. ‘The valves are the 4-yolt type, and are heated by three 14-volt cells. ANSWER: In this particular case, the burning of the filament with the rheostat turned on full would mean that the filaments were getting half a volt of current more than they should do, so that the rheostat would naturally have to be turned back. The set bursting into oscillation would indicate that there are too many turns on the tickler or the set is not correctly neutralised. 2. My earth is a tin chimney of about16 sq. ft., and buried 5ft. underground. It is connected to the set by Tit. of wire of the same guage as the aerial. I have soldered a wire to this earth and carried it about 20ft. under the house to a clamp on the water pipe. Would the water pipe earth be an advantage? ANSWER: Providing the first earth was efficient and kept damp, no. The 20ft. of wire would offset any advantage. Concerning a Portable. " W.R." (Karori) wishes to dismantle * his portable set, and seeks advice on the following points: 1. Could you supply me with the diagram’: of a cireuit, and particulars of components? ANSWDHR: If the correspondent wants a really good portable set, he could no no better than construct the ‘‘Rotorua’’

portable, described by "Pentode" in our issues December 14 and December 21, 1928. 2. I intend using a 6-volt car battery in place of 3 dry cells. What would be the result? ANSWER: This would depend on the type of valve to be used. If valves eonsuming 1 and upwards of an ampere, the accumulator would be necessary. For dull emitters of the .06 type dry batteries would be quite O.K. Otherwise, there is no difference providing the accumulator can be regularly charged. 3. What advantages, if any, other than ‘inereased "A" battery drain, have low filament consumption valves over high filament consumption valves? ANSWER: None. 4. Is the small vertical tuning coil obtainable in New Zealand? ANSWER: If the plug-in type of eoil is meant, yes. 5. I use an ordinary aerial coupled to the frame aerial. Could the frame aerial be obviated by employing different tuning coil from that incorporated in the factory-built set, and, if so, can it be procured in New Zealand? ANSWER: The method of attachment of a frame antenna differs from that of an aerial. The former is made to replace the aerial tuning coil, the outside being connected to the tuning condenser and the inner end to the other set of plates of the tuning condenser and then to the filament negative. An aerial, however, is eounected to an aerial coil which may be a single tapped coil or two coils. If the former type is used, the coil should consist of about 50 turns tapped at the eighteenth from the bottom. The aerial is taken on to this tapping, and the top of the coil goes to the condenser and the bottom to the other set of plates of the condenser. The frame antenna is thus replaced by the aerial coil.