Questions and Answers

Radio Record, Volume V, Issue 47, 3 June 1932, Page 10

Questions and Answers

EK B. (Devonport): As your set takes * 3 of an ampere it would not be a practical proposition to attempt the Daniells Cell vharger. Full instructions appeared in the "Radio Record" of: November 7, 1981.

R L. (Christchurch): I cannot get a * power detector to work. Why? A.: Can you test the voltage on the plate? It will be necessary to use a high resistance meter, owing to the high resistance in the plate circuit. Try the effect. of a choke and two .0005 condens-ers-one on either side of the choke, and between this and earth in the circuit. To get the power detector working satisfactorily it is necessary to put a heavy signal on the grid, and as you do not appear to be doing this it is not altogether surprising that you do not have the stage working. 2. I purchased an alleged 25,000-ohms resistor, but appears to be 21,000 ohms. wil this affect the working conditions? -No. 8. Used as a power detector, approximately what voltage should be on the plate of the 227. and what approximate m.amps. should it draw? A.: There should be about 180 volts on it, and it should draw less than 1 m.amp.

prxVvox (Timaru): What yalue_ of coupling resistors or chokes should be used to give the greatest gain consistent with good quality in the circuit I enclose? The valves are 224 coupled to 227. A.: In the plate circuit 100,000 ohms. in the grid circuit .5 megs. Actually these are not the optimum values for coupling a screen-grid valve to a 227, but as you are using grid-leak detection. and in consequence will be drawing a relatively fairly high current for the plate, it is impossible to put the optimum resistance in the plate cireuit because the necessary voltage would never be developed on the grid. Were you using power detection, you could inerease the value of the plate resistance to somewhere between + and 1 megohm. which would give you a better load resistance, 2. Yes, you can use the circuit, though it is doubtful that it would be as effective as the ordinary resistance coupled stage. 3. Please criticise the converter I haye depicted. In operation there are no repeat points. The B.C. Set is a superhet. "A.: Your circuit is quite satisfactory. but you would get better results if you used an rf, stage ahead of the detector. Are you quite certain that the set will oscillate on the lower frequencies. It may be necessary to reduce the resistance in the B+ lead. Decreasing the value of the .01 condenser may bring about more ready oscillation. W J.D. (Reefton): Is there any cure efor loud hiss in a super-het. receiver? Iam using one which was designed for a 119 valve, but am using other six-volt valves. This hiss is very pro: nounced. A.: It would probably be reduced if you used a 199 in the oscillator position. You can decrease the filament yoltage by inserting a suitable resistance ‘ series with one of the filament prongs. ra W. (Wellington): 2YA comes in over ten degrees. blotting out 5CL, 4QG and an American, and when 2YA is approached a loud crackling noise is heard. Could this be my aerial, "which is 40 feet high and 100 feet long, and the lead-in’ ‘coming down outside two clectric light wires? The broaduess of tuning may be due to your condensers being out of alignment,

or a faulty resistance in the detector eir cuit. However, it would be necessary for a serviceman to make adjustments, It is unlikely that the crackling is being picked up through your lead-in. You would notice it on all outside stations. Probably you are overloading ‘your set with the heavy signals from 2YA. Possibly your proximity to 2YA is giving you trouble. It would not be a bad plan to cut down your aerial. Certainly you would lose volume, but that cannot be helped. . HIZBANG (Frankton Junction): I added another valve and transformer outside my Cossor Melody Maker, but when the "(": battery is connected volume is decreased. I have noticed that the yalves and transformer are not connected as described in "Questions and Answers." — _- . e re oe

AL? tuvidentliy the stage 15 DOt COURSE ed correctly. The diagram in "Questions and Answers" was prepared specially for the Cossor Melody Maker, so check them over and see that they are the same, and you will certainly get better yolume, but take the 220P to the last stage and put in a general purpose valve where the 220P was, cutting out the bias on that valve, or at the most reducing.

(ZREBE (Tauranga): I have my set grounded to an oil drum which I keep filled with water sunk about three feet down, properly connected. Would reception be improved if I installed a further ground connection by gas or water-pipes sunk, say, six feet away and connected to one another by a ground wire? A.: An elaborate ground, particularly in districts where the earth connection is not particularly good, is generally worth while, particularly if a battery set is used. Under these circumstances we think you would get decidedly improved results by making the change. G M. (Christchurch) : See this month’s * "Radio Times" for a circuit of the Loftin White, using a pentode valve. We are hoping to incorporate this circuit in a high-class local station receiver to be described in the near future in the "Times." A ERIAL (Wokitika): The combined + "A" and "B" current must flow through the 4-ohm resistance before the battery is reached, and as the set was no doubt designed for 201A’s this current amounted to approximately 1} amperes, and so would result in the development of anproximately .6 of a volt, (Continued on page 22.)

petemererere. cr Information Coupon " (To be used with all requests f information.) ' Name of set .,..-...-.+- woaccoce Model seen ecco eee eereeeerereneees Name ase cecceneconaceesosevenes ACAreSS -ncccescccccsccccescssee:: eee wee PCT HES HSESHESSESHSEOAO HEROES SPORE EEE OR ESE SOSSESOEHABO ENE: Nom de@ Plume ..cccacesseveeesss To be kept in subsequent inquiries Date _........ Scenes Please Note :-- (1) Be specific and briet, tabu lating, if possible. (2) Write legibly and on one side of the paper (3) We do not design cirenits (8) We do not design circuits. (4) Limit three questions, unless 1/- is enclosed. ‘

Questions and Answers" / (Continued from page 10.) which would provide a slight bias on the audio and at the same time cause the valves to operate at their correct potential of 5.5 imstead of the six volts delivered by the "A" battery. 2. Why will the set perform with the "GO" bias removed from the detector? A.: Detection is probably taking place in one of the valves prior to this, or a leakage is taking place within the valve socket, 8. Why will the set not oscillate above about 350 metres? A.: Because of its design, It is not strictly speaking a regenerative set, and should not oscillate at all. 1"? (Wellington): Could I add a screen-grid detector to the ‘‘Sellens Shortwave" set without altering the audio stage? ; A.: Unless the screen grid detector is followed by a resistance capacity coupling, there is not much to be ghined in using the screen-grid detector. + 2. Is my potentiometer grid return connected correctly? A.: The correct connections are as follow :-One side of the potentiometer goes to A- on the detector valve, the other side to A plus or earth, for A plus in this cireuit is connected with the ground. The centre tap goes to the grid eak, 8. What are the correct voltages of a 8008 "B" and "OC" eliminator? A.: The voltages depend upon the amount of current you are taking from each tapping. Had you let us know how it was connected, we could have told you, the various voltages. It is impossible otherwise. a T K.M. (Christchurch) : I have a three- * valve well-known set, including one stage of screen-gird, and I can get only the YA stations on the speaker. Why cannot I get the overseas stations? A.: There must be something wrong either with the valves or with some component in the set, because we tested this set and were able to bring in many overseas stations. Can you borrow a voltmeter and actually test the voltage which is en the detector valve? Possibly your primary has a high resistance join which is causing the trouble. Have you tested the s.g. valve and made certain that all connections are satisfactory? Are your aerial and earth in good condition? Try improving the earth, and see if this has any result.