QUESTIONS AND ANSWERS

Radio Record, Volume VIII, Issue 24, 21 December 1934, Page 43

QUESTIONS AND ANSWERS

H A.M. (Timaru): I have a two-gang * condenser, .00038 mfd. per section. What value of fixed condenser would I need to connect in series with each section of the gang to reduce the value to .00015 mfd.? A.: .0002 mfd. 2. My broadcast coils consist of 85 turns on iin, formers, with 20 turns wound on smaller formers placed inside each secondary for primaries. The tuning ig rather broad; how could I cure ikis? A.: You are not using a very selective type of set, but at the same time, you cculd easily make it a little more selective than it is at present by reducing the aerial primary turns to 10 or 12. On the other hand, the detector primary is rather on the small side for greatest sensitivity, though if you increase the number of turns on this winding the selevtivity will suffer. Hence it would be hest to leave this coil as it is. "NicHr OWL" (Shannon) : I wish to rewind a magnetic speaker unit which has the coil bobbin and pole pieces arranged as shown in the sketch. In which direction should the winding be put on? A.: Neither-it is quite immaterial. FV, ©. (Auckland) : It is not likely that * one of the neutralising condeasers in your set is at fault; ‘rather we would suspect one or both of the rf. valves. Have these been tested with proper testing equipment? ‘The fact that the filament lights up is no indication that the emission is not faulty. If the tuning coudensers in your set are ganged, another likely cause of the trouble would be incorrect alignment of the gang. For these reasons, we advise you to have the valves tested by a dealer and the set realigned and reneutralised. 2. What are the values of resistors and condensers for a resistance-coupled stage, using a 201A? A.: Plate resistor, 50,000 to 100,000 ohms (depending on the preceding valve), coupling condenser .01 to .1 mfd., grid leak 5 to 1 meg. "(\AMBRIDGH" (Pukeroro): Your best plan would be to fit headphones in the grid circuit of the power valve of your set, thus cutting out the last stage when the phones are being ‘used, The simplest and safest way oi ‘doing this is to "break the lead between the audio coupling condenser and the grid and grid leak of the power valve, and insert a closed circuit jack The separace terminal should be earthed. This method of fitting phones was used in the "AirAce Short-waver" described in the August, 1934, "Radio Times." Unfortunateiy we have not a copy of the circuit your set uses on hand, so we cannot give you more specific details. "PUZZLED" (Livingstone): I have built the "Tramper’s One" and have added. a two-stage amplifier, using two 201A’s and a B605. The set refuses tc oscillate, though the amplifier works very well in conjunction with a microphone. A.: Have you tried reversing the con-

nections to the reaction winding? If the three windings of the detector coil are all put on in the same direction, then the bottom of the reaction winding should gv to the plate of the detector valve, and the top to the reaction condenser. Also, try increasing the plate voltage of the 201A. As a last resource, you could try adding a few turns to the reaction winding, although this should not be necessary. 2. Is it necessary to complete the circuit of a charging dynamo when the ba‘tery is removed while the dynamo is running? A.: No; under no circumstances should the tetminals be shorted, as damage tu the dynamo is likely to result. 3, Is there any method of automaticaily cutting out the dynumo when the battery is fully .charged? A.: Yes, by using a cut-out as described in the September "Radio Times." E G.D. (Dunedin): The most impor- * tant factor in erecting a noise reducing antenna system is to erect it as high and as distant as possible from the source of the noise. As it is usually? impossible to erect the lead-in outside the interference field, a special type of lead-in js generally used. This can either take the form of a shielded cable with the shielding earthed, or a transposed type of lead-in. In the former case, the

interference does not penetrate to the shielded wire, and in the latter the noise picked up is cancelled out. In regard to interference entering through the mains, you could easily check up on whether this is appreciable or not by removing the aerial lead. Then, if your set is properly shielded, you can take it for granted that any noise remaining is entering via the mains. If it is appreciable, then a power-line filter would be beneficial. A suitable type, which you could make yourself; is described in the December, 1933, "Radio Times." W A.B. (Dunedin) : Sorry, but we can*not undertake to design, circuits for readers. However, you will find that the "Midget Three," described in the March "Radio Times," is just the set you want. Ganged tuning condensers could be used. K J.S. (Lower Hutt): .00085mfd. tun- * ing condensers are far too large for short-wave work, and you would find. . tuning extremely critical. To reduce the vapacity to a suitable value, you could pul] out five or six plates from each section of the gang. Also, as you are plauning to use home-made coils, it would be advisable to incorporate a trimmer actoss the r.f. section of the gang to compensate for any slight discrepancies in the coils, A suitable capacity would be .00005 mfd. or thereabouts.

Information Coupon (To be used with all reguests for information.) Name of set Model Name Address Nom de plume To be kept in subsequent inquirie8: Please Note : ~ (1) Be specific and brief, tabu: lating if possible. (2) Write legibly and on one side of the paper. We do not design circuits. 4) Limit three questions unless 1/: is enclosed; (5) Postal queries limit 3' ques- tions: Enclose stamped: and addressed envelope and 1/ fee: