QUESTIONS AND ANSWERS

Radio Record, 12 June 1936, Page 54

QUESTIONS AND ANSWERS

" C,H." (Petone): We have not. yet ~* described a five-valve portable, so that there is no blue print available. However, the "‘Picnic Portable," a threevalve set, was described in the November, 1935, "Radio Times." ""T R.F." (Roxburgh): Your troyble * should be easily curable, provided that the new voice coil is not too large for the frame. Try using strips of thin card or paper as wedges bétween the voice coil and the core to centre the coil before tightening up the serews. This will ensure that the coil is central gs regards the core, "If rattles still persist, then it means that the outer surface of the coi! is touching the outer pottion of the gap. If you cannot get: satisfaction from the agents and can’t get the present speaker fixed by anyone else, you can still get a new speaker to suit the set at a reasonable price. "TFT UMDINGER" (Timaru): We have not been able to get a diagram of this set, so that we, too, gre rather in the dark, The method suggested doves not seem to be of any use to us at ail, but if you have a one or two microfarad condenser handy, you can find where the hum is coming in. Connect one side of this condenser to the chassis and with # flexible lead on the other side touch the plate of the 6B7 and see whether the hum is less, If so the hum is coming in ahead of the 6B7 plate. Then try the diodes in turn, and if the hum persists, it has come in probably from the cathode of the 6B7. Connecting across the cathode resistor should now cut out the hum, and a suitable condenser here will cure the trouble. However, if the hum is not pre sent in the 6B7, you will have to try, with this method, methodically going through the set. One other place where hum may come in is in the fiéld coil itself,’ Short the speaker transformer itself, and if the hum is ‘still there, the hum comes in via the field coil and indicates that the co!l is faylty. You will find this condenser method.a very ‘nseful one. ‘ "HEADPHONES" (Oamaru): This job is one that must be done well or not at all, as there is a real danger of teceiving a bad shock if the insula tion is not perfect. We would advise you .to get a serviceman to do it for you. . "GPEAKER" (Christchurch): Your diagram is correct, and a ten-inch speaker would improve the tone, Ltajher than use a 2500 ohm field, get one for 1500 ohms, as that is standard too. D*64W. (Feilding); We are please to hear of the suecess you are haying with the "Power Pack and Amplifier" and the adaptation of’ the: "Sky Hawk" to suit it © The number of turns for the secoudaries for the broadcast band you ean obtain from the tables at the back

of the "1935 Radio Constructor’s Guide," as they. will depend upon the diameter of the coil form you have. The primary .of the R.F. coil will then be one-qnarter those of the secondary; the primary of the detector coil one-half and the reaction coil d4pproximately ‘one-eighth of the secondary. The reaction cojl will probably need experimenting with to ascertain the best number of turns. "NY J.L." (Waipiro Bay): The portion *of aerial which yoy mark is the lead-in and it ie not necessary to insulate this at all unless it is liable to scrape on some metal object. In that, case it is much better to tie. on an insulator and hold the insulator off the object with a strain-rope. The results gained by insulating would be just the same ai at present. Q." (Rangaroa): The size of wire * iy about 40 atid the spool used was an ordinary layge size cotton reel. As the resonant points of pick-ups vary a great dea] among the different makes it was not necessary to design a coil aecurately, and this one covered all the’ usual resonant points with a little to spare. "MNHOROLA" (Bay of Islands): ‘The ‘best plan would be for you to obtain another transformer and replace the faulty one with it. It seems as though the gain with the one stage is not sut: ficient to run the last valve with the normal bias. The replacement valves for the five-valve set. would probably be those which were in the set when you bought it. The ones that ‘are recommended by ihe makeis are not as good as those you , lave. "PUZZLED" (Ohinewai); Your set should not have this trouble, anil as you have had it so short a time the agents should go over it carefully anid correct the misalignment which is evidently pregent. ‘The "wave-like" sound you get on shortwave may or may not be normal, but without bearing it we cannot express an opinion. "POM THUMB TWO" (Awakino): Transformer coupling is desirable in order to obtain better gain. The tone wil] not be affected much in this set if a fairly good transformer is used. . 2. The automatic bias could be used, 3. Yes, by all meang remove this wire: it is of no uge. , . H.E.K." (Nelson): The apparent * spreading of 4YA oyer 4QG js probably due to the latter fading, and .the’ automatic. volume control coming — inte Play, which is liable to happen especiaily if the receiver ie not accurately tuned to 4QG. The interference you experieyce on short wave cannot be completely diagnosed from the symptoms you give, but it seems like "man-made" interference.