QUESTIONS AND ANSWERS

Radio Record, Volume VIII, Issue 43, 3 May 1935, Page 41

QUESTIONS AND ANSWERS

L H.R. (Christchurch): I would ‘like * to build a one or two-valve shortwave set. Where could I obtain constructional details? A.: From the, 1935 "Radio Guide," now on sale. You could not do better than build the "Tom Thumb Two," which gives very fine results on both the shortwave and broadcast bands. C H.F. (Dunedin): You would obtain * more audio gain by amending the circuit for your duo-diode second detector along the lines indicated in the circuit of the:"Comet Superhet Five," described in the 1985 "Radio Constructor’s Guide." "T R.F." (Dunedin): If you do not * which to make any circuit alterations to your set, replace the 224’s with 224A’s; otherwise make no change 10 the replacement types. You could improve the selectivity by using an aerial tuner, such as that described in the 1935, "Radio Guide," or by taking a few turns off each of the primaries of the r.f. coils; it would not be practicable to convert your set to a superhet. A ©. (Tirohia): It would be. quite * possible to use two 30’s in pushpullto drive a 19-valve, but there would -he no point in doing this, as a single 50 can fully. load a 19. By the way, have you’ had the 380 tested recently? If it is a little "off colour," volume will be poor. H. M.B.K. (Paeroa): Bvidently the * volume control in your set is defective, and should be replaced. A T. (Pokeno): I have an old three- * valve battery set which I intend to convert into a four-valver with a screengrid rf. stage. I would like to use a Browning-Drake circuit. A.: There is no need to use the Brown-ing-Drake circuit if you are using a Sereen-grid r.f. valve. An ordinary "straight" tr.f. set such as the "Midget Three," described in the. March, 1934, "Radio Times," would suit your requirements. , 2. What type of valve should I use? A.: In the 6-volt series, a Philips 642 or a Mullard PM16. 5. What would be the details of the coil windings? A.: You have not given the capacity of your tuning condensers or the size of coil former you want to use. Send this information along and. coil details will be supplied. "TMENDERFOOT" ,(Whakatane): A type B605 power valve would be the best replacement for your output valye. Be careful to apply’ the correct bias.:- , 2. Could yeu give me a line on how to proceed with the calibration.of a homeconstructed oscillator? A.:.This could be done quite easily by beating the output of the oscillator against the carriers of the main broadcast stations, as outlined in the "Radio Times". dated January, 193+. ' 3..-Bbe oscillator: I have constructed used‘a 2A5. When used for such a purpose, does this valve'still require its usual grid) bias?

A.: This depends on the eireuit, which you have not enclosed. EFCY" (Carterton): How could 1 add a pair of phones.to my set? A.: Connect one side of a fixed condenser of from .01 to"1 mfd. and of high test to the plate of ‘the output pentode. Connect’ one side of another ‘condenser of similar capacity to earth, Then the headphoneg_are connected to the remaining two free terminals on the condensers. Fit a switch in one of the voice coil leads

to the speaker to silence the latter when the phones are being used. 2. My set is not as selective as I should like, and there is a great deal of heterodyning on most of the smaller stations. . A.: You could try using a_ shorter aerial, But you would probably lose a fair amount of your signal strength on distant stations. The fact that your set has not an rf. stage accounts for the comparatively poor selectivity. (Continued on page 43.)

Questions and Answers (Continued from page 41.) 8. With any volume at all, the speaker vibrates on all deep notes or on speech. How can this be cured? A.: If the output pentode is in good order, then evidently the speaker wants re-centring.

"PADIO" (Wataroa): If your set is a superhet, then the most likely cause of the trouble you mention is that the oscillator is not functioning over the higher portion of the waveband. It seems: as though the oscillator valve wants replacing, but your safest plan would be to call in a serviceman to check up on this. If, as you say, the "A" battery is a little flat, this could cause,-the trouble. D O. (Auckland) : You have not sent us * the size of the plates of your tuning condensers, and so we can only make an approximate guess at the capacity. The following are the coil details you ask for :-

Band. Pri. See. React. 16 to30 metres... 4 9 65 30 to 49 metres .. 5 16 8