Questions and Answers

Radio Record, Volume VI, Issue 16, 28 October 1932, Page 10

Questions and Answers

APRIL (Wellington) : I intend building a band-pass filter described in the 1932 "Guide."’ Is this a suitable filter for my seven-valve a.c. set? _ Act Yes, but: it would be better to construct a filter along the lines deseribed in the September "Radio Times" in. connection with the "Link. Three." It is. of the capacity-inductance type’ and has a better frequency response than the purely capacity ‘type which was described in the "Guide." . The coil particulars were given in the October issue. a Will it make 2ZW any sharper?3. Is the .01 mfd. mica coupling condenser supposed to be touching the aluminium sereen?-A. : Yes. OUNTRY (Frankton J.):-I have a five-valve commercial neutrodyne. Would it be more efficient and economical and not too expensive to construct another circuit using the same parts? A.: Probably’ a more economical set would be the "Neutron Four’ described in the "Radio Times," with an extra stage of radio frequency amplication. 2. What parts can be salvaged from my present receiver? A.: Any. that fit in the parts list or are fairly near to it. ANNOYED (Taumarunui): When the volume control is full on there is & loud hiss in the speaker which does not appear to be valve hiss. A.: Every set will, to a certain extent, hiss when turned on fully, and if you have had a serviceman on the job it is impossible for us to advise you any further. Your receiver is a standard one and should be quite satisfactory. The only satisfactoty method of getting over this trouble will be to analyse the set thoroughly, proceeding stage by stage. You can loeate the stage which is causing the trouble by pulling out the valves one by one, comInencing with the one nearest the aerial. A HY, (Wellington): An aerial doub- * ling back on itself is not as efficient as an ordinary straight aerial. However, if space is limited you are probably doing the best you can under she circumstances. BERDONIAN (Wellington): I am using." sereen-grid pentode on a short-wave set. Can I successfully use a Band © eliminator? A.: There may be a little ripple on the short-wave due to this, but you eould probably get over it by incorporating a fairly big choke in the detector lead and by-passing, it with a

large condenser of, _Say, 2-4 mfds. You will need probably 75 volts on the screen instead of the 4) you have shown. 2. Will there be any added | signal strength by using an outdoor say, 45 feet long? I now use an indoor antenna, A.: Yes, it would be much better. 734 (Southbridge): What are the particulars of the filament winding , for a tungar bulb rectifier? A: Owing to the high filament current taken by tungar bulbs" they have rapidly gone out of date, being replaced by economical rectifiers. A two amp. charger has a filament current which requires about 16 amps at 2 volts. The secondary would have to deliver 20 volts, as this woule allow for 14 dropped through the bulb. . OI WHARE (Feilding): I wish to bring my "Outspan" up to date. I use a "By and "©" eliminator, and want to do without the resistances as specified for the 1932 "Outspan" in the last "Radio Times." What are the voltages on the plate, and screen of the detector? : If you wish to derive the full benefit from the screen-grid detector you must use resistance coupling. In this the detector plate voltage is fed. through the usual } megohm resistance and an appreciable voltage drop takes place, hence use your B max, tapping. The voltage on the screen should be between 20 and 30 volts. If the last tapping on your eliminator is 50 your best plan would be to use 2 50,000 ohms potentiometer between this tapping and earth, taking the moving arm’ direct to the screen, not forgetting to by-pass it with a .5 mfds. fixed condenser. A. C. (Auckland): Your parts, list is | correct, though the use of .0002 instead of the .0001 in the position C3 may | alter your tuning slightly. | P. C. (Livingstone) : Yes, the shortwave adaptor shown is a perfectly good one, and will work well with the 1982. "Outspan Four.’ It does not use the — whole of the set, but plugs in to the defector socket. 2. Should I change the four-volt valves, would it be best to remove the series connector and then connect the-two cells in parallel? A.: Yes. although being a little under, the required voltage, you may not get the best results, You may have to use.a rheostat in series with 3 volts. 5. If my set uses 3 amps "A" current, how long would a car generator rated at 6 volts 4 amps take to. recharge a 120 ampere hour accumulator; the set being used 8 hours a day and run for a week without charging. : will need to charge your battery four or five hours a week, 4. What would two PM8 in pushpull double biased draw? : On no load the current would be Under operating conditions they ‘Will draw between them from 1 to 2 milliamps. L S. (Christchurch): My set is very * harsh when tuned to either of the local stations. When I disconnect the set all the harshness disappears, A.: Through turning down the yolume you are introducing distortion. It is usually far better ‘to take off the setial where local stations are concerned. 2. How many charging hours should a 90-volt Fdison battery require ‘when using a 60 watt lamp in a chemical rectifier? A.: It will be required to be charged approximately twice as.long as you use the set. That is, if you are using the set for 6 hours, you will require to charge your battery for 12. , .:(Continned on page 13.)

Information Coupon (To be used with all requests for information.) Name of set PPoeeHeBeeeeseseaseese Model sereteroseeeressseceeresoses Name ve ccsanceresacccoceceseseses Address SSCSHHEESESTOSBASESESEE SESE SSCS SSS eSESOSSOEHSHOSSSSESE ES EEOEED SOTO SE SESE EHTS SOTSEOESERESS EO EE OOEO Nom de plume .occccsencevccecece To be kept in subsequent inquiries. Please Note :- 1) Be specific and. brief, tabulating,.if possible «2) legibly, and on one side of the paper. (3) We do not design circuits, (4) Limit three questions, unless 1/- is enclosed. (5) Postal queries limit three questions, Enclose stamped envelope and 1/- fee.

(uestions and Answers (Continued from Page 10). 3. What is the charging rate for a 4 volt 60 ampere hour battery? A.: This is usually stated. About 4 amps. Viti (Manurewa): Soon after my set is turned on a fierce tearing noise develops, and dies out in pops in about 200 seconds, A.: Probably one of the valves is causing trouble. Turn the set on, and when the noise starts pull out the valves, commencing with the one nearest the aerial in turn, and by this means you can locate the valve which is causing thé trouble, for when the noise stops, it is that valve or the associated circuit. Have the valve tested, or move them round if they are movable, 2. Using a pick-up with the set a crackling noise begins, and is influenced, by the potentiometer.

A.: This is probably a broken wire in the potentiometer, but it may also be a break in the armature in the head of the pickup. ‘ 8. The aerial tuner was described in the "Radio Record" dated October 30, 1931. MMICROPHONIC (Westport) : You will get details of the band-pass filter from the September "Radio Times." You will find them in connection with the "ink Three." F{OPEFUL (Karori): Would it be an improvement if I biased the two v.f. stages of my "Outspan Five’? Yes, it would improve the. selectivity. 2. How would I make the connections? A.: Break the two grid returns to earth and take them to a common G.B. negative terminal, taking the positive to "A-." The "Neutron Four’ described in the "Radio Times" will show you the connections. Use a separate primary. @ I sometimes get a loud hum in my — Shifting the speaker often stops this. . ‘A.: This is due to a microphonie detector valve and the only satisfactory way of eliminating this would be to change the valve, BROOKLYN (Wellington) : Can I use a pentode in the last stage of the "Kestrel Three" ?-Yes. 2. Can I make the above set selective enough on the broadcast band to permit of separating 2YA from the other YA stations, and perhaps Australia? A.: Yes, by incorporating a band-pass filter. However, if you are to use it for shortwave you will be in difficulties. CURIOUS (Picton): What power valve do you suggest for a "Celestion" speaker? Could I work a power valve in the last stage, in connection with it? "A.: Hither a pentode or a medium power yalve can be used, the ratio being ad-

~ justable by terminals on the output transformer. 388 (Invereargill) : I have a PMi4 and an $625 and I want to use both in the "Differential Wive.’- Which one would precede the other? A.: Use PM14 first, followed by the $625. 2. T am told the inter-electrode capacity of S625 is reduced if it is tapped to-the succeeding coil. Should I make this connection ? A.: Not if it is the valve next to your

_ detector. If you are using it inthe first stage, it should be tapped in at the eentre of the coil. 8. I have a 442 valve, the top of which has broken off, leaving a small hole, with the plate wire protruding. A white sub- ‘ stance has formed inside the glass of the bulb. The filament appears to be all right. A.: Providing there is no actual gap between the evacuated portion of the valve and the outside of the glass, the valve will be all right, but if there is a hole in it, the valve has been ruined. —

New Zealand Radio \ Institute At the next meeting of the New Zealand Radio Institute, which will be held in Messrs. Nimmo’s Hall, Wellington, at 7.30 p.m, on October ; Mr. Preston Billing will lecture on theory, operation and-servicing of the modern superheterodyne. As this subject is of paramount importance to radio servicemen and enthusiasts, if is hoped that a good muster will attend. The Institute is making this lecture open to all those interested in radio.