QUESTIONS AND ANSWERS

Radio Record, Volume VIII, Issue 37, 22 March 1935, Page 47

QUESTIONS AND ANSWERS

~~ ~ iesoopesmemansisesarcssisse ee ~~ ¢Auekland):: The 9294's ; set ‘ could be. ‘replaced « by 224A's,. but. ‘otherwise ‘we do not advise yousto; suse any other: replacement ‘types of: valves’ than ' those.Jin your. set at present. W: A.V. (Oamaru) and: AM. (Ashbiur- "*’ ‘ton),: You-will-both\find‘that the. "Viking Short-waver," .to be- described in the: 19385.. "Radio Guide". (published: at’ the’ end" ‘ofthis month), would give: you. excellent . -results.- «Alternatively, you could, build up the Lekmek A.C. Converter;. described in the July, 1933, "Radio Times:" Also, if -you vould care to: wait for two months or so, 2 cheap. ‘but efficiént ‘single-valve a.c. converter. will be- described’ in the: "Radio ‘Times. id ) Vv. W. -(Giivtstéqueen):: I. intend. to *: build. the "Comet Superhet Five, " ‘in, the’. November’ "Radio ° Timés." * Could I add:a ‘visual tuning nieter, and, ‘if: 80,: ; where, -would I ‘connectiit? ©". *A.: Yes, .a, visual’ tuning. meter could be. used. ‘The «best: place. to-connect it would :be.inthe’plate return of the-58 Ik, * "amplifier: In: other-'words, -breakthe: lead ‘between "B‘plus" and:the. primary of ‘the second. I.F: ‘transformer and insert. the tuning: meter: here. Also,. it-,would:be’.a:, wise. precaution. to con: nect a. decoupling condenser of from 03 to: 25 mfd. -capacity -between the "B plus’ lead. from: the UF. transformer in- question . and- -earth, . 2. Could I'usé a 5Z3: in. place of the two:80’s specified ? VA: Yes, provided that the rectifier filarnent winding: on. your power transformer ‘will stand .the, extra 1 amp. drain: taken: by.the.former-valve. However, as an adequate margin is usually . allowed . when: -these windings. are. put | on, the extra current dtain- should not endanger. the: -transformer. S: . 8.100" (Marton).: I have: built the International . All-wave. Superhet"™ described ‘in the 1984 "Radio’ Guide," but -find: ‘that’. shortwave stations surge badly. I:could add'more: audio’ stages. but. am’ doubtful, whether ‘this. would.

~ cure the* troubie. rr think that ‘another LF. stage would ° cure «the trouble, theoretically, , at least., A.: But would it,. even. theoretically ? If the-surging:is present in’. the first detector, then’ by> adding: more ampli-’ fication, ‘either audio or. ‘radio, you would only ‘make it more noticeable. Again, you..would’ find: it ‘would: be .almost: impossible : to. stabilise: three’ LI. stages, -with'all three:valves working at. maximum efficiency. An, appreciable. amount of gain. wouid: ‘have to be sacrificed -to obtain stability, ‘and for this reason "it would be-just ‘as. effective and much. more economical to have‘only two IF. stages ‘working; if "possible, "full out’: ‘You. would: find that by fitting:an effective’ A:V.©.-system, surging would-be largely. eliminated. 2.: Why; is one: side .oft the: 2.5-volt_ heater winding: earthed’ in: ‘the "Comet Five?" _. A.: Because ' ‘there. is. 0°. centre-tap _ on ..the. winding to earth. It‘is ‘not strictly. necessary‘to earth‘ either, but in some .sets ° doing so: brings about .an appreciable .reduction in. ‘hum,, and so the, precaution: is: usually. ‘taken with,all recelvete, : ‘Would :a 0003 : "mf. © condenser cover the: broadcast: ‘band? , A. Not. "from, 200+ to: > BBO- -metres, though., it ‘would probably : tune from 225 to. 525‘metres. . ¢ : R. HH. (Napier) : I ‘have a. _six-valve battery-operated ,commiercial port-'-able superhet, using a'129-volt "B" battery... I°find- that a battery. usually lasts about. three. months, but..the last ‘two have ‘only "given two months of service each, though:the‘hours of operation ‘have. been. shorter. -What could cause: this? . ' 'A.: As the ‘bias battery. . incorporated: in: the . "pR "it is scarcely likely ‘that’ the trouble’ lies in the: bias: voltages. ‘One ofthe valves . may haye gone’ a. little "soft". and is. , drawing » ‘appreciably... ‘more. current than. it:should. However, if the set is.. in constant:use, it. is: not. surprising that this -battery runs; down. as quick-ly-as it does. Of. the- valves in. your set, the.S22’s ‘take: 3.5 inils.,: the HL2’s 2 mils.,. and: the PT2 8.5: mils., giving a total current: drain. of: 22: ‘mils, This drain is fdr-too..heavy ‘to’ impose constantly: upon. the" ‘light: portable: ‘type of battery you ‘are | using, and’ this explains. why it is: running ‘down -so quickly, . Afterall, ‘portable . receivers . are. not. for. every-day use, for four or: five: hours¢a day,’ but. only occasionally, . For this reason then, your’. best plan would‘ be’ when. next buying a replacement..to buy ‘three ‘45 heavy duty or* super-heavy, duty ."B" bat- | teries. Though~ the outlay ‘would. be . appreciably | more, you would, be. say- . ing inthe long run,. as-a-set of these batteriés’ ‘should ° last you" at: least a’ year with constant, use, ‘and ‘quite probably... "much: longera: .* The outlay might .even ‘be: double, but. you would get ‘at least: four: ‘times. the‘service from _them. °- Jk ; 0, (Wellington) : I have: just dis- ? manjled: .a!four-valve-portable: re-, ceiver: ‘thatiwas:designed'in:1928. The

‘cabinet, permanent. magnet speaker, batteries and valves, etc, are in good: condition, and I:am-: considering build-: ing the "Picnic Portable," described in’ the December "Radio Times," and fit-ting-it into my present:cabinet. I have’ the. following. valves: on _ hand:-LP2, HL2, S22 and As the filament, drain‘ is. only.a minor consideration, would any of the above valves be suitable for the. "Picnic Portable?" A.: Yes, you could use the $22 screen‘grid: valve as r.f. amplifier. Its characteristics. are. as follows:-Fil. voltage 2, current 22-amps., plate voltage! 150; screen voltage 75, plate and screen current 3.1 mils. You could use either ‘the: HL2 or the PM2DX as detector. Possibly -the latter would suit the coil’ ‘specifications a. little better. Both are. 2-volt. valves, taking -l amp on the filament. The LP2 is a 2-volt triode power valye,-and takes .2:amps, filament current! > At. plate voltage: of 150, -the plate’ .drain’is‘11.5 mils. Bias at this plate voltage should be -4:5v. However, if you are using. only :a- 108-volt "BY" battery, -38-volts bias would: be ample, and ,also the plate drain would be. appreciably reduced, possibly to about 7 ‘mils. ~. 202: The space: for. the set m my case isa. little in excess of the dimensions . for. the "Picnic : Portable!" Could: I utilise- this ‘space, and, at the same time,:use ordinary, ‘tuning condensers in place’ of: the bakelite dielectric ones specified ? A.: Yes-quite easily, if you have the room, The aluminium panels. at both ends of the chassis could .be left out without harmful effects, It.is possible‘for a set to be unstable: without actually howling: or whistling. It may even be without. showing these symptoms, but quality ‘will be largely spoilt. % * The fewer the:turns used on the pri-. mary,’ winding of an; aerial coil, the sharper will :be the selectivity, However, if the number of turns is cut down too much, sensitivity will suffer:

Information Coupon (To be used wIth all fequests or Anformation;) Name ot: set_ Model Name- Address Nom:de 'plume; To; be kept In 'Gubsequent inquiriee: Please Note (1) Be bpeclflc eI11 (' briet;' tebo: latibg I8 posgible. (2) Write legibly and 00: one eide of the paper; '(3) We do not;; design 'circuitb. 4) Limlt tbree queetions tnlegs 1k 'jg enclosed; (5) Posta] querieg' Umit 8. queg-. 'tiong: Enclose' stamped and addregsed enyelope and Ve fee