Silencing Motor-Boating

Radio Record, Volume III, Issue 5, 16 August 1929, Page 25

Silencing Motor-Boating

A Type of Trouble Freduently Encountered

a Ne term "motor-boating" is ope ‘Y which is rather aptly used in order te describe the peculiar and characteristic reiterated "put-put-putting" noise which: is sometimes present during the’ operation of receivers which are work- . ed in conjunction with a "B" pattery eliminator. . i A Common Fault. ; PARTICULARLY | is motor-boating liable to set in when the receiver eontains' a low-frequency amplification ‘ gireuit of the resistance-coupled variety. 4 . Howeyer, the bark of the trouble is, if it may be put so, worse than its bite, and_with a little careful thought and modification of existing apparatus the amateur may generally rid himself of the trouble.

~ Many Causes, . Tisa difficult, matter to ascribe the cause of motor-boating to one particular set of circumstances. The complaint may be due to many causes acting together. In general, however, it may be said that motor-boating is the result *f the set’s amplifier (particularly Hit be a resistance-coupled amplifier) being more sensitive ,to low frequencies than to high Therefore, the traces of the alternating current hum which may, perhaps, escape the eliminator, are considerably amplified, and these, combining with the received modulations, of current, give rise to a periodic impulse of current, which, as we have said above, creates the "put-put-put" effect in the loudspeaker, Motor-boating may be the result of the general. ili-design and construction of A receiver, or, on the other hand, > 7€ Way just.be due to some particular ss component being unsuited to that particular circuit. As an example, in resistance-coupled circuits which are employed with ‘pat- . tery eliminatorg the values of the con-

densers require to be controlled with reasonable accuracy. If a set motorboats, a condenser of some unsuitable value may have been included in the circuit-the capacity value of the condenser generally being on the low side. Careful attention to the details of the set’s construction in the above respects during the course of its. building- will nearly always result in the

. finished receiver being quite free from the objectionable complaint. Finding a Cure. ONE method of attacking the problem, is to leave the set itself untouched, and to increase the capacity of the éliminator cireuit. A 2 or 4-mfd. fixed condenser placed across the output terminals of the eliminator may mend matters. If, however, .the annoyance persists after this trial has been carried out, try reducing the’ resistance of the grid leak on the power: valve. This can readily be effected without touching the existent grid leak on the circuit. All that has to be done is to connect the extra leak in parallel with the existing one. The latter thereupon will be eut out of action, the current leakage. taking place through the newly-added grid leak, which will be, of course, of lower resistance than the standard leak, A good value of grid leak to try out for this purpose is approximately .05 megohm. Very likely this treatment will succeed in eliminating the motor-boat-ing completely, :

Further Tests .Necéssary. JF it does, however, do not be con-. ; tent: to let matters rest there, for the use of a grid leak of very low value may, be detrimental to the reception. On this account, therefore, if other grid leaks are available try them out in place’ of the newly-added leak; until a value of leak which just brings back the motor-boating into the reception is obtained. The next lowest value of grid leak to this will constitute the best one for the purpose, ‘ Difficult cases of motor-boating are often caused by an imperfectly-design. ed resistance in the resistance-coupled amplifying unit of the set. The usual value of the resistance used for this purpose is 100,000 ohms (.1 megohm) This value of resistance may be quite satisfactory for working with dry B batteries, but when it is used in con junction with an.eliminator, the latter may tend to supply an excess of current to the amplying valve, and unless this is combated by the employment of

a Digher resistance, qmotor-boating may set ‘Effect of Current Leakage. -: therefore, the remedies suggested above fail in any case of motorboating, it will be probable that the catise of ‘the trouble wil! lie in this direction, particularly if ‘it is found that the set works satisfactorily when used in eonjunction with an ordinary .B battery. Of course, it must not be forgotten that any small imperfection in the fixed eondensers in: the amplifying circuit may set up the trouble. Motor-boating, for instance, may very well set in if be or more of the small condensers er a leakage of currert. In every case of this trouble, therefore, it is advisable ‘to test every condenser systematically. Take each condenser. and charge it ‘up by placing it in contact with the poles of a single cell. After this, set aside the condenser on a sheet of glass ° or ebonite ‘for a minute or so.’ Then connect ‘the condenser across a pair of . headphones. At the moment of ‘contact, a very distinct click should. be heard ‘in the phones, signifying that the condenser -has been storing a charge and has given it up at the moment of contact. A condenser which wil]. not store a eharge in. this. manner should be regarded with grave suspicion, and, if (Concluded. on page 26.)

Motor-Boating

--eE Cl . 9 (Continued from page 25.) possible, should be replaced by ame other. Suspect. the. Condensers. JN brief, therefore, the cause.of motorboating is generally an affair of eondensers. If the trouble starts in your set, suspect the condensers first of all. They may be too low in eapacity or they may be faulty in construcon . It ig a hundred chances fo one that attention to details such as these will effect a cure of the-trouble. If not, however, a remedy must be sought in other directions; and in the -points enumerated in the above aes s it will almost certainly be found.’