Locating Hum in A.C. Sets

Radio Record, Volume IV, Issue 39, 10 April 1931, Page 16

Locating Hum in A.C. Sets

Stage by Stage Analysis

By

W. T.

COCKING

(In Wireless World" )

Accompanying is an article of great. interest to constructors and, indeed, to all users of A.C. sets. -We all know how extremely difficult it ‘is to locate definitely annoying hum, ‘and this article, by a well known English radio engineer, should prove of considerable value..

VERY reader who. has experimented with mains. receivers must at some time or other have experienced the annoyance. of. hum,.-: The locating, of the: source of difficult part of the problem, trouble is by far the most but once it is found its’ cure is ‘usually fairly easy. In order to illustrate the principles involved, the commonly used circuit of Fig. 1 will be taken as a basis. for discussion, and the: modifications necessary -for other circuits will become apparent. It:will be assumed that the complete. apparatus is built into one unit, and that- it performs satisfactorily with the exception that there is.excessive hum.. . It: will be realised that a systematic investigation of the set is essential, for the hum’ may be introduced in many ‘places at once, and the elimination : of one. source alone often makes little audible difference. The obvious starting point is the loudspeaker; it is very unlikely, although not impossible, for there to be hum in a reed-drive-type speaker, but it is a very probable source.of trouble if it.be of the mainsenergised. moving-coil type. . The procedure to be adopted, therefore, is to connect the moving-coil to the loudspeaker terminals on the set in. the usual manner, and to switch off the set, leaving only the field energised. Any hum must now be due to the field supply, and must be eliminat ed in one of the many well-known ways. . If the field be of the ‘lowvoltage type supplied by a metal. rectifier, the trouble-can usually be cured by. connecting in parallel with. the winding an electrolytic. condenser of high capacity. With a high-voltage field winding supplied by a valve’ rectifier, or D.C. mains, it will usually be necessary to add a smoothing choke in series with the winding, in addition to a 2 mfd: or ‘4 mfd. condenser in parallel with it. The Output Stage, Having made certain that the loudSpeaker ‘introduces no hum one can tackle. the set in earnest. ‘The first step is to make-sure that there is no electro-magnetic interaction between the dutput choke, or transformer, and the mains transformer. . To do this disconnect the choke from the valve and H.T. supply, and connect across it a resistance equal in value to the anode A.C.: resistance of the: power valve, as shown in Fig. 2a for: choke and Fig. 2b for transformer coupling. Under these conditions there should be no audible hum whatever, and if this be the case, the connections may be put back to normal. If hum-be found. however, the choké or transformer must be rotated, -or moved to a different position in the set, until a position is found which gives silent operation. Trouble in this portion of the receiver is rather unlikely, for no amplification follows the output circuit, and any pick-up is not likely to be at all serious The next step is to isolate the output stage, and to do this we shortcircuit the secondary of the intervalye transformer by connecting a length of ‘wire between the points, a, b, Fig. 1, ‘We have proved the loudspeaker and output cireuit to be free. from hum, and

any hum with the inter-valve transformer secondary short-circuited must obviously be due to the H.T. supply, the grid bias supply, or the filament of the power valve. The latter is the most easily tested by heating it temporarily from an ac-

cumulator, when the possibility of the trouble in the filament supply ceases. An accumulator is not always available, however, and in this case the return lead ec to the centre tap on the mains transformer should: be disconnected and connected instead to the slider of an adjustable potentiometer, which in turn is connected across the valve filament yy, as shown in Fig. 3. If a position can be found on the potentiometer at which the hum is absent, then it is due to the filament supply; the mains transformer tapping is out of balance, and the potentiometer should be retained permanently. If no better results are obtained by the use of a potentiometer, however, the trouble must lie in the H.T or grid bias supply; unless the valve used is one with a thin filament: and

itself introduces hum, or there is an excessive capacity between the different windings on the mains transformer. ~The H.T. supply is most easily tested by increasing the capacity, say, by 4 mfd., of the condénser O1; if this results in an improvement it’ is the

smoothing circuit which is at fault, The choke inductance must be increased or the capacity of the condenser Cl, or both, in accordance with wellknown principles; a further possibility, however, lies in electro-magnetic induction between the. smoothing choke and the mains transformer, and the effect of rotating the former should be tried. The grid bias supply may be tested by connecting an additional 4 mfd. capacity across the condenser C2, or across the bias resistance R1. If any improvement results, a condenser of the correct capacity should be connected permanently in these places, or in extreme cases an additional choke can: be ‘used. The Detector. We next come to the detector and its intervalve coupling, and the short

circuit to the L.I.. transformer. secondary should be removed. Now _ this transformer is, perhaps, the most likely source of hum in a. compact set, and. very careful attention should be given to its position. The test is the same as that for an output trans-. former; the primary must be disconnected; and then connected to a resistance whose value is equal to the normal working :resistance. ‘ That is to say, in the case of the circuit of Fig. 1, a resistance equal to the internal valve resistance, some 10,000 ohms for an AC/HL-type. valve; ‘but where a resistance-fed . transformer,,i4 used,, the resistance ‘should: be ed: 4 to the valve resistance.and the couplitig resistance in parallel, some 6000-7000 ohms for an AC/HL and the circuit of Fig. 4. oe, The necessity for this primary shunt resistance during testing is -to. stimulate the working conditions as far as hum pick-up isconcerned; it will-be found that without . this resistance there.is a very large amount of hum, which-is not normally audible, while if: the transformer primary be shortcircuited, all hum vanishes. Having connected the correct value of resistance across.the transformer primary, it should be rotated to the position of minimum hum; and in order to do this it is, of course, necessary to replace the normal connections to it by. sufficiently long flex leads." It may be mentioned: that simple rotation will not always cure hum, and the trans« former must sometimes be turned on tts side, or in some other curious position. When a suitable position has been found for the transformer, its primary should be reconnected, and the grid of the detector valve short-cireuited to the cathode by connecting a length of wire between the points d,.e, in Fig. 1. Any hum is now due to the H.T. supply to the detector, and more smoothing -is indicated. The effect of adding capacity to the condenser O32 should be tried, and if there be an improvement, a large capacity should be connected permanently in that position. In a few cases, of course, an excessively large capacity may be needed, and it is then moré economical to add an additional choke and con;:, denser at the point z in the H.T. sudply. aN ‘The next step is to check the detector grid circuit, which ean be done by removing the short-circuit between the points, .d, e, and applying it to the other side of the grid condenser; that is, to the points e, f. At the same time, the connection between the condenser C4 and the tuned circuit should be broken. The detector grid is now connected directly to the cathode through the grid leak and condenser, and any hum will usually be due to electro-static pick-up on the grid. This will occur if the heater wiring is placed close to the grid of the valve, but more usually it is due to pick-up from a higher voltage source. The~ positions of the wiring in the H.T. rectifier and smoothing systems should be examined, but the most likely source of trouble is the mains transformer. Fortunately, screening is often of great use in eliminating electro-statie pick-

{tec and the effect of enclosing the de-: together with its grid leak and ‘condenser’ in an earthed metal. box should certainly be tried. In this connection it must not be forgotten that interaction between the valvé electrodes themselves can occur, and the writer once traced a persistent hum to interaction between the electrodes of the H,T. rectifier ‘and the detector valves, which were rather close to gether; the trouble was cured by screening, The H.F. Stage. AVING cured hum as far as the grid circuit of the detector, the short-circuit between the points e, f, should be removed. There should still be no hum, and ‘in the very rare case when it is found that this alteration introduces it, it can be cured by rotating the tuning coil itself. Next connect the condenser C4 to the tunA and test for hum. During test the set must be so tuned that no signal is audible, for this may give rise to modulation hum, which will mask other effects. Any hum which can now be heard must be due to the H.T. supply to the H.F. valve, and appropriate. additions should be made to.. its smoothing’:circuit, and also to the grid-circuit bias resistance by-pass condenser C5. In the absence of a signal, the set should now be perfectly quiet, and with no trace of hum, beyond, perhaps, one foot from the loudspeaker cone. Modulation Hum. Now it may quite possibly be found that when a station is tuned in, a fairly strong hum. becomes evident. This is known as modulation hum, Since the hum actually modulates the ‘carrier of the incoming signal, and it is often difficult to cure. There are several causes of modulation hum, and each has its own remedy. In the first place, it may be caused by insufiicient smoothing of the grid bias and H:T. supplies to the H.F. valve. The former .is the more prevalent source of trouble, and it may be necessary to use a total capacity for C5 of Fig. 1 of about 4 mfd. before the hum can be cured. Extra capacity across the other H.F. by-pass condensers shold also be tried. Another source: of trouble lies in the H.T. rectifier valve; this may generate H.F. oscillations which are modulated by the A.C. mains. This state of affairs can be cured by adopting the rectifier circuit shown in Fig. 5, where the valve is prevented from oscillating by the two condensers C, which shoul ve a capacity of 0.1 mfd. and be tested to at least 1000 volts. In the writer’s experience, however, the most troublesome source of modulation hum is the supply mains; and this particular form can occur even. when no H.I, stage is used. The cause of this type of .hum is rather obscure, but it is apparently due to the presence of H.F. currents in the mains leads. The cure is simple and satisfactory; a condenser C6 (shown dotted in Fig. 1) should be connected between one of the mains leads and earth. The best capacity must be found by experiment, but a value of 0.0005 mfd. is often sufficient; in some cases, however, a capacity of 0.001 mfd. / or 0.002 mfd. may be found necessary. The full mains voltage is connected across this condenser, and so it is very important that it should be rated for continuous werk on not less than 1000 volts A.C.

It will usually be found possible to eliminate all traces of hum by a sys: tematic search conducted on _ the above lines, and the whole principle lies in working backward from the loudspeaker, and checking each point in turn, During this process a watch should be kept for high-resistance connections, since these are a prolific source of hum from electro-statie pickup; apart from badly soldered. joints, they will be found most frequently in valve holders and plug-in coils. There is always the chance of a faulty valve, and this is the most likely source if the strength of the hum is variable. In obstinate cases the effect of using a potentiometer for the connection hetween the cathodes and heaters of the indirectly heated valves should be tried, as recommended for an output valve.