A Very Useful Instrument

Radio Record, Volume II, Issue 29, 1 February 1929, Page 25

A Very Useful Instrument

A Home-made Testing Apparatus

3K. the average radio experi- || menter what he would like to possess if funds would meter for both high and low allow, and he would say something like this: "A nice set of measuring instru-

; ments, including a_ voltvoltage and a milliameter." There is no doubt that a range of high-class instruments gives a set builder more assurance and helps to keep a check on his work. This article deals with the «construction of an instrument that has any uses. It tests A, B and C bat- . teries, and has a further use, in as much as it will test each valve in the set under actual operating conditions. "It can be used to determine the D.C. resistance of ’phones or loudspeaker, and also to check up resistances and transformer windings. Of necessity a few simple calculations will have to be described, but because of these the writer does not advise the absolute novice to give up the idea of making one. Anyone who owns-a multi-valve receiver will find this instrument a great help in his search for quality. Almost every listener is more or less interested in his receiver, and is constantly cn the look-out for hints and information relating to its improvement. Why not do as the expert does and go over everything yourself? Radio is not so very technical as ( people imagine. To understand how to check any valve from its published characteristics is not really. difficult. To test a battery is less difficult still, and these are the only two parts of a wireless receiver that should decrease

in efficiency with use. ‘Lheretore, to keep these up to scratch is to be sure of a continuance of the results obtained when the set was first installed. Keeping a check on the valves is of importance when it is realised that one faulty one in the set makes the whole receiver no better than that valve. It ean. be likened to a claim that is no stronger than its weakest link. Because the valve lights is no indication that it is working efficiently. In the case of the old bright emitter, the thin metallic filament gradually decomposed, and the electron stream was made by the filament itself. The filaments of the more recent dull emitters are coated with an ele- . ment called "thorium," and it is this, : when at a dull red heat, that throws off ‘electrons similar to those of the bright filament. If this dull red heat is exceeded at any time the thorium is liable to boil away, leaving the thin

metallic filament on which the thorium was suspended. When in this condition- the valve will light, but is absvlutely useless in a receiver. To weed gut these parasites and substitute new valves will increase the general efficiency of the set three or four hundred per cent. Use of Power Valves. UCH has been said recently on choosing a large enough power valve for the last stage. Overloading is the chief cause of unfaithful reproduction. The listener is inclined to blame his speaker. With the aid of the instrument to be described, it will be evident at a glance that the last valve is behaving itself. Adjustments to the B and C batteries can be made until the valve can carry all the load that is asked of it, and then when this is done, the speaker can De criticised. It will surprise many when they look at the needle of the milliameter shaking about, denoting just when distortion occurs. It is far easier to see the effects of distortion than to rely upon the ear. A person is liable to get accustomed to his particular speaker, and to automatically correct its faults. The human ear can adapt itself to suit its owner, and imagination plays a great part when listening to the reproduction of, say, a singer. rine

Assembling the Tester. AN open-topped box has first to be made to house the milliameter and resistances. This also affords protection for the various leads to the milliameter. It can be made from any kind of timber of 8-8in. thickness and measures inside, 9in. x 6in. x 3in. deep.

Two small strips can be fastened inside so that the ebonite panel can be slipped in flush with the tops of the sides and ends of the box. In the centre at the top of the ebonite mount the valve socket. This can be done from the top or underneath by drilling a hole large enough. to take the valve pins and by fastening the socket with two nuts and bolts. Below this is mounted the double pole, double throw switch. Refer to the diagram for the position of the milliameter, which should preferably be of the panel mounting type reading 0-100 m.a. In the centre at the bottom of the panel are fastened the two valve pin sockets with two 1-8in. holes drilled just above. The use of these will be described later. On either side of the panel at the bottom ends are mounted two terminals. To lessen the chances of accident to the milliameter by accidental short circuit, the terminals are preferably of the insulated top type. Some method will now have to be found of mounting the resistances, and it will have to be largely left to the discretion of the builder, depending on the type and make of resistance used. This must be of the wire wound variety capable of passing 100 milliamperes without warming up and altering the resistance. Specify this when buying these components.

Making a Resistance. F fixed resistances are unprocurable, variable ones can be used, or they ean be made by using suitable lengths of eureka or similar resistance wire wound on a non-inflammable former. If this method is adopted, get the particulars of the wire at the same time as buying. Resistance in ohms per yard is required as well as currentearrying capacities.

For instance Eureka resistance wire No. 82 SWE, has a resistance of 7.35 ohms per yard, and will safely carry 100 m.a. for a short time. A total resistance of 100 ohms can be obtained by using 100 x 7.85 yards of this wire, or 13 yards 22 inches. Wind on a rod of, hard fibre or similar material taking great care that the wire does not stretch. The actual resistance can be checked over afterwards against @ known voltage. The 100 ohm resistance can be made in the same way by winding on a spool ten times this quantity of the same wire. Nichrome wire has a higher resistance per yard than Hureka, so it would be best to use this, and avoid such a large resistance unit. Unless the wire is cotton or silk-covered, do not let adjacent turns touch when winding either resistance, or the reading of the volt meter will be quite inac curate. Yet another way of obtaining this resistance is by the use of 200 or 400 ohm potentiometers. A 200 ohm can be used for the low resistance, and three 400 ohm ones connected in series and neglecting one of the side termix nals in each case, for the high resise tance. These can be mounted undere neath the panel and adjusted to the correct value by comparing the volte meter with a good commercial one bore rowed for the oceasion. If this is not possible, they can be set if two new 45-volt batteries and a tested 6-volt accumulator are on hand. However, more of this later. , Now take the base of a broken valve, This can be obtained from any dealer if one is not to hand. Clean away all glass and file away the solder on the tip of the pins, cleaning the hole down the centre of each pin.’ Two. lengths of twin flex, about two feet each, are cut, and the four ends soldered in the holes in the valve base pins. One twin lead for the filament, and the other two for the grid and plate lead. Mark =. > ee ol -- {

the free ends of the wires corresponding to their termination on the valve socket, The Diagrams. Proceed to wire up according to the diagram. This is a view looking down on the inverted panel, and is not a plan of the wiring as it would be seen if the panel was transparent. It will be noted that the grid and the plate terminals are apparently wrong, but this is due to the fact that it is upside down. The two lengths of flex wire can be fastened underneath by a small clip and a slot cut in

the side of the wooden box, so that it will slide in from the top. When connecting up the millimeter, do this by two short lengths of flexible wire, passing through the small holes in the ebonite, and fastened to the two valve pins outside, so that the connections to the millimeter can easily be reversed by simply changing over. The resistances are marked as Ri, 100 ohms and R2, 1000 ohms. Running over the connections one by one, it is seen that the grid terminals on both the socket and adapter are merely joined via the length of flexible wire. Plate pin or valve base joins te owe end o the switch and one end of 1000 ohm resistance. Free end of resistance to one of the insulated terminals on panel. The adjacent terminal to D.P.D.T. switch and also to plate terminal on valve socket. The two middle pins on the switch join to the two valve pin sockets below. Now, the filament pins on valve base adapter join direct to the filament terminals on tke valve socket. To one wire is joined one end of switch and one free terminal, The last terminal joins to. the other filament lead and one end of 100 ohm resistance, the other end of which is joined to the last pin or terminal on the switch. This finishes the wiring and the whole can be mounted in the box ready for use. Adjusting the Voltages. IRST of all let us try the A and C batteries. Disregard the adapter for a minute and work from the four terminals on the panel. On the left are

the two reading O-10 volts, while on the right are the two reading 0-100 volts. Connect the A or C battery to the two terminals on the left and throw the switch over to that side. The voltage will be given on the milliameter as follows: For every 10 milliamps it denotes 1 volt. Then for 6 volts 60 ma. will be seen. Four volts will show 40 ma., etc. Now try the B batteries. Every 10 volts will be shown as 10 milliamps, so that a 90-volt battery will show 90 milliamps. Connect the B batteries to the right-hand pair of terminals, and throw the switch to the right. If the needle of the meter kicks the wrong way, either reverse the battery leads or alter the polarity by changing over the two valve pins and sockets at the front. If the resistances are not known and can be varied, do this on a known voltage, such as a tested battery, and adjust the low voltage side until 44 volts reads 45 milliamps, and the high voltage side by varying the resistance until 45 volts shows 40 mills or 90 volts, 90 mills, Using the Instrument. Now, assuming that we wish to test the valves in the set, take out the valve and fit into this tester. Plug the adapter into the socket from which the valve has been taken. Throw the D.P.D.T. switch to the left and turn on the set. This will indicate on the meter the voltage that is being applied to the filaments. When the correct voltage is being registered note the position of the rheostat and use in this position while the receiver is being used. Now throw the switch over to the right and the meter will at once indicate the plate current being taken by the valve. Take special note of the last stage, and while the music is being heard watch the needle of the meter. If this kicks with a loud passage, then distortion is occurring. Increase the B and C voltages until no kick is noticed. If this is not successful use a larger power. valve. Consult the characteristic curve as supplied by the makers. Read the grid bias or © voltage horizontally. The anode current in milliamps is seen on the vertical line, and various curves for different plate voltages (B voltages) are given. Select the curve with the longest "straight portion" and bias, so that it is working on a point a little below mid point of this straight portion. Check over by reading off the plate current as given by curve with the longest "straight porshown on test. In either case of testing valves or batteries, the switch thrown to the left gives low voltage reading, and to the right gives plate current or high voltage reading. Regarding the milliameter, a warning which cannot be stressed too strongly should be noted by all those who ‘have not handled this delicate instrument before. It is extremely fragile, and. should never be used to test a battery by connecting direct across the terminals. The only way it can be used to test voltages is by the insertion of a suitable resistance in series. Therefore, after purchasing the milliameter, carefully put it away until it is ready to be built into this piece of apparatus. ; There is still the method of determining resistances with the use of a milliameter to be discussed. Almost all electrical calculations are based on Ohm’s Law. This is defined by the

simple formulae, C eq. E over R, where C eq. current in amperes. E eq. voltage, R eq. resistance, that is R. eq. BE over C. The milliameter and a known battery voltage will have to be used in this ease. Assuming we want to find the resistance of a pair of phones, connect by insulated wire the milliameter, the two small valve pins in this case, the pair of phones and a 45-volt battery all in series, Just make connection long enough for a reading to be obtained. Let us say it was 20 milliamps, which is .02 amprs. . R. eq. B over C, R eq. 45 over .02 eq. 2250 ohms. Similarly any resistance can be determined by arranging the voltage to suit the readings of the milliameter.