The "CATHODE" Super

Radio Record, Volume V, Issue 31, 12 February 1932, Page 12

The "CATHODE" Super

(Part IV)

By

CATHODE

#0 far we have completed the ‘power pack and final amplWi ifier, and progressed with the i receiver proper to the extent fash) OL assembling the chassis 4 and the intermediate bandpass filters; the coils should ; also be under way. The assembly and wiring ‘of the rest of the gmeceiver, while bound to take a fair time, will not be found at all difficult af tackled systematically in the ‘manwer to. be outlined. The screening boxes containing the fmtermediate filters should first be placed temporarily in position and the «omponents adjacent to them-i.e. -walveholders, resistances, condensers, ehoke, and; audio transformer-posi-tioned in the spaces between 'the filters gad at the end of the chassis in the manner indicated in Fig. 1. The alum{nium boxes may 'then be carefully re"moved and the components mentioned gecured in place with the assurance that they will not foul the filter units fwhen these come finally to be mounted. The three remaining valveholders ay next be mounted on the front part of the chassis in the positions indicated. At this stage the small terminal strip shown in the view of the bottom of the chassis should be prepared and mounted. At the other end ‘of the ¢hassis a 60 ohm centre-tapped registance should be mounted either upon an insulated mounting or upon the wooden batten which strengthens fhe chassis at this point. Between

the 24 volts terminals on the terminal strip and the ends of the 60 ohm resistance a heavy twisted twin lead should be run. (wire of aft least 16 s.w.g. must be used) and at suitable points along this lead short twin leads should branch off to the heater terminals on each valveholder. Some little care is needed in joining the short leads to the main one, the proximity of the joints being apt to cause shorts unless care is taken with the insulation. The centre of the 60 ohm resistance will be seen to be earthed direct to the chassis as are numerous other components. The B_ negative terminal on the terminal strip is alse connected directly to the chassis. Completing The Mounting, T this stage all the remaining components which are to be mounted above the chassis should be secured in place and such holes as are necessary for leads to the underpart drilled. it is important that the top components should all be in place before those underneath the chassis are mounted as, whereas the placement of the firstmentioned parts is important, those below the chassis can, within limits, be shifted round so as not to foul etl a?

screws and leads penetrating the chassis. , As regards the aerial and earth terminals, the earth connection is, of course, merely a straightforward terminal mounted directly on the screening strip separating the front section of the receiver from the rear section. It is located at the extreme left hand end where, too, the werial connections are located. As separate aerial connections are required for broadcast and short-wave reception (the higkfrequency stage being cut out on shortWave) it is convenient to provide a pair of sockets at this point, inte either of which a banana plug affixed to the aerial lead may be _ inserted. The leads from the plugs are clearly shown in the diugram. The .0001 mfd. variable condenser used for controlling oscillator feedback should be of the solid dielectric tyne, partly because a condenser of small dimensions is called for, bur principally to avoid any possibility of damage resulting from the two seis of plates shorting. ‘This condenser must be insulated from the chassis, this being most conveniently accomplished by mounting a scrap of insulating material on a little aluminium bracket and mounting the condenser on the tiny panel so provided, It is advisable to attach the leads to the plug-in coil holders before mounting them within the screens. Unless these wires are attached and led through the screen in advance, the utmost difficulty will be found in doing this part of the work. The attachment of the coil-mount and sereen to

the chassis is a rather awkward little job, but can be fairly readily accomplished if the bolts are first secured to the screwdriver with a scrap of Chatterton’s compound or some similar material. The trimming condensers should, of course, already be mounted on the panel and wired in parallel with’ the first two sections of the gang condeuser. Under-Chassis Components. "THE componeuts under the chassis can all be mounted now. As previously explained their exact Jocation is unimportant, being diclated principally by the uecessity of clearing the components on the upper. surface. Naturally, however, some effort will be made to keep the lead as shore as possible, while by-pass condensers will be mounted as near as poxsbile to the point to which the high-potential end of the condenser is wired. The earth terminals of all the by-pass condensers are connected directly to the chassis. A word in connection with the highfrequency chokes will not be out of place here. That in the plate circuit of the second detector clearly has toa Geal with intermediate frequencies, so that. a high-inductance choke should be chosen for this position-eg. x choke of English manufacture designed for use in a long-wave receiver. he cboke in the plate circuit of the osvillator, however, deals with high-fre-quencies only, and a choke having uor too high an inductance will be most suitable for this position. F The remainder of the wiring may now be proceeded with, and in order to avoid confusion the reader is strongly advised to follow a_ definite programme in making the connections. For example, the cathode terminals of all the valveholders--except the oscil-

lator cathode, which goes direct to earth-are earthed to the chassis via biasing resistances of various values. Similarly, with the further exception of the first detector, all these biasing resistances are by-passed with condensers. Now the idea is to make this series of connections before proceeding tv any other part of the circuit. The next series of connections to be made might well be to the screening grids of the type 224 valves. These are made to the grid terminal on each valveholder. Here again the feeds are all through resistances by-passed with condensers, and may be completed as 2 group. The volume control and the resistance in series with it really form a part of this group with the additional by-pass condenser and may be con-, pleted at the same time. The wiring of the oscillator-not forgetting the connection from it via a condenser to the cathode of the first detector-may now be completed as i further group, following which we ay turn attention to the completion of the intermediate assembly. Band-Pass Filter Connections. ¥ we examine our three band-pass filters we find that there are three eonnections to be made from each, and flexible leads to make these may be attached in readiness. It will be convenient to attach the three plate leads {one from each filter) first. The lead from the first filter will be seen to gu to the plate terminal of the first detector valve: this lead will require to be fairly long. The lends from the other two filters go each to a plate terminal on the next preceding if.: yalyeholder. Similarly with ihe three grid leads. The first two will terminate in clips for the grid terminals of the next succeeding if. valves, while the third will connect to the grid terminal on the second detector valveholder. That leaves us with three leads-one from each filter-and these are seen to be all intended for connection to B positive via resistances. At this stage. .then, it may be well to run a B positive lead along the whole length of the

receiyer and start to make the various connections to it. Firstly, there ure these three leads, the resistances in which are already by-passed within the filter units. Then there is a lead to the 25,000 ohm resistance in series with the volume control. In addition there ere leads, each with a series resistance bypassed by a condenser, to (a) the radiofrequency transformer primary, ()) the oscillator rf. choke, and (¢) the primary of the audio transformer. Completing the Wiring. Tf at this stage all the connections which have been made are ticked off on the theoretical diagram it will be seen that not only is there relatively little wiring to complete, but that tuis is confined to two or three well-defined groups. Hxcepting a straightforward connection from plate of r.f. valve to primary of x.f. transformer, these are (a) tuned circuit of rf. valve and aerial connection to tapping on coil, (b) tuned circuit of first detector and alternative aerial connection to coil socket, and (c) plate circuit of second detector and audio transformer connections. These groups may now be completed and a careful check of the wiring made both in groups and as a whole. If everything seems satisfactory it is fairly safe to assume that the receiver is ready for attaching to its power pack and final amplifier

(which should have been checked and tested in readiness) for the purpose of adjusting and tuning. Before finally closing the constructional stage of this description it is felt desirable to Warn readers as to the necessity of adequately insulating and protecting wires which go through the metal chassis or screens. Hyven if the

sharp corners of the holes wre removed, ihe insulation seems to fray to somé extenr, and to safeguard against trouble it is wise to provide at least three layers of insulation if insulating hushes are not used. The very last word of advice under this heading is to test all resistances-especially of the fiexible variety--before inserting them, and to use heavy-duty resistances wherever the current to be carried is uppreciable. ; The process of tuning and adjustment for best resulis will be brietiv described next week.