Eddystone Short Wave Manual de usuario

SHORT WAVE RECEIVERS AND TRANSMITT

EDDYSTONE

SHORT WAVE MANUAL

No. 5

All the sets and equipment described in this Manual have been subjected to thorough tests and trials and can

be relied upon to give excellent results, if built exactly to the instructions given. Should difficulties be ex-

perienced, other than from faulty construction or the substitution of different components, we shall be glad to

help, but to avoid lengthy correspondence, we ask constructors to make a careful check of the equipment before

writing us. Finding out the cause of one's own troubles is, moreover, not only a reason for self-satisfaction

but is also highly instructive.

" Eddystone " components are available through selected Registered Retailers in most areas. In case of any

difficulty, you are invited to write for the name and address of your nearest stockist.

CONTENTS

BAND -SPREAD ALL DRY FOUR RECEIVER .. Page i

V.H. FREQUENCY METER ..

. . I3

SIXTY MEGACYCLE. xs WATT TRANSMITTER ,.

9 f

FIVE -TEN METRE. 5 -VALVE CONVERTER .. , f io

TWO VALVE PRE -SELECTOR .. .. .. ..91 ,xl

28 MEGACYCLE C.W. AND TELEPHONY TRANSMITTER.. .. is

COMPONENT LIST AND PRICES .. .. Inside Back Cover

INT. OCTAL .

VALVE BASE DATA

B 9 G. AM. 7 PIN.

OCTAL BASED VALVES.

MSHDGiG2G3AK

Metallising.

Shield.

Heater.

Diode.

Control Grid.

Screen Grid.

Suppressor Grid.

Anode.

Cathode.

Valve Pin 1234567

Top Cap

EF 36

EF 39 M

HAG2 G3 -GI

L63

EC31 HA-GI K-

EL35

EL37

6V6 MHAG2 GI H

HG3

K(G3) ---

0VO4-20

DF33

HGI

K, G3, S G2 H.C.T. Connect to

Shield H

GI-

AG2 -

DAC32

DL35

HA-D

HAG2 G1

BRITISH 9 -PIN BASED VALVES.

Valve

EF 50

EF 54

EC 52

OVO4-7

2345 6

GI-

8 9

G2 A G3 S S

AG2 K, S. G3 S H

GI K.A -H

AG2 G2 SKH

AMERICAN 7 -PIN BASED VALVE.

Valve 123 4

5 6 7 Top Caps.

4074A HGI K

GI HA

2EDDYSTONE SHORT WAVE MANUAL

in the rear of the cabinet to allow the aerial to be

connected when cabinet and chassis are assembled.

WIRING.

The wiring throughout, except for the screened

lead to V3 and possibly the battery leads, may be

carried out with 20 s.w.g. tinned copper wire en-

closed in insulating sleeving. All R.F and detector

circuit wiring must be kept as short as possible and

this point applies very particularly to the top grid

of the DAC32 detector valve. Both the grid con-

denser C8 and the grid leak R5 should be soldered

extremely close to the top cap - otherwise static

hum and other undesirable effects may be en-

countered.

The top cap of Vi and the stator of CI are

connected to a tag on the midget insulator, from the

underside of which A. wire is taken through the

chassis to pin 4 of the coil base.

Similarly, the grid condenser C8 and the

stat9rs of both C2 and C3 are connected to the other

midget insulator and detector coil socket. All

tuning condensers are earthed with short leads from

the tags provided to tags fitted under the bracket

bolts.Resistors Rio and Rig and condenser Cm are

soldered to the group board and the associated inter-

connections made before the board is fixed to the

rear of the chassis. The other small components are

held in the wiring, 5 -way tag strips being fixed on

available projecting bolts, for anchoring purposes and

to enable H.T wiring, etc., to be commoned up.

C4 and Cie are soldered directly from the appro-

priate valve pins to an earth tag, and condensers

C5 and C7 similarly from the detector coil base.

The double pole switch connects the L.T and

H.T batteries simultaneously. If a non self -shorting

telephone jack is employed, be careful not to dis-

connect the telephones whilst the set is working.

L.T leads are taken up through the chassis from the

switch and a chassis earthing tag and H.T leads

from the switch and the right hand tag strip to the

batteries clipped to the rear of the chassis.

Fig. 2. Illustrating the lay -out of the receiver. Due

to the addition of side brackets, it is necessary to trans-

pose the positions of the batteries.

Fig. 3. Showing the mounting of components on the

underside of the chassis.

OPERATION.

Before connecting the H.T battery, ensure

that the correct voltage is shown across the filament

pins (2 and 7) of any one valve, to avoid possible

damage through an inadvertent error in wiring up.

On advancing the reaction control from mini-

mum, a point will be reached at which oscillation

commences, indicated by the usual slight increase

in noise. The receiver is most sensitive when just

oscillating - advancing the reaction control too

far will cause violent oscillation. The R.F tuning

condenser should be kept in tune with the detector

stage - signals will increase in 'strength consider-

ably at the proper setting.

The frequency ranges, band -spread of amateur

bands, etc., are set out in the table below and close

agreement will be found in a receiver built to the

layout and instructions given above. In the case

of the 6BB coils; oscillation ceases at approximately

28° on the band -set dial, due to the inductance

capacitance ratio becoming too low but there is

still an overlap with the 6LB coil, As will be seen,

the coverage is complete from approximately 35

Mc/s. to 990 Kc/s., using the six coils specified.

The amount of bandspread on each amateur band is

fully adequate and makes the receiver a pleasure to

handle. The noise level is extremely low and weak

signals can be tuned in, held and read with ease.

An earthing terminal, on the rear of the chassis,

will be an asset but, to be effective, the earth lead

itself must be short, particularly when the receiver

is being operated on the higher frequencies.

The aerial employed will, of course, depend upon

local circumstances. For general use, a wire of a

length between 5o and too feet, suspended as high

and clear as possible, connected directly to the aerial

socket, will prove very effective on all ranges. For

restricted frequencies, e.g., any one amateur band,

an aerial cut specially to the appropriate length and

fed to the receiver by either low or medium im-

pedance feeder will ensure particularly good recep-

tion on that particular frequency band, although

directional effects may be noticeable.

The consumption from the batteries is very

low - 25 amperes L.T and 8.5 milliamperes H.T.

EDDYSTONE SHORT WAVE MANUAL 3

PANEL OF FREQUENCIES.

Frequency Range Amateur Bands. Frequency Range Amateur Bands.

Coil. Coil.

Max. Min. Band Set Frequency Band Max. Min. Band Set Frequen Band

Mc/s. Mc/s. Dial. Kc/s. Spread Mc/s. Mc/s. Dial Kc/s. Spread

Dial. Dial.

6 BB 35.6 27.2 18° 30000 15° f6 It 8.8 3.65 13' 7300 30°

(28°) 28000 89° 7000 60°

6 LB 27:4 12 680 14400 15° 6 W 4.65 192 13' 4000 0°

14000 88° 3500 100'

6 Y I15:85 7.05 6 P 2.45 99 16° 2000 10°

1800 90°

CHART OF POSITIO

CHASSIS (looking at front).

Top Chassis Screen 5' high, 7' long plus 1"

fixing flange, mounted 64' from L.H.

side, 94' from R.H. side, fixing flange

on right.

Sub Chassis Screen 2" high, 7' long plus 1"

flange, mounted 710 from R.H. and 84'

from L.H. side.

Adjustable brackets all 24" from front

(fixing centres). R.F. condenser

bracket 2' from L.H. side, detector

bracket 3' from R.H. side, Band spread

bracket 14" to right of- s!*een (i.e.,

dead central). All condensers 24 up

from chassis top.

NS OF MAJOR COMPONENTS, DIMENSIONS, ETC.

V.4 valveholder

from front. lir from R.H. side, 34" R.F. Gain Control 6' from L.H. edge (2' dia.

hole).

V.3 valveholder

from front. 11- from R.H. side, If" Switch 31' to left of R.F. Control (1" dia.

hole).

V.2 valveholder

from front. 51' from R.H. side, ir Jack 31' to right of Reaction Control (

dia. hole).

V.1 valveholder

from front. 51' from L.H. side, 11" Aerial Socket 5" from L.H. side.

1 General Purpose Chassis Eddystone No. 641

1 General Purpose Cabinet .. 609

2 Metal Handles 8' .608

2 Microdensers, 100 pF (Cl. 3) .. 1130

1 Air Dielectric Trimmer, 15 pF (C2) 580

1 Full Vision S/M Dial

2 Precision S/M Dials, 2" Silver ..

2 6 -pin Coil Bases ..

2 6 -pin Coils, 6BB

2 6 -pin Coils, 6LB

2 6 -pin Coils, 6Y

2 6 -pin Coils, 6R

2 6 -pin Coils, 6W

2 6 -pin Coils, 6P

1 R.F. Choke ..

2 Skirt Knobs 11'

3 Large Flexible Couplers

598639964959959959959959959

1010

2416

1009

R.F. coil base 54' from L.H. side, 34' from

front.

Det. coil base 54' from R.H. side, 31" from

front.

(Both are fitted with pins 1 and 4 to the

right).

Reaction control 6" from R.H. edge (1' dia.

hole). LIST OF PARTS.

3 Adjustable Insulated Brackets .. 1007

2 Midget Stand-off Insulators .. 1019

1 Valve type DF 33 (V1) Mullard

2 Valves type DAC 32 (V2, 3) .. Milliard

1 Valve type DL 35 (V4) Mullard

4 Octal Ceramic Valveholders.

1 Intervalve Transformer 3.5: 1 Niclet Varley

1 D.P.S.T. Toggle Switch, S.88 Bulgin

1Insulated Closed Circuit Jack .. Igranic

1 Aerial Socket type L373/Skt. Belling -Lee

1 Aerial Plug type L.373/P.8 Belling -Lee

3 Mica Condensers :001 mF (C4, 5, 13).

2 Mica Condensers -0003 mF (C7, 12).

1 Ceramic Condenser, 100 pF (C8).

1 Ceramic Condenser, 20 pF (C15).

1 Paper Condensers -5 mF (C11).

1 Paper Condenser -01 mF (C9).

PANEL.

Three 4' holes for dial bosses, plus two 4BA.

holes per dial, vertically above and

below large hole and 1 r apart. Holes

for jack, switch and potentiometers as

for chassis. Aerial socket and earth

terminal holes at rear of cabinet.

2 Paper Condensers -1 mF (C10, 14).

1 Electrolytic Condenser 8 mF (C6)

1 Resistor, 4 watt, 5,000 ohm (R3).

2 Resistors, 4 watt, 1,000 ohm (114, 12).

2 Resistors, 4 watt, 2 megohm (R5, 9).

1 Resistors, 4 watt, 50,000 ohm (R6).

2 Resistors, 4 watt, 20,000 ohm (R2, 8).

1 Resistor, 4 watt, 100,000 ohm (RIO).

1 Resistor, 4 watt, 10,000 ohm (R11)

2 Potentiometers, 100,000 ohm (RI, 7).

1 x 5 -way Group Board ; 3 x 5 -way Tag

Strips.

NOTE.-The stated values of resistance

are not critical and the nearest preferred

value, 20% tolerance, will be suitable.

V.H. Frequency Meter

INTRODUCTION.

Some method of measuring the actual operating

frequency is essential when working in the five metre

amateur band. The frequency meter described

below fulfils this function and it also has several

other uses which render it a valuable addition to any

amateur station. It illustrates one use of the small

diecast metal box - No. 65o - for which many

other applications can be found.

Basically, the instrument is of the absorption

type but with the addition of a valve and meter,

which combination gives a more positive and reliable

indication than is possible with the flashlamp bulb

usually employed. This latter method calls for

power to light the lamp and results in very flat tuning,

which drawbacks are absent in the present design.

CIRCUIT.

Referring to the complete circuitdiagram shown

in Fig. I, Li, CI, and C2 form the tuned circuit,

the 6o to 56 megacycle range being covered from 19

to 45 degrees of the dial. Coupling to it is accom-

plished by taking a lead from a point on the coil one

turn from the earthy end to a midget lead -through

4EDDYSTONE SHORT WAVE MANUAL

insulator, to the external terminal of which a short

pick-up rod is fitted.

Connection to the valve is made from an

approximate centre tap on the coil via a to pF

ceramic condenser, thus keeping the tuned circuit

loading low. The indicating meter, a Hobut minia-

ture type reading up to Soo micro -amperes, is shunted

across the valve with a Cat. No. tot I choke in series.

Two versions of the meter have been tested.

The one shown in the photograph, Fig. 2, utilises

a type 1T4 miniature 1-4 volt filament valve (Mullard)

used as a diode with all electrodes, other than the

filament, strapped together. The use of this valve

enables a small cell to be included within the box,

making the instrument completely self-contained.

The circuit employed is given in Fig. i(a).

Improved results are obtainable if a television

type diode (Mullard E.Ar5o) is substituted for, the

1T4, but in this case, it is not possible to include a

heater battery inside the box. Two terminals, one

insulated, are fitted to the lid of the box for con-

nection to an external L.T. supply which may con-

veniently be a 6.3 volt supply from a mains trans-

former indoors and three or four small dry cells in

series for use outdoors. The circuit in this case

becomes the one shown in Fig. t (b).

CONSTRUCTION AND WIRING.

Enclosing the instrument in: the Cat. No. 65o

metal box, which is complete with a close fitting

flanged lid, assists in the maintenance of frequency

stability and calibration and eliminates the effects

of body capacity and other unwanted external

influences. Because of the restricted space, the

components must be located with care, otherwise

the construction is straightforward.

The II" hole for the meter is located 1" from

the left hand side and t i" from the top, looking at.

the face of the box.

The tuning condenser requires a '4" hole 1" from

the right-hand side and t 14" from the top. The

switch is fitted to the left-hand top corner of the box,

near the lid, so that it is clear of the meter terminals.

Immediately below the meter is a midget insulator,

to a tag on which are anchored condenser C3, one

end of the RF choke and the wire to the valve anode.

FIG .I (a).

Circuit diagram of VHF Frequency meter employing

a 1T4 1.4 volt valve.

The aerial insulator is mounted through the top

side, 1" from both side and front, a -" hole being

necessary. The small index strip for the dial should

be fixed at an early stage in the construction. A

small metal strap holds in place the filament cell.

When all holes in the box have been made, first

fit the valve -holder in place. It is located 2i" from

each edge and as close to the face of the box as pos-

sible. Two I" 6 BA bolts with spacing pieces keep

the valve -holder away from the metal side. Loose

leads should be soldered to the valve pins (in the case

of the IT4, the GI, G2, and anode pins are corn-

moned) for connection to the appropriate points

later. The other components are then mounted,

leaving the coil until the valve and meter wiring has

been completed. The coil, directly across the pins

of which is connected C2, is soldered to one stator of

the variable condenser and to an earthing tag bolted

to the side of the box. Finally, C3 and the aerial

lead -through connections are completed.

CALIBRATION.

Before calibration, ensure that the lid is screwed

down and fit a 12" length of stiff copper wire to the

external terminal of the midget insulator. If

assistance is available from amateurs using trans-

mitters of known frequency, calibration can be

carried out quickly. Otherwise it will be necessary

to use an existing transmitter, or build a small

oscillator, with which to excite a Lecher wire system.

Space does not permit of a full description of this

method here and the reader is referred to the " Eddy-

stone " Ultra Short Wave Guide (published before

the War) and to the R.S.G.B. Amateur Radio Hand-

book. The frequency meter itself may be employed

to determine the position of the antinodes along the

Lecher wires.

When a number of dial readings against fre-

quencies have been secured, a graph should be drawn

and kept for use with the instrument.

USING THE FREQUENCY METER.

To measure the frequency of a transmitter, the

box, with its pick-up rod in position, is placed some

little distance from the output tank, and the dial

rotated slowly, since the tuning is quite sharp. If

Continued on page 9

-11C3 RFC

CI C2

TT 111LT

FIG.I (b).

aim

Circuit diagram of instrument when using a

television type (6.3 v heater) diode.

EDDYSTONE SHORT WAVE MANUAL

60 Megacycle 15 Watt

INTRODUCTION. .

Several considera-

tions have been borne in

mind when designing a

transmitter for use in the -

6o megacycle amateur

band. Firstly, something

better than the simple .

self-excited oscillator, with

its inherently poor fre-

quency stability and liabil-

ity to frequency modula-

tion is definitely desir-

able. Secondly, too

elaborate a " line-up " hag

been avoided and the trans-

mitter described is capable

of giving good results without

adjustments.

A suitable modulator is included, together with

the necessary power supply for working off A.C.

mains, the whole forming a complete transmitter

on one chassis.

CIRCUITS.

The complete circuit diagram is given in Fig. 1.

The radio frequency circuits, of the balanced push-

pull type, are built around two type 4074A twin

triode valves, which are designed for use at very high

frequencies and are capable of realising high efficiency.

At the same time, the power requirements, both

L.T. and H.T. are economically low. The trans-

mitter is nominally rated at 15 to 20 watts input, the

R.F. output being between io and 12 watts.

The first 4o74A valve is employed as a self-

excited master oscillator in a somewhat uncon-

ventional circuit which requires only one tuning

control and which, in practice, functions well. The

frequency stability is good, sufficiently so in fact,

to enable the use of C.W. in a particular manner,

of which details are given later. The full high

tension voltage is not required on the first stage

and the surplus is dropped through R5. A closed

circuit jack (J3) enables a meter to be plugged in to

read the anode current of this stage. The master

oscillator produces more R.F. voltage than is

necessary to excite the power amplifier, and this

factor enables the loading to be kept low, with a

consequent improvement in frequency stability.

The second 4074A valve acts as a balanced push-

pull Class C power amplifier with a somewhat

higher L/C ratio in its anode circuit than is employed

in the master oscillator. A o/too "Millimeter is

permanently wired in the anode circuit. Neutralisa-

tion is effected by Cat. No. 481 Neutralising Con-

densers, mounted symmetrically in easily accessible

positions.

Bias for the M.O. valve is obtained through

separate resistors and for the P.A. valve through a

RFC.I

1.1 v --1J3

412 C

RI R3

c;IL

`T .T

calling for critical

nG.1.

Transmitter

RI2 18

R2 C 2 0

combination of resistors. Whilst not essential, a

certain amount of fixed bias, obtained from a battery,

is advisable on the P.A. valve and the jack (J2) may be

employed for the connection of this additional bias.

This jack is primarily included to enable measure-

ment of grid current to be made, this being necessary

when neutralising the transmitter. The resistor in

the cathode circuit of the P.A. valve is normally

shorted out when the transmitter is functioning

properly but it is included for use during tuning

operations and for the,' special method of C.W.

operation.

The modulator conforms to standard practice

and, as shown in Fig. 1, is intended for use with a

crystal type of microphone. If a moving coil micro-

phone is employed, a suitable coupling transformer

must be included. If a carbon microphone is used,

the second valve may be omitted, in which case of

course, the anode of the first valve will be coupled to

the grid of the output valve.

The first two stages in the modulator use triode

valves, the first a medium impedance type and the

second a low impedance type. The beam power

output valve is capable of giving up to 8 watts audio

output, which is sufficient for full modulation of a

16 watt carrier. The switch S3 cuts off H.T. to the

modulator valves when C.W. is being used. The

correct output Toad of the EL35 valve is 2,500 ohms

and this is matched into the P.A. anode impedance

of approximately 5000 ohms by a multi -ratio. trans-

former.With the modulator and R.F. stages all on one

chassis, precautions are necessary to avoid R.F.

getting back into the modulator. Adequate filtering

is included so that no difficulty should be experienced

from this trouble.

The power supply also follows normal practice.

It uses the Mullard type FW4/5oo rectifier since the

total H.T. current consumption is in the region

of 15o milliamperes. When the transmitter is in

operation, all heaters are allowed to run contin-

EDDYSTONE SHORT WAVE MANUAL

uously and S2, which breaks the H.T. centre tap

to earth, is employed as a transmit/standby control.

GENERAL CONTRUCTION.

The transmitter is built on a standard chassis

Cat. No. 617, with a 8r front panel and two side

angle brackets, the whole forming a rigid unit of

pleasing appearance. Placing the transmitter,

modulator and power supply all on the one chassis

means that little space is wasted but no undue con-

structional difficulties will be experienced if the wgrk

is carried out in proper sequence.

The photographs Figs. 2, 3 and 4, show clearly

the lay -out which has been adopted, whilst the

chart, page 9, gives details of the spacings between

various components. It is suggested that a dummy

chassis top and panel are cut out of stiff cardboard

and all the components; screens, etc., laid out there-

on and the positions carefully marked. The card-

board pieces may then be used as templates for

marking out accurately the actual chassis and panel.

The Retailer from whom the items are pur-

chased may be in a position to assist the constructor

in the work of making the larger holes, otherwise the

help of a local metalworker should be engaged.

Note that the jacks and switches are fitted to the

front of the chassis and i" holes to give clearance are

required in the front panel. Two jacks (J2 and J3)

and two switches (S t and S2), being those which are

less frequently used, are mounted at the rear of the

chassis. The smoothing condensers take the form

of a three -in -one block, manufactured by British

Insulated Cables Ltd. This block mounts con-

veniently below the chassis to the rear, thereby saving

top space.

THE RADIO FREQUENCY STAGES.

Tandem construction, with the valves in a

horizontal position, permits an efficient lay -but with

an absence of long leads. To reproduce the excellent

Fig. 2. Top view illustrating the lay -out of the

transmitter.

Fig. 3. Close-up view of oscillator stage.

results given by the prototype this layout should be

followed closely, for which purpose a close-up view

of the master oscillator stage is shown in Fig. 3.

The M.O. valveholder is fitted to its bracket'

with the cathode pin at the top. To each top corner

of the bracket is fitted a midget stand-off insulator,

employed as a lead -through to hold the grid con-

densers and associated wiring firmly in place. The

grid R.F. chokes are similarly held rigid by two

further insulators on the chassis, leads being taken

from the undersides to the grid resistors. The M.O.

tuning condenser is fitted to an adjustable bracket

and its associated tuning coil soldered directly on the

end lugs on the stator sections. One end of the

anode R.F. choke is soldered to the centre of the coil,

the other being taken to the top of a Cat. No. 564 red

lead -through insulator. The timing condenser

rotor is connected to the cathode pin of the valve

via a short piece of thick copper braid.

A 6" square brass screen holds the P.A. valve -

holder and the neutralising condensers, which are

themselves fitted to miniature stand-off insulators.

Two holes in the screen are necessary to enable

the grid connections to pass through to the neutralis-

ing condensers. The P.A. tuning condenser is

mounted on an adjustable bracket in a position which

brings it close to the valve top caps. As before, the

coil is soldered directly across the condenser and the

R.F. choke from the centre of the coil is held by a

midget insulator, from which a lead is taken to the

meter. Another red lead -through insulator carries

the H.T. supply through the chassis, an R.F. choke

(RFC2) and by-pass condenser (C7) being fitted as

closely as possible to this insulator to prevent stray

R.F. being carried below the chassis.

As a result of keeping the wiring short, the

insertion of the valves is a little difficult but this is no

real disadvantage since it is unlikely that the valves

will require changing over long periods. The valves

should initially be fitted in their holders before the

EDDYSTONE SHORT WAVE MANUAL

adjustable brackets are fixed in place and should it

become necessary at any time to change a valve,

the nut holding the tuning condenser should be

slackened off and the condenser itself gently lifted

sufficiently to give clearance for the removal of the

valve and the insertion of a new one.

Aerial coupling is effected by a single turn coil

of 18 s.w.g. wire, enclosed in sleeving and looped

around the centre of the P.A. coil. The ends of the

loop are soldered to a Belling -Lee coaxial aerial

socket, fitted to a brass bracket which, in turn, is

mounted on the angle bracket opposite the coil.

MODULATOR AND POWER SUPPLY.

The modulator valves are grouped together

near the master oscillator and the associated com-

ponents are arranged on group mounting boards

fitted beneath the chassis near the appropriate

valveholders. The power supply components and

the modulation transformer are fitted at the rear of

the chassis and mounting of them should be left to

the last, to allow the chassis to be easily handled for

wiring up. Details of this part of the construction

are clearly shown in the photographs.

WIRING.

To begin with, the M.O. and P.A. radio fre-

quency stages should be wired, followed by the group

boards and audio circuits and finally the power

supply circuits.

The cathode, one heater and the two blank pins

on the M.O. valveholder are all soldered to a tag

bolted on to the brass bracket. Condenser C4 is

soldered directly across the heater pins and a flexible

lead from the " live " heater taken through a hole in

the chassis. The other components associated with

this stage are then wired up.

The silver plated tuning coil, a Cat. No. 602 of

four turns, is soldered to the condenser lugs nearer

the M.O. valveholder and to the same lugs are soldered

stiff pieces of wire from the grid lead -through

insulators. Anode caps on short flexible leads are

soldered to the other condenser lugs. Note that the

anode connections are crossed over so that the grid

condenser goes from the grid of one triode section

to the anode of the other. The valve will otherwise

fail to oscillate.

The P.A. stage wiring is very similar. In this

case, an additional condenser is soldered across the

cathode pin and earth tag, and leads through the

chassis are required both for the cathode and live

heater connections. The tuning condenser rotor

is not earthed but is allowed to " float." The tuning

coil is one of five turns. It is again necessary to

cross over the connections between the anodes of

the P.A. valve and the neutralising condensers.

The chart gives details of the components

which are fitted to the group boards, No. t being the

board mounted on the side of the chassis and the

others numbered reading outwards from this. The

small components are of course fitted to the boards

before the latter are bolted in place below the chassis,

using spacing washers to prevent any possibility

of short circuits occurring. The wiring of the R.F.

stages can then be completed. If a suitable power

supply is available (one giving between 15o and

25o volts will serve) it will be well to give the R.F.

stages a preliminary test as outlined below, to ensure

they are functioning correctly.

Next the audio stages may be completed. The

majority of the components are already in place on

the group boards but some, R9, Rio and RH for

example, are held in the wiring and should be con-

nected with very short leads. The electrolytic

by-pass condensers are connected between the

appropriate valve pins and the nearest chassis

earthing point. The lead from the underside of the

red lead -through insulator to the modulation trans-

former should be of screened cable and likewise the

lead from the modulation transformer to the anode

of V5. The H.T. connections for the three modulator

valves are brought to a common tag from which

a by-pass condenser is connected to earth. From

this tag also a lead is taken to S3 via a radio fre-

quency choke.

Tags t and 6 form the primary of the modulation

transformer and are connected to the anode of the

EL35 valve and to H.T. via the switch S3. The

secondary tags, connected in series with the H.T.

supply to the power amplifier, are 7 and to. It is also

necessary to connect up the following tags :-

2 tO 5 ; 7 to 9 ; and to to 12.

The audio circuits may now be tested but,

before doing so, connect a dummy load - a high

wattage resistor of 5,000 ohms or near - across the

modulation transformer' secondary winding.

Finally, it only remains to mount and wire up

the mains transformer and smoothing choke. The

safety fuse fitted in series with the primary of the

transformer is fixed to the rear of the chassis, between

the two switches, a t ampere fuse being adequate.

Fig. 4. Under chassis view of the Transmitter.

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