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RESEARCH DEPARTMENT

PERFORMilNCE TESTS ON A G.E.C. OVERSEA3 TEN RECEIVER MODIFIED FOR 10 kcls AUDIO B.4.NDWIDTH

Report No. G.042 Serial No. 1948/37

Research Department

Viork carried out by:-

Report written bYI-

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REFORI' NO.:J..042

Serial No. 1948/37

J. G. Spencer G.R. Frood

J.G. Spencer

1st November, 1948.

Drawing Nos.

G.042.1 to G.042.9

PER1!'ORNlANOE TESTS ON A G.E.O. OVERSEAS TEN REOEIVER, MODIFIED FOR 10 kC/ s A.,.UDIO BANDWIDTH

1. General Remarks

In his report dated 20th June 1947, E.i.O. Caversham described . a series of modifications to the G.E.C. Overseas Ten Receiver to e~able it to be. used as a high fidelity receiver for transmitter quality checking.

A receiver modified by Research Department in accordance with these instructions, although satisfactory from the point of view of bandwidth and audio performance, was rather poor in sensitivity and signal to noise ratio, It was therefore decided to design a similar modification in this department and endeavour to improve these latter factors.

A test report on a receiver modified to the hesearch Department specification follows and an appendix is attached gi ving details of the modifications required.

Summa:c'.L of Modifications

The overall R.F. and I.F. bandwidth has been increased and since the resulting adjacent channel selectivity is too low for satisfactory S.W. reception the short wave R.F. coils have been removed.

The tone control circuits and associated L.F. amplifier valve have been completely removed and a tunable heterodyne whistle filter has been fitted.

A 600 .n output jack and a loudspeaker muting switch ere provided.

-1-

-2-

3. Summary of Tests

The tests made on this receiver conform to Research Report No. G.024 unless otheTI~ise stated.

It may be mentioned that details of the performance of the standard production model Overseas Ten are given in Report No. G.026.

(1) Signal to Noise R!jtio (See Fig. 1.)

The input voltage, modulated to a depth of 3rf/o at 400 c/s, required to produce an output signal to noise ratio of 20 db is of the order of 140 IJ.V on medium waves and 400 tJ.V on long waves.

(2) Selectivity (See Figs. 2 and 3)

S1uce the shape of the overall resonance curve between the ! 10 kcls points is of great importance in 6 receiver of this type, and depends largely on the accuracy of alignment and tracking of the R.F. circuits, these curves have been plotted in greater detail ,than is required by Report G.024.

The medium wave range bandwidth is + 12 kcls for -3 db, ~ 20 kcls for -20 db and ~ 28 kc/s~or ~40 db.

The long wave range bandwidth is +11 kc/s for -3 db, + 19 kc/s for -20 db and ~ 26 kc/s for -40 db.

(3 ) Ima~e Ratio f~

765 kc/s • 57 db . 220 kC/s r >60 db

(4 ) A..V.C. (See Fig. 4L The knee of the A.V.C, curVe occurs with an input of 1 mV,

above this point an input increase of 60 db gives an output increase of 10 db.

(5) Overall Fidelity (See Fig. 5)

These curves show the overall fidelity on both wave bands and the attenuation introduced by the whistle filter at its resonant frequency,

... 3-

(6) ~udio Input/Output Characteristic (See Fi~

The maximum undistorted output measured by this method is 4.0 watts (see Table 1) and the mBximum output 5.8 watts.

(7) Overall Total Harmonic Distortion (Se~ Table 1)

The figures given in this table include the distortion generated in the audio oscillator and signal generator used for the test.

With an output level of 250 mW in 1 .n load fo:' 40% modulation at 1 kc/s, the distortion is 1.3% rising to 2.7% with 90% modulation.

(8) Brief Specification

Wave Ranges

Long W0ve 143 kcla - 333 kcls (2100 m. - 900 m.) Medium Wave 545 kcls - 1500 kcls (550 m .... 200 m.)

Controls Facing Panel (Left to Right)

(1) ( 2)

(3 )

(4 ) Rear (1)

Volume Control Inner ... Whistle Filter tuning Outer ... Mains sw::.tch. Inner - RIF. gain control. Outer ... Wave Change SVJi tch. Tuning Control,

of Cabinet Loudspeaker muting switch.

Inter.mediate Frequency

445 kc/so

JoS/DMO

and on/off switch.

-4-

APPELDIX

Modifications to G.E. C. Overseas T(~n Rtlceiver to secure 10 kc/a Aul1io B"ndwidth

The component numbers used in this specification were obtained from the original G,E.C. circuit diagram and the modified circuit diagram (Fig. 7).

R.F. St;.ge

R(,mOV8 coil unit and replace wave change wafer switch by YuxJ..ey 2 pole wafer. M.ake up new coil unit.

=

=

1000 IlH

190 \lH

Mutual Inductance :::: 56 ~H

LW

= 3100 ~

Wire up coil unit as shown in modified circuit diagram.

~.F. Gain Control

Disconn3ct cathode of V from of R.IB. Connect one end ot R.1B

chassis and wire Lt to slider to chassis and the other end Eypass c[)thode to chassis with through R.5b (25,0000) to H.T.+.

O. 4B ( 0.1 llF).

Mixer Strge

Romove coil unit and replace wave change switch by Y,-lx1c:-l 2 pole wafer. l'ktke up new coil unit.

150 !JlI

~utucl Inuuctance c 45 ~H

LW Lp :::: 2BOO llH

Ls = 3100 IJ,H

Mutucl I:1l1uctance "" 630 IlH

e-

-5-

The values of inductance given for the tuned secondary windings of the mixer and R.F. coil units are nominal values. Ir. order to ensure ganging accuracy permeability tuned coils must be used~

T11cru fire no modifications to the oscillator coil unit or circui t.

I..F. TruwfQrmers

LF.T.l. Replace RIO HHd R13 by 100,0000. tririulJ.sr from anode of mixer to grid of first R:nove leads from SI to primary.

Wi re ;':;-30 J.i~F I.F. hinplifier.

I.F.T.G. Replc-,ce RIt> ., ... cl R20 by- 47,0000. trimmer as above.

Aa~ top capacity

I.F.T.~~ Replace R22 by 47,000n. as above,

Ada to~ capacity trirr@er

If b6.ndwidths other than 10 AC/S ere required the following information is offered regarding the constants of the I.F. tr~~sformers prior to modification.

I.F.T.l. Open circuit inductance Coupling factor K

Q,.

LF.T.2 [1[lU J.

, Ope!! circuit inductance K q,

I.F. Amplifiers V3 Pond V4.

Cor~~ect cathodes to chassis. Disconnect HIS Remove C22

2nd Detector

Rcnuve R50 R(;}:lvce Ci::8 by 50 Il~ RJmove C26, R25, R26 and leads to S2.

"" 095 IlH = 0.0145 = 140

::: 440 IlH := 0.0123 == 150

Remove R24 nUl .. replace it by 100 m g!1i .. ~ control fixed in posi tion which R25 yreviously occupied. Corillect slider of gain control to griq of 2nd detector through 0.01 ~F condenser.

Cennect llfJ.\1 resistor from the grid of V5 to earth •. Replace R~;:; by a 22,0000 resistor, and wire two 33,00041

re~istors in series between its negative end and the anode of VB. . ..

An acceptor circuit consisting of Cjl and Ll is wired from the junction of the two 33,0000 resistors, Ro9 and RoS, and chassis. It acts as a whistle filter vvhich can be tuned betw6eIi 6 and 12 kcf;;,. The variable condenser CbI is fixed in the 'position which R:35 previously occupied, and when this is in its minimum capacity posi tion it is arranged to open switch So. TIle circuit is shown fully in the modified circuit diagram.

Remove completely together with R:14, C;jl, R.J5, 033 ,R~6, R40, C67. R:37, R38, R39, C39. Oonnect positive of 0~~8 to negative end of R~)3.

Oonnect 029 from junction of R59 and R60 to grid of V1. R"-'fi,ove 0~4 and disconnect R/,!,l..

Tuni~ Iudicator Stage

Remove completely together 1IJi th C46, R3l and VlO. Tue new tuning indicator VII, is an EM4, and its buffer stage, an EF50, is mounted on top of the chassis. 11; is fed from the anode of the last I.F. amplifier,thnough a lMn resistor Ral which is mounted in the I.F. can. TIle circuit is shown fully in the modified

. circuit diagram.

0l1t12ut Stage.

Connect R'72 and R73, 22,000.0 resistors in series with 0,*1 and C4~.

The whole of the secondary circuit of the output transformer is to be rewired as shown in the mod~fied circuit diagram. 57 is mounted on the back of the chassis beside the me ins input pl.ug, the l5f.V600.n transformer ~119 is mounted on the inside side panel of the cabinet.

Tx)(;.'l mechanical layout of the modifications is shown in the attached photographs. (Figs. 8 to 9 l.

J~1ignment of I.F. Circuits

(1) Iaj ect output of frequency modulated S. S. G •• tuned to 445 kC/s into grid of V4 4nd connect oscilloscope to output of V5. J'..c.just T23, T;~.:1 and T7;l to obte in a symmetrical overcoupled resonance curve with a peak separation of 15 kC/s and 8 0.5 db dip in the centre.

Aojust T3:. so that the tuning indicator circuit resonates at the I.F. mid-band,frequency.

(2) COunect S. S. G. to grid of V;~. A\l.j ust T:n, T!:,2 and T;JO until overall resonance curve (I.F.T. 2 Bnd 3 in cascade) has peak separation of 15 !cc/s and 1 db dip.

(j) COlwect S.S.G. to signal grid of V2, short circuit R15, and adjust T19, T~O and T:9 for peak SGI)aration of 15 kC/ S Bnd 1.5 db dip.

~Uignment of Sienal Frequency Circuits

For these adjustments either short -circuit C21 and C:":'6 to render the .'-i..V.C. system inoperative, or ensure that the amplitude of carrier is below ~~.V.C. threshold.

L'ij ect llnmodulated S.S. G. output into receiver aerial socket through a dummy aerial.

(1) Tu.ne S.S.G. and receiver to 560 kc/s, adjust M.W. coils' inductDnce trimmers to maximum output.

(~) Tune S.S.G. and receiver to 1400 kc/s, adjust T25 and T27.

(j) Rc~eat (1) and (2).

(4) Tune S.S.G. and receiver to 165 kc/a. adjust L.W. coils' inductance trimmers.

(5) Tune S.S.G. and receiver to 300 kc/s, edjust T26 and T2~.

(6) Repeat (4) -;..nd (5).

It is advisable to repeat the trimming adjustments on the input tuned circuits with the receiver connected to the aerial on which it will be operated, using broadcast transmissions on or near the appropriate frequencies as test signals. Tnis will give optimum ganging conditions and will tnke into account the aerial reactance.

Tn.o component numbering on this list follovm on from that on the original G.E.C. circuit diagram.

Components whose value has been nltered end 'which retain their original G. E. C. component numbers.

RCt'li'stors Condensers

Number Old Value New Value Number Old Value New Value

RlO 220,0000 100,0000, C28,' 0.0001 ~r 0.00005 \.LF R13 220,0000 100,0000 R16 77,0000 47,0000 R20 77,0000 47,0000 R22 220,0000 47,000.0,

New components with new component numbers

-~istors Condensers

Number I Value Number Vclue

R5l 800 T25 2-30 ~IJF Trimmer R52 4700 T26 2-30 IJ+.lF Trimmer R53 800 T27 2-30 \.L\.LF Trimmer R54 4700 T28 2-30 \.q..lF Trimmer R55 25,0000 T29 2-30 J.tIJF Trimmer R56 100,0000 Volume Control T30 2-30 WF Trimmer R57 IM.G T3l 2-30 \.L\.LF Trimmer R58 33,000.0 T32 2-30 ll\.LF Trimmer R59 33,0000 C48 0,,1 ~ R60 22,0000 C49 0.01 \.LF R61 1 Iv"lfl C50 0,1 IlF R62 1 ]t£l C5l 0.0005 IlF Variable R63 0 .. 5 Nill C52 0.25 tJ,F R64 50,000.0 C53 0.5 \.LF R65 1 Nifl C54 0.0003 v.F R66 4000' C55 0.1 ~tF R67 lMn C56 0.1 \.LF R68 0.5 Mn C57 100 I-tI-LF R69 IMn C58 300 I-tI-LF R70 lMn .C59 0 .. 1 \.LF

..

Number

R'll R72 R'73 R74 R75'

-10'"

Resistors

25,0000-22,000tL 22,00.00

l.n 14.0

~--------------------------~----------------.-----.----

M,1flcellaneous

Number Description

S4 Yaxloy 2 pole switch wafer.

S5 Yaxley 2 pole switch wafer.

S6 S.P.S.T. Toggle switch . ' S7 S.P.D.T • Rotary switch

Ll 1 H inductance L2 400 J.IH inductance :-

wiX6 Westector R •• 1..119 15-60on Transformer V 11 1l.\14 "" V 12 E]'50

One output Jack

-11 ...

T.A.:3LE 1

OVEItUL R'~RMONIC DISTORTION (Measured with Marconi Instruments Ltd., Distortion Factor M.::;ter Type TF142E)

~ .. ll tests made with 10 mV input modulated with 1 kC/s tone at 7'65 kC/s through standard Dummy ;~erial.

(1) i .. udio gain set to give zero level into 600 n lo9d from line jack: ...

% Modulation ~ Dis~ortion,

40% 1.4% 60% 1.5% 80% 2.Cf1/c 90% 2.6%

(2) .mdio gain set to give 250 m;', in 1 n resistive load at 40% modulation;-

cg, Modulation ~. Dbtortion

40% 1.3% 6rP/o 1.6% 80% 2.1% 90% 2,7%

(3) Constant 4rP/ modulation; audio gain varied to give stated output into 1 n resistive load:-

OutEut % Dis~ortion 0.5 I.W 1.3~ 1.0 if 1.6% 2,0 11 2.3% 3.0 If 3.5% 4.0 W 4.8%

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