Decca 6 valve set service manual

Page 1

April, 1934

iii/P , _

Service Engineer Supplement to The Broadcaster

Both the Decca Grmphnnc Co. Ltd. and

its associated company, Brunswick Lid.
markel this six valve radiogram.

Circuit.-This set is a superhet on the
medium waves, and a. straight receiver
on the long. The terms used will apply to
the superhet features.

The H.F. valve, VP4 (V1), is preceded
by is tuned secondary aerial transformer.
Bias is a plied to the rid by a resistance
in the catlhode lead and y the A.V.C. volt-
a e. Coupling to the next valve is by H.F.
c oke, and a free end winding wound with
the tuned grid coil.

The first detector oscillator, AC/SG (V2),
is used with reaction applied by a coupling
coil in the cathode circuit. Wave-band
switching causes the oscillator coil to be cut
out of circuit, and the tuning is transferred
to an L.W. transformer.

DECCA SIX-VALVE SET

The IF. valve, VP4 (V5, is coupled to
the second detector by a. ban -pass I.F. trans-
former (LF. frequency 183 KC) on the
medium waves and by an unturned H.F.
transformer on the long. Bias is supplied
solely by the A.V.Cl system.

For the second detector 8 double-diode
triode, AC/HL/DD, is used as a detector and
also to provide delayed and amplified A.V.(
The anode of the triode section is resistance
capacity coupled to the output pentode.

This valve is a seven-pin AC/Psn. The
manual volume control is a potentiometer
across the grid. input, but separated from the
grid leak by the inclusion of a condenser,
(:17, between the slider and the grid leak.

A tone correction circuit, consisting of Clo,
R18 and a small iron-cored choke, is con»
nected ecross part of the V.C. resistance.
Pentode compensation is provided 'by a .005
condenser directly between the anode and
earth.

Bins for the valve is derived from 9. poten»
tiometer across the L.S. field (in negative
H.T. lead), consisting of R22, R21 and R25.
The tapping between R21 and 323 provides
the delay bias for the anode of the diode.

The output is fed to two speakers, which

VALVE READINGS
No signal.
vuve Typo. Electrode. Volts. MA.
1 Vl4 anode 200 .5
aux.grld so
2 AC/BG anode 200 .5
screen so
3 vm nnode 200 3.5
lux.grld .. so
4 AC/HL/DD Triode
anode 130 3.25
AC/ren anode 275 83-40
5 (7pm) uux,gi1d 1275 543

have the speech coils connected in parallel,
while the 1,000 ohm field coils are connected
in series in the negative H.T. lead.

Mains equipment consists of a transformer,
a full-wave indirectly-heated rectifier, W3,
and electrolytic smoothing condensers.

Special Nolan-The controls on this set
are original. The large knob operates the
tuning and the gram switch at the rear
end of the gauging condenser. The small
concentric knob controls the volume and by
push-pull action the wave-change switch.

The tone remains the same whatever the
setting of the V.C.

Visual tuning is obtained with dimming of
the pilot lamp (on reception of a carrier) by
means of altering the inductance of chokes in
series with the AC. supply to the lamps.
This is brought about by having two chokes
in series with the lamps, on the same care as
a much larger choke which is included in
the H.T. supply to the controlled valves.

As the current to the valves decreases by
the A.V.Cl action the A.C. voltage drop
through the lamp-chokes increases owmg to
the higher inductance brought about by the
de»saturation of the core.

In the first models it was found that this
might increase the risk of modulation hum,
but in later models a 25 mid. electrolytic
condenser connected in place of C7 (nomin-
ally ,1 mid.) eflectively cures this tendency.

Quick Teeth-Between casing of outer
electrolytic condenser (-) and chassis (+),
135 volts.

Between second terminal from inside on
LS. transformer (-) and chassis iii), 155
volts (same point as above, i.e., H. .- .

Between third terminal tag (+) and chassis
(-), 287 (H.T. smoothed).

(Continued on next page.)

Page 2

Vintage Service Data CD-Flom

Service En

AERIAL COIL IDI
CZI

Oczo
VI 0

O

V2

-+i
GRAM
SWITCH
V3

The Dm-Brunmx'ck six-value receiver is
[table in that it is a mullet on the
Elm wavdmgllu and a straight " in-
slvumenl an the long. Here an the dumb
layaulx (above) and the circuit diagram

(Continued from previous page.)

Between fourth terminal tag (+) and
chassis (-), 275 (V5 anode).

Output transformer primary is between
terminal tags 3 and 4.

Removing Chmis.-Pull 011 the large
knob and remove the smaller one (grub
screw). Remove foam screws from under
neat}: platform and undo the cleats holding
the motor leads and cable.

Ease chassis out a little and unsoldel- the
motor leads from the mains transfonnelr (end
terminal tags). The L.S, cable is sufliciently
long to allow afnassis to be examined on the
floor.

For bench work the leads to the output
mfomer must be unsoldered. Connec»
time are, counting terminals from inside:
2, orange; 5, red; 4, white.

General Notes-The pilot lamps
on to a plate.

The wiring round the AC/HL/DD
AC/Pen valves is rather com Heated, and
the layout drawing can my give the
sppmximate positions. In case of doubt
compare the code values of mesistanoes with
those given in the table.

The connections to the block condenser
near the pentode valve-holder are (our

clip

and

MAINS TRANS I IVISUAL TUNING APPAR.

Cl CZI~
as 0
cs , ._ 37 cg 0
R22 cx . - R2
R21 >3 g
R24 RCECIlez<§It C r";
6 ' O
R km a
l R $2 .I .417 L.W.W| mes Q
7-3 Ill5I8 " 5
90 13.6 .99 RI! 522 [7R4 i
CI7 4. I
V
. . 323% 5.66. 2:3
R20°i=l 1:! ml-l no
CI4CIZ RIG RIO C15 RIB '

C22

numbers) :-1, C5; 2 and 3, E; 4, 019; 5,

ca; 6, c4; 7, cm; a, (118;9, E.
Replacing the Chassis.-La,y chassis

slautwise on platfiorm and resalder the motor

RIG

C2
R

leads (shorter one is joined to outside taxi).
Replace four holding screws and clip t. 0

cable and motor:- leads. 'Beplace knobs and
reconnect P.U. and A and E plugs.

RESISTANCES
R.
1 V1 grid leek .
2 v1 cathode bias .
3 V2 Cathode bias
4 Lower put «ii 8!} pl:
5 Mlddle p811? qt 5G 1)
0 Top pm. of 36 pix. .
7 Load Across tenor diode
8 Load across Vector diode
9 Cathode mm m ...
10 Provide: bibs potential for P
11 V wthodglxbl f lfi d. 2,000
01 e p,ngorelnpie
A% u? 25,000
12 Decoupling AVG 500,000
13 Across PU : 30,000
14 Decoupling twin power pack
negative 25,000
15 LF coupling Y4, V5 50,000
10 HF stopper unocle V4 ,000
17 Var. Vol, who] ... 500,000
18 In tone correction Circuit 5,

10 VSgIldleuk 2
20 Voltagedxoppingtavfi ix grld 2,000

21 Pm of 0m ptr. 50,000
22 Part a: bias pn. 350,000
23 Part of blu . 5,00

24 Decoupling m 1 mg

CONDENSERS

c. Pumas mu.

1 v1 grid .005

2 v1 cathode .1

3 v2 cathode .ool

4 Decoupling screens of v1 and v2 .25

5 Dewupllng anode v1 .25

0 Decoupling anode v2 2

7 Across HT wludlng on visual

tuning choke 1*

s Dewupling AVC to v1 and v3 .25
13 HF by-pau gnd (Triode) v4 . . .0001
11 Deooupling amplified AVC pl)-

teutialfol V4 8 e1,
12 v4 triode anode by-pass .0005
13 V4 cathode 2
14 HF by-pm betwe V4 and v .0005
15 LF coupling .02
16 mm of me correction menu .02
11 V5 grid condenser .02
18 Tom: compensation anode vs .005
10 Dewupling bins v- 1
20 HT smoothing . 7
21 HT smoothing . a
22 HF bypass from mains .005

t In our model an additional condenser of 25 mfd.
el. was mlmected across 07.

www. Vin tage-radio. com