Decca 55 service manual

Page 1

11 (Supplement)

TRADER SERVICE SHEET

ESIGNED so that it can easily be
Dcarried about, the Decca 55 is a

4-valve (plus rectifier) A.Ci/D.C.
3-band superhet with a shortwave range
of r949 metres, and suitable for mains of
200-250 V, 50-60 C/S. It has sockets (or
connecting a gramophone pick-up and a
pair of phones, while there is also a
switch for cutting out the internal
speaker. A length oi wire is supplied [or
use as an aerial.

The chassis fitted in the 66 receiver is
identical but that model has a large
walnut finished table type cabinet.

This Service Sheet was prepared on a
55 model.

CIRCUIT DESCRIPTION

Aerial input via series condenser (11
and coupling coils L2 (S.W.), L4 (M.W.)
and (via 342 metre rejector circuit
L1, 03) L8 (LiW.) to single tuned circuit.»
L3, 030 (SiWi), L5, 030 (M.W.) and
L7, 030 (LiWi).

First valve (V1, Mazda metallised
mam) is a triodc-hexodc operating as
frequency changer with internal coupling.
Oscillator anode coils L9 (S.W.), L11
(M.W.) and L13 (L.W,) are tuned bv
036; parallel trimming by C31 (S.W.),
032 (M.W.) and 033, 011 (L.\V.) ; series
tracking by 09 (SW), 010. 034 (M.W.)
and 035 (L.W.). Oscillator grid reaction
coils L8 (S.W.), L10 (MW) and L12

(L.w,).

Second valve, a variable-mu RF,
pentode (v2, Mullard metallised VPISC),
operates as intermediate irequcncy
amplifier with tuned-primary tuned-

secondary transformer couplings 037,
L14, L15, G38 and 039, L16, L17, 040.

Cl

53

Circuit diagram of the Decca 55 A.Ci/D.C. superhct receiver.

'lllewmmsu urn-um

July 3, 1937

5E§CA 55

ND 66

Intermediate irequenoy 465 KC/S.

Diode second detector is part of
separate double diode Viilve (V3, Brimar
10D1)i Audio frequency component in
rectified output is developed across load
resistance R11 ziiid passed viii stopper
resistance R10, Al. coupling condenser
017 and inanuiil volume control R12
to C..(-. M puiiimle output \4th(: (v4,
Brimar 7D6). 1" Oil (one (.m'rt' ion in
anode circuit by 021. Mo .nii (or
connection of low impedance external
internal

speaker across secondary of
speaker input transformer 'llr Switch
819 permits internal speaker speech

coil circuit to be broken. Provision for
connection of gramophone pick-up across
R12.

Second diode of V3. fed from tapping

on L17 via 018, provides. 11C. potential
which is developed acre load resistance
R13 and [ed buck Ihrougli decoupling
circuits (in (LE. to 1?.(1 and 11. valves.
giving ziiiionialic volume control, Doliiy
mlingv is obtained from «Imp across V4
catholic ii-sisiziiiccs R14, R15.
is used with AC.
mains lifl. current i supplied by a hall-
ivave rectifier (V5, Mullard URIC) th11.
with l).C. supplies, behaves as a low
resistance. Smoothing is effected by iron-
cored choke L19 and dry electrolytic
condensers 022, 023.

\alve heaters are connected in series
together with current regulating biirretter

(Philips 01), across miiins input. One
scale lamp, \vhic illuminates the wave-
band indicator coloured slide, is con»

nected across the heater of V3. Another
lamp, which fioodlights the tuning scale,

(a Sic M
|E

is connected across the mains input.
Filter comprising chokes L20, 1421 and
condensers G24, G25 suppresses mains-
borne interference.

COMPONENTS AND VALUES

Values
CONDENSERS or)
('r Aerial suits condenser owns
(:2 [Earth blocking condenser a»:
as An lair it 3.2 III. roiuctor
inning .. .i .i o-oooorz
a V1 sci decoigiling .. ,. 01
(:5 VI hexorle c. icondenscr crew!
or. V: hexode cc. decoupling .. 0-02
C7 Vx cathode lay-pass .i .. 0!
ca Vr osc. cc. condenser i . o-oooz
c9 Osc. circuit s.w. fixed tracke woo:
Cm Osc. circuit iiLw. fixed tracker moms
Cir 05c. circuit LWi fixed trimmer ooooos
sz Vi an. anode coupling o~ooox *5
C13 v; on. decoupling . . am
2 V2 cathode lay-pass i. .. 0-!
r5 o-ooox
cm }LF. bypasses .. .. { mm
c A.F. cou ling :0 R12 .. om
cm V3 Aiv. idiodeieed 0000:
C19 V4 cc. decoupling 0'!
Cm V4 cathode hyrpass 25.0
En. V4 anode tone correclor i. 04:06
22 . 80
513. li-m. smoothing { m
5:; Mains m: mm condensers<[ :32
cm Vr heater RF. 1: mass 00:
C27: Aerial circuit 5 --
(2233 Aerial circuit .-
-��29: Aerial circuit -
Cami Aerial circuit lu g .. a
can 05:. circuit sw. trimmer ~
C32: 05:, circuit M.w. trimmer -
C33 Osci rlrcuit Liwi tn'mrna' -
cm 05:. circuit M.w, tracker 0-900
(:35: Osc. circuit L.wu tracker D'ooozz
C361 Osc. circuit tuning .u w
:37; xst r transi ri tuning -
E38: ~ _
39 r -
cw: us. see. tuning .u -
Electrolytic t Variable. IPmset.

Icu

L20

M.
H mm:
LII

VVA

520

Note the arrangements for the scale and wavcband indicator lamps.

Page 2

July 3, 1937

Under-chassis
view. Most of
the trimmers are
mounted at the
ends of the indi-
vidual coil units.
The two switch
units are indi-
cated by num-
bers in circles,
and the arrows
show the direc-
tions 'in which
they are viewed-
in the diagrams
on this page.

i

Values
RESISTANCES (ohms)
Rx Vr so. in. leed .r 15,000
R: V! hexode .resistanoe . 500,000
n; Vr hexode c . decoupling
Iesistnnoe i. .. .. 500.000
R4 Vr fixed 0.13. resistance i. 200
R5 Vr osc. cre, resistance ,. 50,000
R6 Use, circuit s.w. stabiliser .. 75
R7 V1 050 anode H.1. iced . . 40,000
Rs v: are decoupling resistance 5min
R9 v: fixed as, tesistanm . . :00
Rio in. rtop r .. r. .. 70,000
Rn v; sigmi diodeload .. 300,000
R12 Manual volume control 500,000
Rm v.1 Aivlc, llimle lolul .0 .. 500,000
Ru )A.V.C. delay voltage and i lxu
Rxs vi on. resistances .. r50
mo v, anode circuit. stabilisor .. r 50
A ptox.
OTHER COMPONENTS nines
(ohms)
Lr Ar-rlill rinuiit 342ml rfljtKInl'
' lt-u

cull .. .,
L2 Aetigll circuit s.w. coupling

onl . . . . . . . .
L3 Aerial circuit SiW. tuning coil

mi L4 Aerial circnlt M.w. coupling coil

gt L5 Aerial clrcu t MlW. tuning coil

1 L6 Aerial 0' tL.W.eonpling coil
L7 Aerial u t L.w. tuuiug coil
L3 000 0' s.w. grid 00il

L9 000. circuit s.wr tuning ooil . .
Lro 000 circuit M.W. grid coil .
Lxr 000. circuit M.W. tuning coi , .
Lr: Osc. circuit L.W. grid coil .
L13 use. circuit l..w. Inning (70A . .

L } xst LF. trans. {.ch

'ITlewurri m. H rrnnnll

Next remove the pilnel carrying the aerial.
earth and plck-up sockets (three round.
head wood screws), and the scale lamp
and its holder (two roundehead wood
screws) and nllsoltlor the speaker leads.

The l'llassls can now be withill'nwn
from the cabinet and when replacing,
connect the speaker leads to the two outer
tags and (10 not forget to replace the felt
washers. beLween the knobs and the
cabinet.

Removing Speaker-.7 To remove the
speaker lrlun llu- lulliillet, remove the
panel carrying the sockets [or yllolles
(two f0illlllvhmtll 00d screws) and the
switch (null, unsolder the leads coming
from the eh' and remove the nuts
and washers lroni the {our screws with
ornamental heads holding the speaker
to the cabinet. When replacing, see
that the ll llsinruier is pointing: to the
luu-lr (ll llu: trui ill the mliiiu-l.

VALVE ANALYSIS

Valve voltages and currents given in
the tuble below are those measured in
our receiver when it was operating on
AC, mains of :30 \'. The receiver was
tuned to the lowest wavelength on the
medium band and the volume control
was at maximum, but there was no
signal input.

Voltages were measured on the 1,200 V

Iii? (end in trans, {5133: mm scale , of an Avometer, clra being
Ln; 5 alter speech coil again/6-
in r. smoothing choke. . .

no - Anode Anode Smell Semen
L2! i 3" fin" mm ' i>r' i Valve Voltage Current Voltage Current
1'; Speaker mput trans. { 52:. . (V) (m) (V) (W

5»st Waveband switches. ,. . . vI TH2320 23¢, 26 (,0 l 5-4
Sig Internal speaker swttch . V2 VPiiC 230 150 :30 6-0
5:0 Mains switch, ganged Rx: v3 roDi ., _ i ,
F1 Mains drouit (use, 75o m/a v4 7m. . . :m :30 230 1
vs URtC: l 7 , ---

DISMANTLING THE SET
Removing Chassis-elf it is desired to
remove the chassis from the cabinet,
remove the three control knobs (recessed
grub screws) and remove the two bolts
(with washers and lock washers) holding
the chassis to the bottom of the cablneL

sescillntiu- muuit siiv, 3-5mA.
:Cullinrle lu I'llllssls, 255v oc.

GENERAL NOTES
Switches.-Sl-818 are the waveband
switches, ganged in two rotary units
beneath the chassis. The units are
indicated in our underechns. ' view and

(Supplement) III

are shown in detail in the diagrams
below, as seen looking in the directions
of the arrows in the under-chassis View

The table below gives the switch
pas 'ons for the three control settings,
starting irorn fully clockwise. 0 indicates
open and 0 closed

Switrli s M L
o l o

0 c o
o 0 c
o c
c n c
c c o
c o o
t) C O
o o c
c o 0
o c o
0 () c
o c c
c 0 c
c u o
C 0 0
o (1 ~ 0
o 0 c

S witch dia-
grams, as seen
I looking at the
: underside of
the chassis in
, the direction of
' the arrows in
t h e u n d e r-
ehassis View
above.

919 is the Q.M.B, internal speaker

switch, mounted on the rear of the cabinet.

520 is the Q.M,B. mains switch. gauged
with the volume control 1112.
Continued overleaf