Decca deccalain auto service manual

Page 1

DECCA
AUTO-DECCALIAN

Self-contained portable record "producer.
consisting of a Z-valve amplifier with metal
rectifier and a Garrard «aromatic record
changer, type RC70A, taking ten 10in ar 12in
records, no! mixed, and finzd with Decca
frr lightweight pickup with sapphire
stylus. Brown or grey lent/rerclnIh-covererl
case with lea/her carrying handle and gold pr
silver»finished speaker grille. Suitable jar
200-250 VSOGOC/x AC. A madelfor 110-130 V
50-60c/s AC is available. Made by Decca
Radio and Television, Ltd. 1-3, Brixton Road,
London. S .

Vl-6SL7GT. VZ-GVGGT

230v
62 32MA

VOLTAGE ACROSS

VOLTAGE ACROSS
L5 = 90v EMS

H TOTAL HT
CURRENT. aaMA

Cl| C12: 250V.

L4

MPLIFIER consists of a twl riode V1 resist-
ance-capacily coupled to a beam tetrode
output valve V2. the output of which is fed to a
in. PM speaker. Negative feedback from
secondary of output transformer is applied to
cathode of VIE. A threestep switched tone con-
trol is incorporated in the input stage. HT is
provided by a metal rectifier connected in voltage ,
doubling circuit.

Pickup is a Decca fTrr " fitted with a sapphire
stylusl It has a substantially linear frequency
response from 30 to 14,000c/s with an output of
approximately ,2V and has a DC resistance of
3,500 ohms. Pickup signal is fed via bass com-
pensating network R1, R2, C1 to g] of input
amplifier VIA.

S] with C2. C3, which are shunted across R2,
Cl gives three degrees of tone control. Cathode
bias for VlA is provided by R3 decoupled by C4.
R4 is anode load resistor. C5 feeds signal from
anode VIA to volume control R6 and thence to g1
of V113. Cathode load resistor is formed by R7.
R8 and negative feedback, from secondary L3 of
01], is fed through R11, C7 to cathode circuit.
R5 is the anode load resistor.

Output stage-C6 feeds signal at anode of V113
to grid of beam tetrode output valve V21 R9 is its

grid resistor and R10 decoupled by C8 provides
cathode bias. Screen (g2) voltage is obtained from
Junction of voltage dro ping resistors R12. R13,
and is decoupled by 10, L2, the primary of
output matching transformer OP! is in the anode
circuit, the HT for which is obtained direct from
the reservoir capacitors. Secondary L3 feeds
srgnal to a 6§in. PM speaker L4. Negative feed-
back from secondary L3 is fed by R11. C7 to R8
in the cathode circuit of VlB.

HT is provided by a voltage doubler selenium
type rectifier fed from secondary L5 of the mains
input transformer MT]. C11, C12 are the Voltage
doubling reservoir smoothing capacitors and R12,
RI], C9, C10 give resistance-capacity smoothing,

Heaters of V1, V2 obtain their current from
secondary L6 of MT]. Primary L7 is tapped for
input of 200 to 250V 50-600/5. A transformer
Euttaible for 110-130V 50-60c/s supplies can be

ttc .

$2 is the amplifier ONJOFF switch.

Automatic record changer mechanism is a
Garrard RC70A which will play up to ten 10in.
or 12in. records. not mixed. The pickup tone
arm is fitted with bayonet socket to accommodate
the plugdn type Decca firr lightweight pickup
head. Driving motor is a constant-speed induction

E.

O
M T ». or.

0

Continued overleaf

RESISTORS
R Ohm:

VOLUME

wN-Owwuamwa-

TONE

Ter-

IANUARY 1950

MANS INFUr
SOCKET

INDUCTORS

MTI.

R7
m3 m2 .
r" - - _

mo

OPI. c9
L2

.5 4 l \
cos

no

TONE

MR MR -CI ( c2 ( )st 1: ohm:

R8 n2 as R
its

3.5K
400

Very low
75 Total

CAPACITORS
C 114/ Type

02 Tubular 350V
5 Tubular 500V

02 Tubular 500V
20 Electrolytic 12V

LE

L3 C0

cit

.os Tubular soov
.05 Tubular 500v
.05 Tubular 500V
(Not fitted on model

c
I:

tested)

103.725134 6

.. so Elcctrolytic 25v
l6 Electrolytic 350V

ts re ll.r2,2 534

16 Electrolytic 350V

L 562 37

. 16 Electrolytic 350V
. 16 Electrolytic 350V

Electrical and Radio Trading

Page 2

L ECCALIAN-Continued

type prowded with dual-voltage field winding for
100-130, 200-250V SOC/S. The motor is
normally fitted with a SOc/s driving pulley, but
special pulleys for 40 or EOc/s mains are available.

For details of maintenance and adjustments to
the changer reference should be made to Garrard
Booklet or to our Service Chart of August. 1949.

Removal of pickup-Gently press head of pick-
up inwardsand rotate an eighth of a turn in an
aitti-clockWise direction and withdraw from
socket in tone arm.

Replacement of sapphire needle. iRemove pickup
head from tone arm as described above. Remove
the two screws situated in the top of the head and
caretully open needle plate to full extent of its
hinge. _Lift out the rubber-bonded armature in
which is located the sapphire needle. Replace
With a new one and ensure that tip of needle passes

Sentercel Miniature

The Sentercel miniature selenium rcctifiers have
been developed .° meet the demand for a small size
component for incorporation In modern radio and
electronic eq uipment.

In the HT types RM1,RM2,RM3. size has been
greatly reduced by employing a tubular rivet
instead ofthe conventional spindle with end clamp-
ing nuts. The rectifiers measure approximately
filn. in length with cooling fin diameter of lsins.
tor RM I, and litns. for RMZ and RM3.

The units are of half-wave type, designed for a
maximum input voltage of 125V RMS with a peak
inverse voltage of 350. At a temperature of 35
deg. C. the maximum output currents are 60mA,
lOOmA, lZOmA respectively. At 55 deg. C. the
outputs fall to 30mA, 60mA, 90mA. The maximum
instantaneous peak current is claimed to be un-
limited and the rectifiers will withstand heavy over-
loads for short periods without damage.

These rectifiers are supplied in tropical finish

More Page: from

OMPLAINT: distorted sound on Murphy
V136 and inability to obtain correct focus at
any setting of the control. This was found to be due
to low-emission sound output valve, the cathode
current of which flows through the focus coil. The
same effects have also been caused by a leaky
cotfiflJing capacitor to the sound output valve.~
C. . .

Symptom : Radiogram with radio normal, sound
fading in the middle of the record.

HE pickup head and wiring were first tested,
T and found in order. The switching and
Wiring were also checked. and no fault tound.
The DDT was substituted without improvement.
The fault was finally found to be caused by the
cathode by-ptiss condenser of the DDT valve,
which was open-circuit. Fitting a new one cured
the trouble.~W.A.

Symptom : Radiogram with radio normal, distor-
tinii on records.

HE set had a magic eye" tuning device, and

_ on inspection itwas found that the mode
section of this was switched in, on records, to act

Vi

\

through the existing hole in latex seal on underside
of head. Close down needle plate, replace fasten-
ing screws and plug pickup into tone arm.

Removal of auto-changer.7Lift off turntable and
record spindle. Remove mains leads from voltage
changeover panel and unscrew small spring clip
zit right-hand side of inside case. Remove the
three screws fastening sloping panel adjacent to
auto-changer platform and lift otT panel. Un lug
pickup leads from input sockets on amp ifier
chassis. Remove the six screws fastening changer
platform to case runners. Lifting rear edge of
platform first, withdraw auto-changer front case.

Removal of amplifier chassis-Remove auto-
changer as described, Remove top panel above
amplifier and unscrew ONJOFF switch from the
anel. Remove the four chassis bolts accessible
ron-i bottom of case and undo nuts on LS bolts.
Remove mains lead from input socket at rear of
case and remove cable cleat. Chassis and LS can
now be withdrawn.

HT and HF Rectifiers

either as single units or with two on a bracket.

The HF type Ml rectifier consists of a single
element mounted in an aluminium case and fitted
with wire connections. Its dimensions are l by «.lin.
and it is undoubtedly the smallest rectifier of its
kind. it will operate as a normal diode detector up to
5 mc/s and with inputs as low as 0.5V. Below this
input satisfactory rectification cannot be obtained.
It has a self capacity of 20 pF, a forward resistance
of 12,000 ohms, and a reverse resistance of 20
megohms. Maximum peak inverse voltage is 50.

The M! is suitable for use in contrast expansion
circuits, level and modulation depth indicators.
detector and AVC circuits, and as an interference
limiter and DC restorer.

On test the rectifiers were used to supply HT and
to function as a detector in a typical radio receiver
and both types operated satisfactorily. They are
made by Standard Telephones and Cables. Limited,
Aldwych. London, WC2.

Service Casebook:

as a first AF amplifier. This was resistance
coupled. through a condenser, to the grid of the
output valve. and the condenser was faulty. causing,
when the set was switched to gramophonc repro-
duction, positive bias on the output valve grid.
The faulty condenser was not in circuit on radio.
which consequently was not aflectediLH.

Symptom : Radiogramidead on both radio and
records.

HE spcakcr, power pack, and output valve illld
T Circuits were checked, and found to be in
order, and all valves were lighting. The set was
of the three-valve and rectifier type of superhet,
and when checking voltages the resistance feeding
the oscillator anode of the frequency changer was
found to be burnt out. and was replaced, This
curcd both faults, as the oscillator section of the
frequency changer was in use, on records, as a
first AF amplifieriLH.

Dont forget, if you experience an unusual and
interesting fault, that your fellow engineers nill
all be interested too. Write a short note to
ELECTRICAL AND RADIO 189, High
Holborn, London. WCI.

TRADING,

Head Office: 11 OLD BROAD $7.. IONDON, ELI
Telephone: LONdon Wall SMI

ALI. rELcoN CABLES are scientifically
designed. EVERY con. is TESTED to ensure
that: the Impedance and Attenuation
characterisrics are uniformly main~
tained within close limits. This back-
ground of technical knowledge and
care in manufacture is a guarantee that
Aerial Installations using 'I'ELCON
TELEVISION camps are a source of
maximum signal and ratixfizcti'on. There
is a TELCON CABLE designed for every
Receiver and for all reception areas-
ITIM Co-axial for high signal strength
and AS 60M Co-axial for fringe
areas, are two examples.

THE rificniirii c NSTRUCTION a. MAINTENANCE co. LID.

tawm l854

Enqumes to: TELCON WORKS.GRFENWICM. S E ii)

Telephone: Greenwich 31

ltlhltx all "I!
tale tn-nittiit
let's trnpn it will
tie rnmzttiint
intuit,

Wail. what it'ymi

know

Just mini In I

rm
e-Iciaeklnrim're
nueut-grLuxlrti"

For all SOLDERING work you need FLUXITE
-the paste flux-with which even dirty metals
are soldered and tinned." For the ioiiitirig
of lead-without solder and the running" of
white metal bearings-without "tinning" the
bearing. It is suitable for ALL METALS-ex-
cepting Aluminium-and can be used with safety
an Electrical and other sensitive apparatus.
with FLUXITEJnInts canlievtlttell"suecesstully
that are impossible by any other method.
OF ALL IRONMOIIGERS. in tins-101.. 1/6 & 3/-.

Used tor over 40 years in Government works and
by leading Engineers and Manutactureu,

The "FlUXITE GUN " puts Fluxite where you want i| by
a simple pressure. Price 2l6 or filled 3l6.

TfliflffiT

SIMPLIFIES I SOLDERING
V i ' T

Write for Lullurl on mammals... smi .na Temp-tin: nah
.im PLUXITE, aI-o an ma-v Iolnll. rm. u. «on.
FLUXITE LTD.. [DapLT.E~). lumendsey $9., Ionian, S.E.l

Electrical and Radio Trading

Technical Books

English and Foreign Books on Radio
and Electxical Engineering supplied
from stock or obtained to order.

LENDING LIBRARY
Annual Subscription from 0N! GUINEA.
Derailed pro-pectin fro: on roqneot.

H. K. LEWIS & Co. Ltd.

136. Gowor Sir-cl, London. W.C.I
hum; HUS! us: (5 um)

I.
Bewlnds
PROMPT RETURN

H Comprehensive stocks of Laminations
Ind Wire for NEW TRANSFORMERS

and CHOKES in lug: or smell

quantifier to your requiremmu.

Coastal Radio Ltd.

Radio Works, hope Cram, Edinburgh 7
flat-on: units

JANUARY I 950