Decca deccalain auto 88 service manual

Page 1

Electrical and Radlo Trudlng, July 12, 1958

1143

FIVE-VALVE portable record reproducer,fwhich
can be fitted with 1ch if required. The model
has a five-stage, push-pull amplifier, three-loop
feedback chain, ZOdB feedback over main
loop, two speakers and provision for 3 ohm
or 15 ohm extension speaker. There isapilot
light at the front.

Date released. August, 1957.

Prlce at date of release, with legs £29 9s. 3d.
plus £11 9s. 9d. purchase tax; total, £40 195.;
without legs, £27 195., plus £10 185. purchase
tax; total, £38 17s.

Maker. Decca Radio and Television, 1-3
Brixton Road, London, SW9.

Service department. 18 Brixton Road,
London, SW9.

Controls. Volume controllon-ofl, bass and
treble at side of cabinet.

Speakers. 8 x Sin. elliptical; 4in. treble
unit.

Output. 8 watts.

Mains. 100-125V, 200<250V 50c/s AC
supplies.

E R T 652 E CHART

Consumption. 75 watts.

Auto-changer. Garrard or Collaro.

Pick-up. Acos Hi-G GP 59-] turnover
crystal.

Sapphire Styli. SKl (green spot) 78rpm;
SKZ (red spot) microgroove.

Indicator lamp. 6.3V 0.3A MES fitting.

DISMANTLING

Removal of chassis requires the removal of
front and rear sloping panels, secured with
wood screws. The wood screws holding

» motor board should also be withdrawn and

mains leads disconnected from motor terminal
box. After which, pull out the motor board
and remove the three-pin plug from amplifier.
Also disconnect mains leads from three-pin
mains socket.

Next, unscrew fixing nut for extension
speaker socket with a box-spanner-nut is
on rear outside of cabinet. Withdraw wood
screw securing pilot light bracket. Dis-
connect speaker leads, tape exposed motor
leads, slacken amplifier fixing screws and then
pull amplifier clear of cabinet.

SERVICE NOTES

Output transformer tertiary winding was not
provided on the first 1,000 models, negative
feedback being taken from point C on second-
ary winding of '1].

C17 on earlier models is not enclosed in a
large electrolytic can with C6, C12, but is a
separate capacitor (see underside layout).

Voltage readings on circuit were taken with
an Avo model 8 under no-signal conditions.

Continued overleaf

COMPONENT RATINGS

CI writers--
50V: C2 7111316.

Electrolytic 350V: C6 12 17.

Electrolytic 50V: C14 18.

Electrolytic 25V: C9.
Resistors-

Awau: R29.

lwatt: R11.

{waltz RIO 14 21 22.

Close tolerance: R24 25 27 28.

All others Awutt.
Potentiometers-

VRl, VR2, VR3.

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llEllL'A Alllll DEEEALIAN 88

VOlTAGE READINGS TAKEN WITH
AVOMODEL 8 UNDER NO SIGNAL
CONDITION. . ,

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Page 2

If machine is to be used on lOO-l25volts,
links in outer terminal box next to motor on
underside of changer must be connected to
agree with diagram embossed on outer mould-
mg.

CIRCUIT

Pickup input. Signal from crystal is fed
to volume control VRl with a compensation
network composed of R1, C1, R3, R2.

First AF amplifier and tone control network.

VlA is a conventional triode amplifier,
cathode bypass capacitor being omitted to
provide a degree of current feedback.
/ \mplified signal goes to tone control
rizietwcuk designed on the Baxendall feedback
principle. The component values have been
,chosen so that, with C8, sharp attenuation
occurs below 40015.

Second and third AF and phase splitter.

Second half of the first doubie~triode (V 113)
amplifies the corrected signal which is then
fed to grid of V2A, the third AF amplifier.
VZB is a conventional anode/cathode phase-
splitter, equal and opposite voltages being
developed across R20, R22.

Output stage incorporates two EL84 pen-
todes in a pushpull circuit. R30, R31 are
anode stoppers which damp out parasitic
oscillation.

Output transformer secondary feeds a twin
speaker system, the elliptical unit handling the
lower frequencies, the treble speaker being
fed through C18, which provides a crossover
at about 4,000c/sr

Negative feedback of about 20dB is applied
from tertiary winding on output transformer
to the cathode of V2. Low tolerance feedback
resistor R23, together with C16, R18, C10
forms a phase-shifting network.

®@@.@ .2

PU
SOCKET

Ti

Electrical and Radio Trading, July 12, 1953

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