Decca 90 service manual

Page 1

issued with British Radio and Television, November, 1951

Test Report.
DECCALIA

GENERAL

The Deecalian Model 90 is a high
fidelity console radiogramophone fitted
with a 6-valve 3-waveband radio chassis
and a Garrard RC 70 record changer.
Although similar in appearance to
Deccalian Models 91 and 92, besides
being equipped only for 78 r.p.m.
records, Model 90 uses a somewhat
different circuit arrangement. The same
chassis was used in the Deccalian table
radiogram, which, however, had only a
imited production. The table radio-

/gram is dealt with under a separate
heading.

Wavebands covered are: 13-50, 190-
550, and 1000-2000 metres. The circuit
includes a separate triode gramophone
pre-amplifier to enable the use of a
Decca lightweight magnetic pick-up.
External aerial and earth are required,
and Model 90 is made only for use (in
200-250 volts A.C. 40.60 c/s. Current
consumption is: Radio, 60 watts;
Gramophone, 90 watts.

Model 90 is priced at £67 4s. including
purchase tax, and is made by Decca
Radio: & Television, 1-3 Brixton Road,
London, S.W.9.

CIRCUIT

Aerial input via coupling condenser
C2 and Si to coupling coils Ll (S.W.)
and L2 (M. and L.W.) C3 provides
additional top coupling on Long and
Medium waves, and C4 'is shunted
across L2 on Long waves only to prevent
Medium wave break-through. L11 and
C1, in parallel with the aerial coupling
coils, form a resonant circuit at the
intermediate frequency and thus act

, as an LP. rejector. 011 Gram. the aerial
input is taken down to earth.

The first tuned circuit consists of
L3, C5 and C8 on Short waves; L3 and
L4, C6 and C8, on Medium waves;
and L3, L4, L5, C7, C8 on Long waves.
Signal voltages appearing across C8
are applied via C9 to the hexode grid
of V1 (ECH35 or 6K8G) which is a
frequency-changer using the triode
section as a tuned-anode oscillator.
On Short waves the oscillator feed-back
coupling is by means of L6 and L7, but
on Medium and Long waves the
respective tuning coils, in conjunction
with their associated padders, provide
sufficient anti-phase voltage to maintain
oscillations at correct frequency. R4
and R6 are included to improve oscil-
lator stability. C10 is provided to
neutralise oscillator voltages which
might otherwise reach the frequency-
changer control grid. V1 has no

Model 90 Radiogram with notes
on the Deccallon Table Radiogram

standing bias, but is A.V.C. controlled.
An intermediatefrequency transformer
using capacity trimmers and tuned to
465 Kc/s conveys the output from V1
hexode to the gridof the intermediate-
frequency amplifier V2, which is an
R.F. pcntode, 6U7G. This valve is
also A.V.C. controlled, and its output
is fed to the second intermediate-
frequency transformer, LF/T. 2. The
secondary winding of this has atapping
which helps to reduce the damping
efiect on the circuit produced by the
detector and A.V.C. diodes. V3, a
6Q7G double-diode-triode, has one
diode used for signal rectification, and
the other for the supply of A.V.C.
voltages. R10 is the detector load
resistor, and AF. voltages developed
across this component are fed, via the
LE filter R9/C27 and the coupling
condenser C28, to the volume control,
R14. R13 is the A.V.C. load resistor,
and R12, C21 and R2 are A.V.C. filter
components. The amplified output of
V3 is taken to the grid of V4, the output
tetrode (6V6G), and from the anode
circuit of this valve the A.F. signal is
fed to an 8-in. permanent magnet
speaker.

Tone control is provided by the series
circuit R24/C37 from the anode side of
the output transformer primary to
chassis. R22 is an anode stopper. H.T.
is supplied by a full-wave rectifier, V6
(524G), and the smoothing circuit

electrolytic condensers, two of which
have a common positive terminal.

0n gramophone the output from the
pick-up is switched to the grid of an
L63 triode (V5), which has a bass-lift
circuit consisting of R17, R18 and C33
in its input to match the amplifier to
flrr recordings. The output of V5 is
taken by way of the coupling condenser
C32 to SS, and thence to the top of the
volume control. Three stages of pick-up
amplification are therefore provided.

CIRCUIT ALIGNMENT

Using a modulated signal generator,
it is recommended that an output meter
be connected across the extension
speaker sockets.

LF. circuits. After switching on,
allow a few minutes for the valves to
warm up. Set the signal generator to
465 Kc/s, and feed in a signal directly
to the grid (top cap) of V2. Adjust
C25 and C24 for maximum output.
Short out the oscillator section of the
gang condenser (furthest from chassis).
Transfer signal generator output to

1 n 21;

grid (top cap) of V]. and adjust C12
and C11 for maximum output. '

Wavetrap. Without altering fre-
quency, transfer generator output to
the aerial socket of the receiver, using a
dummy aerial. Remove oscillator short-
circuit and switch to M.W. Adjust slug
of L1 1 (above aerial socket) for minimum

comprises a choke, L10, and three ;.defiection.

'R.F. circuits. Set tuning control so
that .pointer is at the extreme (550)
mark on the scale when gang is fully
closed. If required, the pointer should
be moved along the tuning cord to
ensure that this is so.

M.W. Set signal generator to
1500 Kc/s. Tune receiver to 200 metres
and adjust C19 and C6 for maximum.

L.W. Set generator to 250 Kc/s and
receiver to 1200 metres. Adjust C18
and C7 for maximum.

S.W. Set generator to 6Mc/s, tune
receiver to 50 metres, and adjust core
of L7 for maximum. Alter signal
generator frequency to 15 Mc/s, tune
receiver to 20 metres, and adjust C5
(air-cored trimmer below chassis) for
maximum.

Padding on M.W. and L.W. is fixed,
and the alignment is now complete. It
is advisable to repeat the above opera-
tions in the same order if the circuits are
found to be very much out of line.
TABLE RADIOGRAM

Although its circuit is identical with
the Model 90, the table version differs