Decca decola service manual

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24.0 RADIO SERVICING

DECCA _ DECOLA

General Description : High-fidelity record player incorporating a four-
stage push-pull amplifier. In some models radio facilities are provided.
The amplifier consists of eight triode valves in four push-pull stages with
independent bass boost and top-cut controls and a bass-compensating
volume control. The total harmonic distortion should be less than % per
cent at 5 watts output.

Power Supply : A.C. mains, zoo-250 volts (three adjustment positions),
50 c/s. Consumption approximately 125 watts.

Record Changer : Garrard, type R.C.6o.

P.U. : Decca needle-armature type with semi-permanent sapphire or
diamond point for range 3o-x4,ooo c/s.

Loudspeakers : Three, 12-in., high-flux permanent-magnet type.
Later models have one dual concentric unit and two 12-in. units.

Valves: (VI) L63; (V2) L63; (V3) L63; (V4) L63; (V5) L63; (V6)
L63; (V7) PXZS; (V3) PXZS; (V9) 5U4G-

Pilot Lamp : 6-3 volts, 0-3 amp.

Circuit Notes : Matched pairs of valves should be used in each stage.
The cathode bias resistors for valves 1-6 are not by-passed, in order to in-
troduce negative feedback. The grid-to-anode capacitance of each of these
valves is neutralised by the connection of 4-pF. capacitors from the anode to
the grid of the opposite valve. This is because the input capacitance of a
triode valve is high. This capacitance is the grid-cathode capacitance plus
the produce of the grid-anode capacitance and the gain of the valve;
by neutralising the grid-anode capacitance, the effective input capacitance is
reduced from approximately 90 to 4. pF., thus greatly reducing attenuation of
the higher audio frequencies.

Loudspeaker Phasing : The D.C. resistance of each of the three loud-
speakers is I 5 ohms, and they are normally wired in parallel. The loud-
speaker plugs should therefore be in the sockets on the power unit marked
parallel output with the two links across the remaining sockets so that the
letters P are covered. If external loudspeakers are used, it is generally
more convenient to wire them in series, in which case the external leads
should be connected to the series output sockets with one link used to
cover the letter S on the panel.

If it is necessary to replace a loudspeaker, care must be taken to ensure that
the new loudspeaker is correctly phased. This can be done as follows:

Disconnect the loudspeaker from the amplifier and connect a D.C. milli-
ammeter across the loudspeaker-input terminals. Gently but sharply push in
the loudspeaker cone and notice which way the milliammeter needle kicks.
Repeat with the other loudspeakers, and thus determine for each the way the
milliammeter has to be connected in order that they all produce a kick in the
same direction. A polarity is thus established enabling the loudspeakers
to be connected in series or parallel as required.