Decca 81 service manual

Page 1

238 RADIO SERVICING

DECCA DECCALIAN Model 8|

General Description : Portable, dual-speed (33% and 78 rpm), record
player with self-contained amplifier and loudspeaker in laminated wood
case covered with leather cloth. Weight 33 lb.

Power Supply: A.C. mains, zoo-250 volts (three adjustment taps),
50 c/s.

Valves : (V1) 6SL7; (V2) 6V6G; (V3) 5Z4G.

Motor : Constant-speed induction motor with dual-speed drive.

P.U. : Two interchangeable plug-in heads with sapphire styli:
red for 33% r.p.m.; brown for 78 r.p.m. Automatic weight and circuit
compensation when changing speed. Spring-balanced arm.

Ext. Loudspeaker : Impedance 3 ohms.

Circuit Notes : The 78-r.p.m. pick-up head has a connecting link that
joins CI to chassis to provide correct bias compensation. SI forms a three-
position top-cut tone control. The first section of the double-triode VI
forms a conventional voltage amplifier, while the second section has negative
feedback applied by the absence of by-passing on the cathode-bias resistors,
and by the feedback link through R10. The power amplifier (V2) has an
anode stopper (RI4) and reduced screen voltage by the action of Ru. The
secondary voltage of the output transformer is connected to the internal
loudspeaker via a jack socket: inserting a jack plug for an external loud~
speaker automatically disconnects the internal one. The secondary volt-
age of T1 is also reduced and applied as negative feedback to V1 by
Rio/R4.

Modifications : The 22-kilohm decoupling resistor (R8) is now con-
nected to C8 instead of to CI I in order to improve stability.

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