Beomaster 1900 Service Manual

Page 2

Bang &Olufsen

INTRODUCTION

For Service Manuals Contact
MAURITRON TECHNICAL SERVtCES
8 Cherry Tree Rd, Chinnor
Oxon OX9 4QY
Tel:- 01841351694 Fax:- 01844352554
Email:- enquiries@mauritron.w.mt

In the diagrams each component is provided with an individual position
number and a co-ordinate designation, e. g. BS. This designation refers to
the position on the PC board which is provided with a printed division into
spaces and with letters and figures along the edge of the PC board.

For the two amplifier channels the two last digits of the position numbers
are identical. Left channel begins with 200, right channel with 300.

Page 23

Bang &Olufsen

ON

PROGRAM SELECTION

6-2

When one of the control fields is touched, the EL. SWITCH, 2TR31 and
2TR32, is activated via ZIC9; also, the relay is energised, applying supply
voltage to the rest of the receiver; i 31V unstabilised is applied to the
output amplifier, -12V to BL. VOLUME CONTROL, 27V to FM tuning,
and 15V to tuner, IF, decoder, pro-amplifier, tone amplifier, and
PROGRAM INDICATION. The stand by indicator turns off when 15V
appears at the cathode.

In addition to the application of supply voltage, a pair of switch diodes,
ZDZOO, 2D300, are activated, enabling the signal to pass. on to the amplifier.
Furthermore the 15V are applied as supply voltage to 2ICS and 21C6
through 2D20. Apart from reducing the voltage, 2D20 prevents noise
impulses on the supply rail from causing the IC to change program. This
will happen if the supply voltage briefly becomes higher than the voltage on

the input.
EL. SWITCH
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100 KHZ 05

The heart of the circuit is a 100 kHz oscillator, 3TR7, 3L1 and 3C7.
The oscillator coil supplies 100 kHz power to the transistors of the .
PROGRAM SELECTION circuit, ZTRIS - 2TR22.

ZRIOO

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The amplitude of the 100 kHz voltage at the emitter of 2TR21 is approx. 3V
rms whilst approx. 6V ms is present at the top of 3L1.
This voltage is fed to the base of 2TR21 through 2C85. The base is
connected to the FM 5 control field through a solder terminal and a bronce
spring.
The capacitance of 2C85 is 3.3 pF because C spread is 3.3 pF. This provides
a division of the 6V voltage so that 3 V will be present at the base just like at
the emitter. Hence balance exists between the emitter and base, and the
transistor is OFF.