Cambridgeaudio Alpha 9 9P Service Manual

Page 1

ALPHA 9/9P SERVICE MANUAL

ARCAM ALPHA 9/9P SERVICE MANUAL
issue 1 Serial No. 001 - (Paul Newton July 96)
Circuit Description

Input/Output Selection

The Alpha 9 integrated amplifier has 6 line level
inputs and optional phono input selected by use of
the motorised listen switch, SW201.

The use of the 'record' switch SW202 allows
recording of one source whilst listening to another.
The Processor/T ape 2 switch, SW207, allows tape
monitoring of a 3 head tape deck connected to the
Processor/T ape 2 sockets. It can also be used to
allow input signals to be routed to an external
surround decoder for processing before being
routed back to the Alpha 9.

Tone Control & Preamp Out/Power Amp In

The tone control stage on the Alpha 9 is based
around a dual op-amp 22. The first half provides
a buffer stage for the tone controls and the second
one with its frequency dependent feedback is used
as the tone control itself.

The output of the tone control then passes through
the balance control FiV201 and then onto the
power amp stages and the preamp out buffer
stage formed by Z203.

The tone controls and balance pot can be
bypassed by use of the 'direct switch SW205.

The Alpha 9 can be driven from an external
preamplifier source by moving the internal switch
SW206 to disconnect the Alpha 9 preamp stages
from the power amp.

Power Amplifier

The signal from either the Alpha 9 input sockets or
an external preamp enters the power amp stage
via C12 and R35.

Q11, Q16 form a long, tailed pair driven by the
current source formed by Q18 and Q19.

QB is the Class A stage driving the quasi
complementary output stage of Q4, Q5, and
mosfet's Q1, 2.

The Alpha 9 and Alpha 9P utilize an auto bias
circuit to maintain the correct quiescent current
(IQ). The circuit works by monitoring the voltage
across the 0.22 ohm resistors R1, RA1 and
comparing it to a voltage derived from the - 44v rail

via R24. Both these voltage are fed into the inputs
of comparator Z3. If the voltage across the 0.22
ohm resistors increases the output of the
comparator goes high impedance and Q15 is
turned off. This causes capacitor C72 to discharge
and reduce the bias on the base of Q17 which
reduces the IQ. Conversely if the voltage across
the 0.22 ohm resistors is low then Q15 is turned on
more and C72 charges and increases the bias on
Q17 and so increases the lQ. Under signal
conditions a low frequency, variable mark/space
ratio, waveform can be seen on the collector of
transistor Q20 as the bias is being automatically
adjusted.

Z1 is a dc. servo used to maintain a low d.c. offset
at the speaker outputs.

Power Supplies

The Alpha 9/9P is powered by a toroidal
transformer with 2 sets of secondary windings.

The main windings are used to provide +/-44V
D.C. after rectification and smoothing by diodes
D201-204 inc. reservoir capacitors 0201, 202.

The second winding is rectified to provide an
unregulated supply voltage for the motor driver ics,
Z402 (for the volume control) & Z403 (for the listen
selector).This unregulated supply is also smoothed
and regulated with a simple stabilizer circuit (zener
diode D402 and associated components) to
provide a stable voltage for the micro controller
Z401.

Protection/ Mute circuit

The protection and mute functions are carried out
by 2501. This performs turn on delay, d.c. offset
detection and thermal cutout. it also displays the
status of the amplifier with the use of a bicolour
LED - 'Green' for ready and Amber' for when the
speakers are disconnected internally because of a
fault condition or on initial power up whilst the 5
second switch on delay mute is operating.

If the unit suffers a current overload the protection
circuit may latch and the unit will not reset until it
has been switched off and on again. if the
protection is activated by a thermal problem the
unit will begin to operate again when it has cooled
down.

The thermal cut out operating point is set by the
thermistor TH501, mounted close to the output
device heatsinks.

Disc Stage (optional)

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ALPHA 9/9P SERVICE MANUAL

The optional phono stage board available for the
Alpha 9 provides amplification and RIAA
equalization for both moving coil (MC) and moving
magnet (MM) cartridges.

Transistors 02-5 and their associated components
provide the extra gain required for moving coil
cartridges when required Z1 forms a flat response
gain stage with the gain set by R3 and R4. R5 and
C2 provide a low pass filter used as the high
frequency part of the RlAA de-emphasis. The
negative feedback loop around 102, R7, 8, 9, 10
and 03 provides the active low frequency RIAA de-
emphasis.

The pcb mounted switch selects between MM or MC.
The board is powered by +/- 15V regulated rails from
the main pcb and the +15V rail is then regulated to
6.3V by Q1 and D1, D2 to power the moving coil
stage.

Disassembly For Servicing

1. Remove the top cover and front panel.

2. Remove 2 screws securing the sub front to the
chassis sides and 3 screws holding the
subfront to the chassis base.

3. Remove 3 screws holding the pcb to the
chassis. These can be found close to the large
heatsink and also near to the mains inlet
socket.

4. Remove 5 screws from the rear panel, one at
each end and 3 along the bottom edge.

5. Remove the transformer bolt.

The pcb, with the rear panel, transformer and
subfront attached, can now be pulled backwards out
of the unit.

TEST POINT DC VOLTAGE TEST POINT DC VOLTAGE

TP1 + 5V TP10 0.5V

TP2 +9.6V TP11 0.5V

TP3 0V MUTE ON +2V MUTE OFF TP12 -2V

TP4 0V VOL UP +5V VOL DOWN TP13 -2V

TP5 +5V VOL UP 0V VOL DOWN TP14 +2.2V

TP6 +15V TP15 +2.2V

TP7 >15V TP16 +6.8V

TP8 +44V TP17 0V MUTE ON +2V MUTE OFF
TPQ >44V The 10 voltage measured across the 0.22 ohm

resistors R1, RA1 or R1 01, RA1 01 should be about 6-
7 mV.