Arcam 8R int sm

Page 1

ALPHA ONE/7Ff/8R/8P AMPLIFIER SERVICE MANUAL

Arcam Alpha One/ 7R/ 8R/ 8P Amplifier Service Manual

issue 1 Serial Number 001 - (Paul Newton/ Haider Bahrani Sept 98)

Circuit Description

The Alpha One / 7R/8R and BF share a common
pcb. The circuits are very similar and the main
differences are the remote volume facility and extra
power output on the 7R and 8R and a switch to
bypass the phono stage to enable its use as an extra
line level input and the AV processor switch on the
Alpha 8R integrated amplifier. The 8P power
amplifier utilises the same power amplifier circuit as
the Alpha 8R integrated amplifier.

Power Supplies

The ac output of the toroidal mains transformer is
rectified and smoothed by D201-204 and 0204, 205.
This is then regulated to +/-15V by Z201, 202 to
power the phono and tone control stages and the
dc. servo.

The Alpha 7R and 8R transformers have an extra
tapping which is used for the remote volume circuit.
The ac in is regulated to 5V by zener D902 and
associated components and this is then used to
powerthe microcontroller Z901 and remote receiver.

Phono Stage (Not applicable to Alpha One)

The phono stage consists of a dual opamp 2305, one
opamp per channel. The RIAA equalization is
achieved by the feedback components R703, 704,
702 and C704, 705, 702 on left channel (R603 etc.
as above for Right channel). R703 determines the
gain and C703 the low frequency roll off.

On the Alpha 8R this stage can be bypassed by use
of switch SW302 to enable use as an extra line input.

Tone Controls

The tone control stage is based around Z304 a dual
op amp.

The range of the controls is set by R507, 508 for the
bass pot RV302 and R509, 510 for the treble pot
RV303.

The tone controls and balance pot can be bypassed
by use of the direct switch, SW304.

Power Amplifier
The input to the power amplifier from previous stages

is fed to the differential input of 0112 and 0113.
0110 and 0111 form a constant current

source for the differential input stage.

The output devices are 'n' channel mosfets SMF045.
Zener diodes D1 03,1 04 limit the voltage swing within
safe limits for the output devices & fusible resistors
R105, 106 provide overcurrent damage to the
devices.

A proportion of the output current appears across the
protection network R41 ,42. if this becomes too high
Q17 is turned on and mutes the output. High D.C.
levels at the output will also mute the output. There
are no output fuses on the Alpha One, 7R and
8R/8P amplifiers. The muting is done by the mute ic
Z303 and its associated compontents. The mute
operates on switch on/off and if an over current or
over voltage condition occurs. The power led glows
amber when the mute circuit is operative and glows
green in normal use.

Removing the Alpha One/ 7R/ 8R/8P Transformer

The toroidal transformer is extremely well secured to
the main pcb with a double sided adhesive gasket.
To remove the transformer you must break this
adhesive bond by cutting through the gasket. This
will be easier if the transformer is warm.

Use a cheese wire under the transformer, pulling
backwards and forwards under tension to gradually
cut through. Once most of the adhesive is cut, you
should be able to pull the transformer away from the
pcb.

A less satisfactory substitute for cheese wire is to use
some 22 gauge tinned copper wire, stretched first to
remove kinks, and two temporary handles.

Alpha One/7R/ 8R/ 8P Quiescent Current (IQ)
Settings

When replacing the output transistors of the above
amplifiers it is important to reset the quiescent
current (IO) to the correct value to prevent the
amplifier blowing again.

The IO should be checked and set by following the
procedure below:

From cold and with no load or signal connected

Turn both lQ presets (RV1 and RV101) down to
minimum (fully anticlockwise) and set the voltage
drop across resistor R1 (R101 for right hand channel)
to between 3.0 and 3.5mV forthe Alpha One and 7R

Page 2

ALPHA ONE/7R/8R/8P AMPLIFIER SERVICE MANUAL

amps or between 2.3 and 2.8mV tor the Alpha 8R/8P Disassembly For Servicing
amps.
1. Remove the top cover and front panel.
Leave the unit for 1 minute and reset to these values 2. Remove 2 screws securing the sub front to the
if required. chassis sides and 3 screws holding the
subfront to the chassis base.
Leave the unit for a further 5 minutes and reset it 3. Remove 1 screw holding the pcb to the
required. chassis. This can be found near to the mains
inlet socket.
The IQ is now set and the unit can be connected to 4. Remove 5 screws from the rear panel, one at
speakers and fully checked out. each end and 3 along the bottom edge.
5. Remove the transformer bolt.
6. Release the PCB pillar located near to the

serial number on the pcb.
The pcb, with the rear panel, transformer and

subfront attached, can now be pulled backwards out
of the unit.

TEST POINT VOLTAGES

Test Point DC Voltage Alpha One DC Voltage Alpha 7R DC Voltage Alpha 8R/8P
1 32.8V 35V 37V

2 27.8V 29V 31 V

3 15V 15V 15V

4 -32.8V -35V -37V

5 27.8V -29V -31 V

6 -15V -15V -15V

7 N/A 0.09V 0.09V

8 N/A 0.10V 0.10V

9 0.3V 0.27V 0.27V

10 3.4V 3.2V 2.9V

1 1 32.8V 35V 37V

12 -3V -2.7V -2.5V

13 32.8V -35V -37V
Between 14 & 15 3.0 -3.5mV 3.0-3.5mV 2.3-2.8mV
16 (protection oft) 0.0V 0.0V

16 (protection on) 0.6V 0.6V

17 0.0V 0.0V

18 0.0V 0.0V

19 0.0V 0.0V