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ALPHA ONE/7R/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 8P 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 C204, 205.

This is then regulated to +/-15V by Z201, 202 to

power the phono and tone control stages and the

d.c. 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

power the microcontroller Z901 and remote receiver.

Phono Stage (Not applicable to Alpha One)

The phono stage consists of a dual opamp Z305, one

opamp per channel.

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 Q112 and Q113.

Q110 and Q111 form a constant current

The RIAA equalization is

E MAN

source for the differential input stage.

The output devices are 'n' channel mosfets SMF045.

Zener diodes D103,104 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 (IQ) to the correct value to prevent the

amplifier blowing again.

The IQ should be checked and set by following the

procedure below:

From cold and with no load or signal connected

Turn both IQ 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 for the Alpha One and 7R