Arcam Alpha One, 7R, 8R, and 8P Amplifier Service Manual

Introduction

This service manual provides detailed information for the Arcam Alpha One, Alpha 7R, Alpha 8R, and Alpha 8P amplifier models. It covers circuit descriptions, power supply details, phono stage operation, tone control mechanisms, power amplifier specifics, and essential servicing procedures, including quiescent current settings and disassembly instructions. The document also originally contained comprehensive test point voltages and extensive parts lists, which are summarized here due to length constraints.

Circuit Description Overview

The Alpha One, 7R, 8R, and 8P amplifiers share a common printed circuit board (PCB) with similar circuits. Key differences include remote volume control, increased power output on the 7R and 8R models, a phono stage bypass switch for extra line-level input, and an AV processor switch on the Alpha 8R. The 8P power amplifier utilizes the same power amplifier circuit as the Alpha 8R integrated amplifier.

Power Supplies

The AC output from the toroidal mains transformer is rectified and smoothed, then regulated to +/-15V for the phono and tone control stages and the DC servo. Alpha 7R and 8R transformers include an extra tapping for the remote volume circuit, regulated to 5V for the microcontroller and remote receiver.

Phono Stage (Not applicable to Alpha One)

The phono stage features a dual op-amp (Z305) per channel. RIAA equalization is achieved through feedback components (R703, 704, 702 and C704, 705, 702 on the left channel, with R603 etc. for the right channel). R703 controls gain, and C703 determines low frequency roll-off. On the Alpha 8R, this stage can be bypassed via switch SW302 for use as an extra line input.

Tone Controls

The tone control stage is based on a dual op-amp (Z304). The control range is set by resistors R507, 508 for the bass pot (RV302) and R509, 510 for the treble pot (RV303). The tone controls and balance pot can be bypassed using the direct switch, SW304.

Power Amplifier

The input to the power amplifier feeds the differential input of Q112 and Q113. Q110 and Q111 provide a constant current source for the differential input stage. Output devices are 'n' channel MOSFETs (SMF045). Zener diodes D103, D104 limit voltage swing, and fusible resistors R105, R106 protect against overcurrent damage. An output current protection network (R41, R42) turns on Q17 to mute the output if current is too high. High DC levels at the output also mute the output. There are no output fuses; muting is handled by mute IC Z303. The mute circuit operates on switch on/off, and during overcurrent or overvoltage conditions. The power LED glows amber when muted and green during normal operation.

Servicing Procedures

Removing the Transformer

The toroidal transformer is securely attached to the main PCB with a double-sided adhesive gasket. To remove it, the adhesive bond must be broken, which is easier when the transformer is warm. A cheese wire pulled under tension can gradually cut through the adhesive. A less satisfactory alternative is 22 gauge tinned copper wire, stretched to remove kinks, with two temporary handles.

Quiescent Current (IQ) Settings

When replacing output transistors, it is crucial to reset the quiescent current (IQ) to the correct value to prevent future damage. The procedure is as follows:

1. From cold, with no load or signal connected, turn both IQ presets (RV1 and RV101) down to minimum (fully anticlockwise).
2. Set the voltage drop across resistor R1 (R101 for the right channel) to between 3.0 and 3.5mV for Alpha One and 7R amps, or between 2.3 and 2.8mV for Alpha 8R/8P amps.
3. Leave the unit for 1 minute and reset to these values if required.
4. Leave the unit for a further 5 minutes and reset if required.
5. Once IQ is set, the unit can be connected to speakers and fully checked out.

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 sub-front to the chassis base.
3. Remove 1 screw holding the PCB to the chassis, located near 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.
6. Release the PCB pillar located near the serial number on the PCB.

After these steps, the PCB, with the rear panel, transformer, and sub-front attached, can be pulled backwards out of the unit.

Additional Technical Information

The original document includes detailed "Test Point Voltages" tables for Alpha One, Alpha 7R, and Alpha 8R/8P models, listing DC voltage readings at various test points. It also contains extensive "Parts Lists" providing reference designators, descriptions, part numbers, and applicable product models for numerous components (capacitors, resistors, diodes, transistors, etc.) used in these amplifiers. These detailed tables and circuit diagrams are not fully transcribed here due to their extensive nature and to adhere to output length limitations.