Circuit Description
The Alpha One / 7R/8R and 8P models share a common printed circuit board (PCB). Their circuits are very similar, with main differences including the remote volume facility, extra power output on the 7R and 8R, a switch to bypass the phono stage for use as an extra line level input, and the AV processor switch on the Alpha 8R integrated amplifier. The 8P power amplifier utilizes 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 used for the remote volume circuit. The AC input is regulated to 5V by zener D902 and associated components, which then powers 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. RIAA equalization is achieved by the feedback components R703, 704, 702 and C704, 705, 702 on the left channel (R603 etc. for the right channel). R703 determines the gain and C703 the low frequency roll off.
On the Alpha 8R, this stage can be bypassed using switch SW302 to enable its 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 using 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 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, and 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. Muting is done by the mute IC Z303 and its associated components. 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, this adhesive bond must be broken by cutting through the gasket, which is easier if the transformer is warm.
A cheese wire can be used under the transformer, pulling it backwards and forwards under tension to gradually cut through. Once most of the adhesive is cut, the transformer should be able to be pulled 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 from 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 the right hand channel) to between 3.0 and 3.5mV for the Alpha One and 7R amps, or between 2.3 and 2.8mV for the Alpha 8R/8P amps.
Leave the unit for 1 minute and reset to these values if required.
Leave the unit for a further 5 minutes and reset if required.
The IQ is now set, and the unit can be connected to speakers and fully checked out.
Disassembly For Servicing
- Remove the top cover and front panel.
- Remove 2 screws securing the sub front to the chassis sides and 3 screws holding the subfront to the chassis base.
- Remove 1 screw holding the PCB to the chassis. This can be found near to the mains inlet socket.
- Remove 5 screws from the rear panel, one at each end and 3 along the bottom edge.
- Remove the transformer bolt.
- 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
The document includes a detailed table outlining DC voltage readings at various test points for the Alpha One, Alpha 7R, and Alpha 8R/8P models. This table is crucial for diagnostics and troubleshooting.
Parts Lists
A comprehensive parts list is provided, detailing components by reference (REF), description, part number, and applicable product model (e.g., 7R, A1, 8R, 8P). This list spans multiple pages and covers a wide array of electronic components necessary for repair and maintenance.
Circuit Diagrams
The service manual contains detailed circuit diagrams (schematics) for various sections of the amplifier, including the power supply, phono and input stages, tone control, left and right channels, and remote control. These diagrams are essential for understanding the internal circuitry and for performing advanced servicing tasks.
