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. 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 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 & 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 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 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 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
This service manual includes a detailed table of DC Test Point Voltages for the Alpha One, Alpha 7R, and Alpha 8R/8P amplifier models. This table provides critical voltage readings at various test points (1 through 32, including protection states and volume adjustments) to aid in troubleshooting and servicing the amplifiers. For specific voltage values, please refer to the original document.
Parts Lists
The document contains comprehensive parts lists for the Alpha One, 7R, 8R, and 8P amplifier models. These lists detail components such as capacitors (e.g., ELST, PEST, PCRB, CERD, PPRO, SUPPR CAP, ELEC, ELLL), diodes (e.g., RECT F ER, ZENER, SSDIODE, ULTRAFAST RECT FER), fuses, heat sinks, inductors, LEDs, jumpers, switches, transistors (e.g., TRANS LF SS N/P, TRANS PWR MOSFET), potentiometers, and resistors. Each entry includes a reference (REF), description, part number, and the applicable product model (e.g., 7R, 8R, 8P, A1). For a complete list of parts and their specifications, please consult the original service manual.
Circuit Diagrams
This service manual includes detailed circuit diagrams essential for understanding the internal workings and facilitating repairs of the Arcam Alpha One, 7R, 8R, and 8P amplifiers. These diagrams cover key sections such as the power supply, phono and input stages, tone control circuitry, left and right audio channels, and remote control interface. They illustrate component connections and signal paths, providing a visual guide for technicians. For detailed schematics, please refer to the original document.
