Aiwa AX-7400 - Service Manual

The AIWA AX-7400EE (UK model) is an FM/AM Stereo Receiver, designed to provide high-fidelity audio reproduction and versatile connectivity. This service manual outlines its technical specifications, disassembly procedures, adjustment steps, and parts lists, offering a comprehensive overview for maintenance and repair.

Function Description

The AX-7400EE serves as the central hub for an audio system, integrating an FM/AM tuner, a pre-amplifier, and a main amplifier into a single unit. It allows users to:

Receive FM and AM broadcasts: The integrated tuner section provides access to radio stations across the FM (87-109 MHz) and AM (515-1650 kHz) bands.
Connect and amplify various audio sources: The pre-amplifier section supports multiple inputs, including:
- Phono: For connecting a turntable, with RIAA equalization and appropriate sensitivity for magnetic cartridges.
- Tape-1 and Tape-2/AUX: For connecting tape decks, CD players, or other line-level audio sources.
Control audio characteristics: Tone controls (Bass and Treble) and a Loudness response feature allow users to tailor the sound to their preferences and listening environment.
Drive loudspeakers: The main amplifier section provides sufficient power output to drive a pair of stereo loudspeakers.
Monitor audio signals: Tape monitor functions allow for real-time monitoring of recording signals.
Connect headphones: A dedicated headphone jack is provided for private listening.

Important Technical Specifications

The AX-7400EE boasts a robust set of specifications, indicating its performance capabilities:

General:
- Semiconductors: 1 IC, 1 FET, 47 transistors, 25 diodes, and 1 LED.
- Power Source: Switchable 120V/220V (50/60 Hz) for the EE model and 120V/240V (50/60 Hz) for the UK model.
- Power Consumption: 205W (MAX).
- Dimensions: 420(W) x 150(H) x 360(D) mm.
- Weight: 9.0 kg.
FM Tuner Section:
- Frequency Ranges: 87-109 MHz.
- Intermediate Frequency: 10.7 MHz ±0.1 MHz.
- Noise Limit Sensitivity:
  - AX-7400EE: 3±2 dB (SN26 dB, div 40 kHz) at 88, 98, 108 MHz.
  - AX-7400UK: 7≤10 dB (SN30 dB, div 75 kHz, THD 3%) at 88 MHz, and 7≤9 dB at 98, 108 MHz.
- Image Frequency Interference Ratio: 45≥40 dB (98 MHz).
- Intermediate Frequency Interference Ratio: 75≥70 dB (98 MHz).
- Effective Selectivity:
  - AX-7400EE: 60≥55 dB (tune out 300 kHz, div 40 kHz).
  - AX-7400UK: 65≥60 dB (tune out 400 kHz, div 75 kHz).
- Capture Ratio: 1.5±1 dB.
- Separation: 40≥35 dB (1 kHz).
- SN Ratio: 63≥58 dB (input 60 dB).
AM Tuner Section:
- Frequency Ranges: 515-1650 kHz.
- Intermediate Frequency:
  - AX-7400EE: 455 kHz ±5 kHz.
  - AX-7400UK: 468 kHz ±5 kHz.
- Noise Limit Sensitivity (SN20 dB): 46≤50 dB (600, 1000, 1400 kHz).
- Image Frequency Interference Ratio: 40≥35 dB (1400 kHz).
- Intermediate Frequency Interference Ratio: 32≥25 dB (input 74 dB, 1000 kHz).
- IF Selectivity: ±25/+6 dB (1000 kHz).
- AGC Characteristic: 44≥36 dB (input 74 dB, 1000 kHz).
- Tuning Hum: 50±5 dB (1000 kHz).
Pre Amp Section:
- Phono Amp Section:
  - Sensitivity/Impedance: 2.5 mV -52 +2 dB/47 kΩ.
  - Gain: 35.5±1 dB (1 kHz).
  - Allowable Input: 120≥100 mV (1 kHz 0.5%).
  - Distortion: 0.4≤0.5% (input 100 mV≥, 1 kHz).
  - RIAA Curve Deviation: ±0.8 dB (30 Hz-15 kHz).
  - Separation: 35≥30 dB (1 kHz).
  - SN Ratio: 65≥60 dB.
- Tape-1, AUX Section:
  - Sensitivity/Impedance: 150 mV -16.5 +2 dB/470 kΩ.
  - Gain: 0±0 dB (1 kHz).
  - Distortion: 0.05≤0.1%.
  - Frequency Response: +0/-1 dB (20 Hz-50 kHz).
  - Separation: 40≥35 dB.
  - SN Ratio: 70≥65 dB.
Control Main Amp Section:
- Gain: 37.5±3 dB (1 kHz).
- Tone Controls:
  - Bass: +8/-7 dB/+6/-5 dB ±1.5 dB (100 Hz), with 400/200 Hz turnover frequency.
  - Treble: +8/-10 dB/+5/-6 dB ±1.5 dB (10 kHz), with 2.5/5 kHz turnover frequency.
- Loudness Response (volume at -40dB): +7±1.5 dB (100 Hz), +4±1.5 dB (10 kHz).
- Continuous Power Output (1 kHz, both channels driven, distortion 0.5%): 32W + 32W ≥ 30W + 30W (4Ω).
- Harmonic Distortion: 0.5%≤0.8% (1 kHz, 30W + 30W), 0.05%≤0.1% (1 kHz, 1W + 1W).
- Power Bandwidth (40 Hz-10 kHz, distortion 0.5%): 27W + 27W ≥ 25W + 25W.
- Residual Noise: 0.6≤0.8 mV (4Ω).

Usage Features

The AX-7400EE is designed for user-friendly operation with several key features:

Selector Switch: Allows easy switching between various input sources (Phono, FM, AM, Tape-1, Tape-2/AUX).
Tone Controls: Independent Bass and Treble controls for sound customization.
Loudness Function: Boosts bass and treble frequencies at low listening levels to compensate for human hearing characteristics.
Stereo/Mono Switch: For optimizing FM reception, allowing users to switch between stereo and mono modes.
FM Muting: Reduces inter-station noise during FM tuning.
AFC (Automatic Frequency Control): Helps stabilize FM tuning.
Speaker Selector: Allows selection of different speaker pairs (A, B, or A+B).
Headphone Jack: Convenient front-panel access for private listening.
Dial Scale: Clearly marked frequency dial for precise tuning.
Tuning Meter: Provides visual indication of signal strength and accurate tuning.

Maintenance Features

The service manual provides detailed instructions for maintenance and repair, making the AX-7400EE serviceable:

Disassembly Instructions: Step-by-step guides for removing the top panel, front panel, tuner circuit board, switch circuit board, main amp/power circuit board, and lamp circuit board. This facilitates access to internal components for inspection and repair.
Dial Cord Stringing Diagram: A clear diagram for correctly re-stringing the dial cord, a common maintenance task for older tuners.
Exploded Views: Detailed diagrams showing the arrangement of all mechanical parts, aiding in reassembly and identification of components.
Parts Lists: Comprehensive lists of mechanical and electrical parts, including part numbers, descriptions, and quantities, essential for ordering replacements.
Schematic Diagrams: Electrical schematics for all circuit boards (Tuner, EQ, Main Amp/Power, Control, Switch, Muting, Transistor, Speaker Terminal, ANT, Lamp, Meter), allowing technicians to trace signal paths and troubleshoot electrical issues.
Wiring Diagrams: Illustrate the interconnections between different circuit boards and external components.
Adjustments: Detailed procedures for AM-IF, AM-RF, FM-IF, FM-RF, Regulator, and MPX adjustments, ensuring optimal performance after repairs or component replacements. These include specific signal sources, connection points, and adjustment steps.
Level Diagram: Provides a visual representation of signal levels at various stages of the audio path, useful for diagnosing signal loss or distortion.
Hardware Nomenclature: Explains the naming convention for screws and other hardware, simplifying parts identification.

The AIWA AX-7400EE is a well-engineered stereo receiver, offering a blend of performance, features, and maintainability, making it a valuable component for any vintage audio enthusiast.