This document is a service manual for the Toshiba 28N33D Colour Television, which utilizes the AK37 Chassis. It provides comprehensive information for servicing, including safety precautions, technical specifications, circuit descriptions, and adjustment procedures.
Function Description
The Toshiba 28N33D is a remote-controlled color television capable of operating in various international standards including PAL, SECAM, and NTSC, with multiple transmission standards such as B/G, D/K, I/I', and L/L'. The chassis, 11AK37, is designed to drive 25", 28", 29", 33" 4:3 tubes and 28", 32" 16:9 tubes with appropriate currents.
The television incorporates a sophisticated small signal part with the STV2248 video processor, which handles vision IF amplification, QSS sound processing (for QSS versions), FM demodulation and audio amplification (for mono versions), video switching, synchronization, chroma and luminance processing, and RGB output. The STV2248 is a fully bus-controlled IC that integrates PIF, SIF, luma, chroma, and deflection processing, minimizing external components and manual adjustments.
The sound system is capable of delivering 12 watts RMS output into an 8-ohm load. It supports various sound processing standards, including NICAM digital sound, and features a virtual surround sound option (3D-PANORAMA) in some configurations. The television is equipped with a double-deck 42-pin SCART connector for AV input/output, a front-AV input, one back-AV output, one SVHS, one headphone jack, and one subwoofer output.
The micro-controller, ST92195, manages all receiver functions, including remote control and front panel operations. It generates on-screen graphics (OSD) for program number, channel number, TV standard, analogue values, sleep timer, and parental control. It also controls tuning, customer and engineering settings, and Teletext functions. The ST92195D1 version supports one-page Teletext, while the ST92195D7 supports seven-page Teletext.
Important Technical Specifications
- Chassis: 11AK37
- Tube Compatibility: 25", 28", 29", 33" 4:3 and 28", 32" 16:9 tubes.
- TV Standards: PAL, SECAM, NTSC.
- Transmission Standards: B/G, D/K, I/I', L/L'.
- Sound Output: 12 watts RMS into 8 ohms.
- Teletext: One-page (ST92195D1) or seven-page (ST92195D7) SIMPLETEXT, TOPTEXT, FASTTEXT, and US Closed Caption.
- Connectors: Double-deck 42-pin SCART (AV input/output), front-AV input, back-AV output, SVHS, headphone, subwoofer.
- Tuner (UV1315, UV1316, UV1336): Voltage synthesized tuning (VST) for VHF, UHF, and cable channels.
- Gain Control Voltage (AGC): 4.0V, Max: 4.5V.
- Tuning Voltage: Max: 5.5V.
- Band Switches (High, Mid, Low): 5V, Min: 4.75V, Max: 5.5V.
- Supply Voltage: 5V, Min: 4.75V, Max: 5.5V (for PLL supply voltage) or 33V, Min: 30V, Max: 35V (for tuning supply voltage).
- Sound Output Stage (TDA7269A): Class AB dual audio power amplifier.
- Supply Voltage: Up to ±20V.
- Output Power: 14W + 14W @THD = 10%, R_L = 8Ω, V_S = ±16V.
- Vertical Output Stage (STV9306): Fully I²C controlled vertical deflection IC for 110°, 4/3 or 16/9 CRT applications.
- Video Output Amplifier (STV5112): Three monolithic video output amplifiers.
- Bandwidth: 8MHz TYPICAL.
- Supply Voltage: 220V typical.
- Rise and Fall Time: 50ns typical.
- Power Supply (SMPS - MC44608): Controls switching transistor for 150V (FBT input), ±12V (audio output IC), +5V and +8V (ST92195).
- Demagnetization: Zero cross detection voltage: 50-mV typ.
- Supply Voltage (VCC): Min: 7.5V, Max: 18V (Operating range 6.6V-13V).
- Line Voltage: Min: 50V, Max: 500V.
- EEPROM (24C08): 8Kbit electrically erasable programmable memory, 4 blocks of 256*08 bits, I²C compatible.
- Supply Voltage: 4.5 to 5.5V.
- Headphone Driver (TDA1308): Class AB stereo headphone driver.
- Positive Supply: 5V, Min: 3.0V, Max: 7.0V.
- Subwoofer Driver (TDA7261): Class AB dual audio power amplifier.
- Supply Voltage: Up to 50V ABS max.
- Output Power: 25W @THD = 10%, R_L = 8Ω, V_S = ±20V.
Usage Features
- Program Storage: 100 programs from VHF, UHF, and cable bands can be preset.
- User Interface: Menu-driven system for easy control.
- Sound Systems: Stereo sound (German, Nicam, German+Nicam) and Virtual Dolby/3D Panorama options.
- Teletext Functionality: Fastext, Toptext.
- Direct Channel Access: Available.
- Automatic Programming System (APS): Automatically stores channels in frequency order.
- Program Naming: All programs can be named.
- Program Swapping: Between the last two watched programs.
- Automatic Tuning: Forward or backward.
- Sleep Timer: Available.
- Automatic Mute: When no transmission is detected.
- Stand-by Mode: Automatic switch to stand-by after 5 minutes of no valid signal.
- WSS (Wide Screen Signalling): Supported.
- NTSC Playback: Supported.
- AV Switching: Automatic switching from TV mode to AV1, AV2 based on signal detection from SCART pins. External AV input can be selected via remote control or by applying 6 to 12 volts to SCART pin 8.
- Tuning Modes: VST tuning, PLL tuning, and frequency tuning, with both manual and automatic options. Automatic Fine Tuning (AFT) correction is integrated.
- Volume Control: Pulse width modulation output controlled via I²C BUS.
- Mixed Mode and Fast Text: Available with 8-page memory.
Maintenance Features
- Service Mode: Electronic service mode accessible via a combination of button presses (4-7-2-5) from the main menu. This mode allows adjustment of OSD position, IF central frequency, AGC, vertical linearity, size, position, horizontal position, R.G.B gains, APR, tuner settings, and various TV set feature configurations.
- EEPROM Replacement: The chassis software allows operation with an unprogrammed EEPROM. In case of EEPROM defect, a new EEPROM of the same type can be installed and all options adjusted via the manual adjustment procedure.
- Safety Precautions: Emphasizes servicing by qualified personnel, replacement of critical components with identical parts, and specific procedures for handling the TV set when switched off (e.g., short-circuiting HV-CRT clip, discharging C809).
- Measurements: Guidelines for voltage readings and oscilloscope traces under specific conditions (60dB antenna signal, color bar pattern, normal picture performance, 220VAC, 50Hz mains supply).
- Beam Current Limiter (BCL): Protects circuitry, CRT, and prevents excessive X-Ray radiation by monitoring current drawn through the fly-back transformer and adjusting picture brightness and contrast accordingly.
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