TOSHIBA 28W23B
Service Manual

EEPROM INITIALIZATION If the E²PROM IC500 is replaced it will come fully programmed and therefore it is not necessary to initialise the new device. In some circumstances the E²PROM may become corrupted in use i.e. static discharge or lightning strike. If this happens, it is advised that the E²PROM is replaced. OFF AIR SIGNAL PATH TUNER A UV1315 voltage controlled tuner is used on the AK37 chassis, operating from a 5volt supply, line (pin6). A 0 to 33 volt rail is used for tuning (pin 2), controlled by the microprocessor IC501. The AFT pin on the tuner is not used, instead the Automatic Fine Tuning is achieved by modifying the tuning control line. This is done by software in IC501. The gain of the tuner can be altered by the AGC control voltage fed in to pin1. The tuner produces a balanced output on pins 10,11. Neither side is connected to earth. This is fed via a surface wave filter Z402 to the IF input of IC403 (pins 6 and 7). IC403 incorporate the IF amplification, AFT, AGC, video and sound detectors as well as AV switching. The IC requires both 5 and 8 V tuned circuit for these functions, L401, L402, L403, and L406. VIDEO PATH The detected video signal is output from pin 13 of IC403, to sound traps Z403/404. The video is taken from the other side via the appropriate filter to pin 18 of IC403. (1.2 p to p) Video to the scart connectors is taken after R473 to Pin 40 of the scart connector. The CVBS_TXT output pin 29 output is fed to IC501 pin 34 (for teletext). The video signal is sometimes labelled CVBS on the circuit diagram. This stands for Composite Video Blanking & Sync. The composite signal is input pin 13 (Video input) of IC403. This IC carries out all of the luma/Chroma processing internally and also provides the customer control functions of brightness, contrast, sharpness and saturation. IC403 is I²C bus controlled and incorporates auto greyscale circuitry and internal luma/chroma delay lines. The resulting R.G.B drive is output on pins 30,31 and 32. The R.G.B passes via connector PL405 to the CRT base PCB. Here the R.G.B signal is amplified by IC900 to provide drive for the cathodes of the CRT. IC900 produces a feedback signal which is fed to IC403 (pin 33) for blanking and auto grayscale correction. SOUND PATH The demodulated mono sound is taken from pin 55 of IC403 directly to the sound output stage IC301 Pin 7. The output signal from IC301 is Volume controlled achieved within IC403 using the I²C bus line from IC501. To limit the volume at the specified out put the A_out pin 55 is fed to IC301 through a voltage divider R455 and R454. Muting of the output stage is provided from Pin 46 of IC501 to pin5 IC301. In the stereo model the IF from pins 10 & 11 of the tuner passes through Z401 and the output signal goes through pins 1&2 of IC403. The output QSS signal from IC403 is taken from pin 11 and sent to audio processor IC700. The left channel is output on pin 29 and the right channel output is on pin 28.Then this outputs are fed directly to IC301. IC403 handles also the AM modulated signals in L/L� systems at pins 1&2. AV INPUT SIGNAL PATH Video and Sound IC403 has three CVBS inputs at pins 18,20 and 22.The composite video signal of AV1 is taken from pin 41 of the scart connector to pin20 of IC403. The mono sound signal is taken from pins 23 and 27 of the scart sockets to the switching transistors Q101. The transistor switch the audio depending on the source, and is then fed to pin14 of lC403. The CVBS coming from AV2 or AV3 is taken from pin20 of scart connector, from the JK101 for BAV or FAV, from the JK102 for SVHS. Then these signals are switched by transistors Q141; Q142 depending on the source by the microprocessor�s pins 5,6,7. The resultant signal is given to pin 22 of IC403. Scart two supports also SVHS signals and then the chroma comes from pin 15 of connector PL101 directly to pin 23 of IC403, whilst the luma uses the same path as the CVBS of AV2. When AV input is selected pin 5,6,7 of the microprocessor IC50 I is taken high, this switches the IC403 to external input mode via I²C BUS. This connects the video inputs on pins 20 or 22 to IC403 and the audio input on pin 14 to the audio out on pin 55 (via the internal volume control circuit) The signal paths are then as for off air. The chassis can detect the video signals on scart 1 and 2 using pin 8 switching voltages at pins 56 and 55 of IC501. R.G.B The R.G.B signals from pins 28,32 and 36 of the scart connector (PL101) are fed to the R.G.B input pins (25,26,27) of IC403. R.G.B operation can be enabled by either taking pin 37 of the scart connector high, this high is fed to Pin 28 of IC403, or via the l²C bus the microprocessor sets IC403 to forced R.G.B mode in which the video processor generates its own fast blank signal. This puts the IC into external R.G.B mode and selects the inputs on pins 25,26 and 27, overriding the video input on pin 20/22. Note: When using R.G.B input the contrast, brightness and colour controls will still operate.

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