Arcam T51 Service Manual
This is the 15 pages manual for Arcam T51 Service Manual.
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Extracted text from Arcam T51 Service Manual (Ocr-read)
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Contents List ! Contents list ! Circuit description ! Circuit diagrams ! Component overlays ! Circuit board parts list ! General assembly parts list
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T51 Tuner Circuit Description Power Supply Mains voltage selection is made by placing a fuse, of appropriate value, in the 115V or 230V position. The single secondary is rectified and regulated and supplies four regulators. Z7 is always powered when the mains is connected and keeps the preset memory of the micro controller intact. The high-value capacitor (C84) will keep this data safe, for a limited time, in case of mains power failure. Z5 provides the main 12V supply rail for the RF and audio circuits. This is always connected, except for the supply to the FM front end, which is disabled by Q2 when the unit is turned 'off'. Z4 powers the LED back light on the display PCB. Z6 is the 5V digital supply for the micro controller. FM Tuner & IF The FM aerial input is mixed in the front end FE1 which provides a 10.7MHz IF output. This is filtered and amplified by ceramic filters CF1 & CF2 and the circuit based around Q9 & Q10. A third filter CF3 can be put in circuit by fitting jumpers JP1 & JP2 across pins 2 & 4 and 1 & 3 of SK6 respectively. This effectively narrows the IF band to improve selectivity. Transistors Q17 and Q21 are part of the AGC circuit. The position of the jumper link JP7 to either SK10 or SK11 determines from which point the AGC circuit is triggered. The default is for the jumper to be fitted to SK10 which minimizes cross modulation and prevents the IF amp becoming saturated. The LA1266 (Z9) demodulates the IF at pin 1 into an audio output at pin 12. The quadrature coil is a double-tuned device for best distortion performance. When a station is tuned correctly the 'On Tune' signal goes low and drives a green LED on the display PCB. This signal is also used by the auto-stop detection and muting circuits. AM Tuner & IF The LA1266 (Z9) contains the AM RF amplifier, oscillator and mixer circuitry. Switching between FM and AM modes is controlled by the 'FM' signal from the micro controller which disables the AM circuitry of Z9 and switches the path of the audio signal through Z8. MW/LW switching is performed by switching transistors Q12, Q14 & Q15. Each band has its own oscillator and aerial tuning coils, tuned by a varicap diode all contained within the AM module L5. The 450kHz IF signal is filtered by coil and ceramic filter combination T5. Birdie Filter The Birdie Filter (L2) is a four pole low pass filter, flat to about 53kHz, then with a very steep roll off from then on.. This is designed to reduce co-channel interference in FM Stereo and cut down on irritating birdie noises. Stereo Decoder Stereo decoding is performed by LA3401 (Z8). The 38kHz reference signal is accurately created by ceramic resonator X3 and does not require adjustment. Stereo separation is adjusted for maximum by preset RV1. This IC also contains FM/AM audio switching and muting functions. Audio Filters & Output The left and right channel audio outputs from Z8 are filtered by the de-emphasis network of R1, C1 etc. The correct de-emphasis network is selected by the position of jumpers JP3 & JP4 on the pins of SK8. C2 & C102 (220pF) are selected for 50us and C3 & C103 (560pF) for 75us de-emphasis. The final filter, based on Z3, is a two-pole low-pass circuit with a cut-off frequency of around 20kHz. Also included is a 19kHz notch filter, L1 & C4, that removes most of the residual pilot tone. Muting When the power switch is turned off Q5 is switched off, enabling the mute circuitry of the stereo decoder (Z8) via diode D9. This removes the need for output muting transistors and eliminates switching clicks and pops. At switch-on the mute is enabled while C21 is charged via Z8. In normal use the mute inside Z8 is enabled in three other ways: The AMUTE signal from the micro controller is active at power-on and for a short time when a preset is changed. When the band is changed the output is muted by charging C52. The audio output must be muted whenever the tuner is in stereo mode and is also off-tune. This is to eliminate noise during tuning and is provided by Q8 and the MONO and OFF TUNE control signals. In Mono mode this mute is disabled so that weak stations can be heard, despite the noise. Micro controller Control of the DiVA T51 tuner is performed by a single micro controller Z10, LA7230. This also provides keypad input and drives a LCD directly. At power-on a matrix of diodes is scanned to set-up the controller for the correct options: D7 selects the correct frequency ranges. D10 enables the use of Auto tuning as well as manual tuning. D8 allows the Auto-tuning mode to use the IF Count method of searching. The position of jumpers JP 5 & JP6 fitted to SK9 are used to disable or enable LW and also whether the AM band steps are 9 kHz (UK) or 10 kHz (USA). Z10 provides control signal outputs for the following functions: MONO - To defeat the stereo decoder