Theory Of Operation - Bose Life CD5 Series I Service Manual

THEORY OF OPERATION

oscillator tuned circuit. The tuned output appears on pin 12 and is fed to AM buffer FET Q300.
The buffered output is sent to U301, pin 27. U301 contains the AM RF amplifier, mixer, IF
amplifier, AM detector and AM stop level detection. Potentiometer R339, which is connected to
pin 16, sets the AM stop level to 70dB uV/M (nominal). The IF output signal appears on pin 26
and is filtered by IF filter T307. The signal is then fed back into U301, pin 24 for AM detection.
The AM detected output (pin 5) is filtered by C315, R316, and C314. The filtered output is fed
back into U301, pin 6. Finally, it is sent to the L/R outputs (pins 14 and 15) when "AM" is
selected.
U302 controls the AM and FM local oscillators. U402 sets U302 so that it can select the AM or
FM band and can tune to a particular frequency. The PLL reference oscillator originates from
7.2MHz crystal X301. This frequency is divided down to 400KHz (U302, pin 7). U302 divides
down the local oscillator frequencies and compares them to an internal reference frequency.
The error signal resulting from this comparison appears at pin 18. This error signal is inte-
grated and filtered by Q304, Q305, and their associated components. This produces the tuning
voltage which appears at Q304's collector.
The tuning voltage is further filtered by R323, C326, R322, and C319. This signal is then sent
to AM front end module T303, pin 14. It is used to vary the capacitance of two varactor diodes.
This first diode varies the frequency of the AM local oscillator. The second tunes the AM RF
input section to the desired frequency. Similarly, the tuning voltage is filtered by R330 and
C333. Then it is fed to the FM front end module. The front end uses this voltage to vary the
local oscillator frequency and to tune the RF input sections.
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