Bose CD-20 Service Manual page 12

THEORY OF OPERATION
5.0 Tuner Electronics (continued)
The signal from the AM loop antenna enters the unit through the 2.5 mm AM jack (J300) and is
fed to the AM front end module (T302). This device contains an RF tuned section and the AM
local oscillator tuner circuit. The tuned output appears on pin 12 and is fed to the AM buffer
FET transistor Q303. The buffered output is sent to pin 27 of U300 which contains the AM RF
amplifier, mixer, IF amplifier, AM detector, and AM stop level detection. The AM stop level is set
by the variable resistor R348 on pin 16 and is nominally set at 70 dBuV/m. The AM IF output
signal appears on pin 26. This signal is filtered by the IF filter T306 and fed back into the chip
on pin 24 for AM detection. The AM detected output appears on pin 5 where it is filtered by
C327, R328 and C328. The filtered output is fed back into U300 on pin 6 where it is sent to the
L and R outputs (when AM is selected), pins 14 and 15.
The AM and FM local oscillators are controlled by the PLL IC (U302). This device is instructed
by the micro to select either AM or FM and to tune to a particular frequency. The PLL reference
oscillator is clocked by a 7.2 MHz crystal (X301). This frequency is divided down to 400 kHz
and then appears on pin 7. U302 divides down the Local Oscillator (LO) frequency and com-
pares it to an internal reference frequency. An error signal from the comparison appears on pin
18. This error signal is integrated and filtered by Q304, Q305, and associated components.
This produces the tuning voltage which appears at the collector of Q304.
The tuning voltage is filtered by R312, C316, R311, and C309 and fed to pin 14 of the AM front
end (T302). Here it is used to vary the capacitance of 2 varactor diodes. The first diode adjusts
the frequency of the AM local oscillator. The second tunes the AM RF input section to the
desired frequency. Similarly, in FM mode, the tuning voltage is filtered by R316 and C321 and
fed to the FM front end. As in the AM case, the FM front end uses this voltage to vary the LO
frequency and to tune the RF input sections.
12