Marantz 2275 - Amplifier Sections Overview

grounded, thereby establishing monaural operation. Transistor H302, connected externally to pin
(, is intended to kill the 76kHz VCO (oscillator) which interferes with AM signals during the
reception of an AM stations. When the function switch is set to “AM” position, a positive bias is
applied to the base of H302, turning it on and pin is grounded. Thus, the oscillation of the
VCO is stopped, ending the interference with AM reception.
3.3 Suggestion for Troubleshooting the FM Tuner
3.3.1 Symptom: No FM Reception
Rotate the fly-wheel tuning knob slowly and observe the FM signal strength meter. If the
signal strength meter deflects at several frequencies received, the tuner and IF circuits preceding
the discriminator circuit are functioning. When no reading is obtained on the meter, check the
FM local oscillator circuit, using an RF VTVM. The normal local oscillator voltage is
approximately 500mV at the tuning capacitor, depending on the tuning capacitor position. If the
local oscillator voltage is present, then check all voltage distribution in the FM Front End and IF
amplifier unit comparing them with those shown in the circuit diagram. When the signal strength
meter deflects but no sound is obtained, check the audio circuits using a high sensitivity
oscilloscope.
3.3.2 Symptom: No Stereo Separation
First be sure the ‘‘MODE” switch is in the normal 2 CH position. Connect an FM RF signal
generator with output modulated by a stereo pilot signal to the rear FM antenna terminals,
and check for stereo light operation. If not turned on, check for 19kKHz VCO output signal
(J310), using an oscilloscope and frequency counter.
4. PHONO AND TONE AMPLIFIERS
Program source signals from the PHONO jacks on the rear panel are fed to the input circuit of
the Phono Amplifier through the selector switch, and the output of the Phono Amplifier is fed to
another section of the selector switch. This amplifier provides a gain of 40dB.
All signals selected by the function switch (SO01-2R, 4R) are fed to the balance and
volume controls through the MONO (L, R) and Hi-Blend switches.
Signals properly attenuated by the volume control are led to the tone amplifier and are
subjected to tone control by the bass, mid and treble controls, and high and low cut filters.
These processed audio signals are then fed to the PRE OUT jacks on the rear panel.
5. POWER AMPLIFIER
The signal from the tone amplifier is applied to the differential amplifier (base of H701)
through the coupling capacitor C701. The differential amplifier provides a high input impedance,
and its collector output (H702) is connected to the base of H703 which in turn feeds its output
to the following stages: H711 through the network of R720, C711 and R721, and H712 through
the network of R720, C712 and R722. The outputs of H711 and H712 are fed to H713 and
H714, respectively. HOO1 and HOO2 are power transistors used in a complementary configuration
and mounted on heat sinks.
To maintain overall amplifier stability and linearity, degenerative feed back is utilized
throughout the amplifier. This feedback is also necessary to reduce distortion to withina
specified limit. The RC network of R724 and C709 conditions the feed back signal for audio
signals. R723 and C708 also comprise a feedback loop provided to obtain a stable zero DC offset
voltage at the speaker output terminals. R741 is a potentiometer resistor to adjust the DC offset
voltage to zero.
A dynamic bias is applied to the bases of driver transistors H713 and H714. This dynamic
bias circuit is comprised of H709, H710 and R742. This provides a variable base bias for the
driver transistors that automatically maintains the proper base voltage with temperature change.