Marantz 2270 - Phono and Tone Amplifiers; Power Amplifier

transistors through de-emphasis network consisting of C315 and R335, and C316 and R336.
L305 is a low-pass filter networks having very sharp cut off characteristics and eliminates
undesirable residual switching signals. Transistors H313 and H314 are buffer amplifiers and their
outputs are led to the function switch.
3.2 Suggestion for Trouble Shooting of FM Tuner
3.2.1 Symptom: No FM Reception
First turn on the power switch and try to tune FM stations. Rotate the fly-wheel tuning knob
slowly and observe the FM signal strength meter and FM center tuning meter. If the center tuning
meter deflect at several frequencies received, the tuner circuits preceding the discriminator circuit
may have no failure. If the signal strength meter deflect but no deflection is obtained on the
center meter, there may be some defects around the detecting circuit consisting H501, L501,
H503, H504, etc. When no reading is obtained in both meters, check FM local oscillator circuit,
using a RF VTVM. The normal local oscillator voltage is one or two volts (rms) at the tuning
capacitor, depending on the tuning capacitor position. If the local oscillator voltage is normal,
next check all voltage distribution in the FM Front End and IF amplifier unit and compare them
with those shown in the circuit diagram. When both meters deflect but no sound is obtained,
check audio circuits, using high sensitive oscilloscope.
3.2.2 Symptom: No Stereo Separation
First check the ‘‘“MONO”’ switches are in normal out position. Connect a FM RF signal
generator output modulated by a stereo modulator to the rear FM antenna terminals, and check
the stereo beacon is turned on or not. If not turned on, check for 19KHz pilot signal and 38KHz
switching signal, using an oscilloscope.
4. Phono and Tone Amplifiers
Program source signals from the PHONO jacks on the rear panel are supplied to the input
circuit of the Phono Amplifier through the selector switch and the output of the Phono
Amplifier is applied to another section of the selector switch. This amplifier provides a gain of
40dB.
All signals selected by the function switch (SO02-3F, 4F) are led to the balance and volume
controls through the MONO(L,R) and Hi-Blend switches.
Signals properly attenuated by the volume control are applied to the tone amplifier and
subjected to the tone control networks such as bass, mid, treble contro! and high and low cut
filters. .
Thus controlled audio signals are then led 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 H751)
through the coupling capacitor C751. The differential amplifier provides very high input
impedance and its collector output (H752) is applied to the base of H753 which in turn applies
its Output to the next stage; to the H756 through the network R766, C762 and R771, and to the
H757 through the network R776, C763 and R772. The outputs of H756 and H757 are applied to
the H758 and H757 respectively. HOO1 and HOO2 are power transistors used in complementary
symmetry configulation and mounted on the heat sink.
To maintain overall amplifier stability and linearity, degenerative feed back is utilized
throughout the amplifier. This feed back is also necessary to reduce distortion to within specified
limit. The RC network R775 and C756 condition the feed back signal for the audio signals. R759
and C755 are also a feed back loop provided to obtain a stable zero DC off set voltage at the
speaker output terminals. The R762 is a trimming resistor to adjust the DC offset voltage.
Dynamic bias is applied to the base of driver transistors H758 and H757. This dynamic bias
circuit is comprised of H761, H760 and R763. This provides a variable base bias for driver