Marantz 2245 - FM Tuner Troubleshooting

MPX Stereo Decoding Circuit
The buffered and non-equalized audio signals are applied to the first amplifier H301 which
serves as a tuned amplifier for the pilot signal in the composite signals and as a buffer amplifier for
the audio signals. The amplified 19KHz pilot signal is led to the second 19KHz amplifier H302
and further amplified if switching transistor H303 in turned on by the controlling BC signal as
described in the preceding chapter. The final 19KHz pilot signal is rectified by the doubler circuit
consisting of the H315 and H316 to obtain synchronized 38KHz amplifier driving signal.
The H304 is the 38KHz tuned amplifier and supplies its output to the switching matrix circuit
consisting of four diodes. While the composite signals are applied to the center tap of switching
transformer 1/2 L302. The right and left stereo signals decoded by the switching circuit are led to
the crosstalk cancelling amplifier which utilizes complementary configuration with NPN and PNP
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.
Suggestion for Trouble Shooting of FM Tuner
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. |f the center tuning
meter deflect at several frequencies received, the tuner circuits preceding the discriminator circuit
may have no failure. |f the signal strength meter deflect but no deflection is obtained on the
center meter, there may be sore 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 tocal oscillator voltage is one or two volts (rms) at the tuning
capacitor, depending on the tuning capacitor positon. If the local oscillator voltage is normal,
next check all voltage distribution in the FM Front End and !F amplifier unit and compare them
with those shown in the circuit ciagram. When both meters deflect but no sound is obtained,
check audio circuits, using high sensitive oscilloscope.
Symptom: No Stereo Separation:
First check the MONO switch is 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 funetion switch (S002-3F, 4F) are led to the balance and volume
controls through the MONO switch.
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 control and high and fow cut
filters.
Thus controlled audio signals are then led to the PRE OUT jacks on the rear panel.