The output of L153 is led to the IF amplifier input Pin (9) through a coupling capacitor C162
and amplified to the sufficient level to drives the detector. The detected audio signal derived from
pin (7) is filtered and amplified, and the final audio output is obtained from the collector of H153
and applied to the TAPE MONITOR OUT jacks through the function switch.
The DC component of the detected IF signal is used as an AGC voltage to control emitter
current of RF amplifier through the AGC amplifier incorporated in the IC. A part of the DC
component is also led to the signal strength indication amplifier H154. The output appearing at
the collector of H154 is level adjusted by R178, indicated on the signal strength meter M002.
2.1 SUGGESTIONS FOR TROUBLESHOOTING AM TUNER
Check for broken AM bar antenna, next try to tune stations by rotating the fly-wheel tuning
knob slowly and observe the AM signal strength meter whether it deflects or not. If the signal
strength meter gives a deflection at several received frequencies, no failure may exist in the stages
at least preceding the detector circuits. Next, connect an oscilloscope to the test point ® or
J161 and check for audio signals with the tuning meter deflected. If the signal strength meter
does not deflect, check the local oscillator circuit. Normal local oscillator output voltage at the
hot end of the oscillator tuning capacitor is about 1.5 or 3 volts, varying with the tuning
capacitor position. When measuring the local oscillator output voltage use an RF VTVM, no
circuit tester gives correct indication. If the local oscillator output voltage is normal, check all
voltage distribution in the AM circuits by using a DC VTVM and compare the measured values
with those given in the schematic diagram.
3. FM TUNER
The FM Tuner section in the Model 2325 consists of four functional blocks: FM Front End,
IF Amplifier & Detector, Muting Control and MPX Stereo Decoding Circuit.
An FM signal induced by the FM antenna is led to FM antenna coil L101 through the balun
coil. The signal is then applied to the FET RF amplifier which in turn feeds its output to the next
FET Mixer H102 through the triple tuned high selective circuit. The FET Mixer converts its input
signal into 10.7MHz IF signal and amplifies it at the same time. The H103 is a local oscillator, the
output of which is injected into the source of the FET Mixer, the injection voltage being about
700mV. The 10.7MHz front-end output is led to the next IF amplifier unit through the coaxial
cable.
The IF amplifier unit consists of seven IF amplifier stages and one AGC amplifier stage. Eight
pieces of ceramic filter are also used to obtain high selectivity, and four symmetrical diode limiter
stages are also employed for the best limiting characteristics, improved capture ratio and good
AM suppression.
A part of the FM Front End output is fed to and rectified by the AGC amplifier H209, and
the rectified output is fed back to the gate of the FET RF amplifier to decrease the gain with
increase of the signal strength.
The signals required for multipath indication are obtained from the five IF amplifier stages
through the coupling capacitors C252, C211, C214, C223 and C228, respectively, and are
rectified by five pairs of full-wave diode circuit. Thus these AM components of the FM signal are
approximately mixed and amplified by the transistors HU01 and HU02 and the output is again
rectified to obtain DC current required for actuating the Multipath Indication meter.
The IF signal sufficiently amplified through each IF amplifier stage is finally led to the
Detector Amplifier H208. The detected audio output is led to the buffer amplifier H210 and its
buffered output is led to: (a) the noise amplifier H310 through the resistor R378 and capacitor
C333, (b) the QUADRADIAL OUTPUT Jack on the rear panel through the resistor R379, (c) the
MPX stereo decoding IC (H321) through R301 and H301.
The DC current caused at the third windings of the discriminator transformer is directly led
to the FM center-tuning meter.
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