Luxman T 310 Service Manual

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h T- 316
SERVICE MANUAL

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MODEL: 13- 310

CIRCUIT DESCRIPTION

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The mains input to the tuner goes via a 2 pole power switch to the mains transformer which has
two secondary windings:
1. A 12V winding for the panel lamps.
2. A 13V center tapped winding providing after full wave rectification +17V and -l7V supplies.
A potential divider using a zener diode which is used to obtain ~12V DC supply for the AM and
FM muting sections. The ZSD235 transistor is used as a ripple filter on PB328 providing 12V to

the AM, FM and Dolby sections. Thereis a fuse in the secondaries for transistor portection.

AM SECTION

A superhet design using a tuned RF amplifier, a mixer/local oscillator and two stages of IF
amplification at 455KHz. A ferrite rod antenna with 3 windings is used, the first winding is
connected to the external antenna terminal, the second is connected to the first section of a

3 gang tuning condenser, the last winding feeding the base of the RF transistor amplifier.

A tuned RF transformer is used to couple the collector to the base of the self-mixing oscillator.

The oscillator operates at ASSKHZ above the incoming signal to produce the intermediate

frequency, which is passed through a double tuned IF trans and then amplified by two further
transistor IF stages. The audio is then recovered by a germanium diode detector and passed on to the
active filter, and the signal strength meter and the muting are driven from the audio detector output.

A (-) voltage detector is used to provide an AGC voltage which controls the gain of the first IF
amplifier; the collector of this provides an amplified AGC back to the emitter of the RF
transistor. This method used provides an audio output relatively constant with varying RF signal

strength.

FM SECTION

Front End

An input balun transformer matches either BOO-ohm or 75-ohm antenna input to the Front End, which
has a 4-gang tuning capacitor and consists of a dual gate FET for the tuned RF amp feeding, via
two-section transformer, the bi-polar transistor mixer. A bi-polar transistor "Colpitts"
oscillator operates at 10.7MHz above the incoming signal. C115 is a negative temperature
coefficient condenser to stabilize the oscillator to less than 25KB: per 10 deg. Celsius. The
output is then fed via a lpf condenser to the mixer, the resultant 10.7MHz passes through a
double tuned IFT included in the front end module. The front end module is well shielded to

prevent any spurious radiation, and to offer good image and selectivity responses.

IF

The IF strip is contained on the same printed circuit board as the stereo multiplex decoder and
muting circuits. The 10.3in IF intermediate frequency is amplified by a transistor then passed
through a block filter with a side chain AGC amplifier to provide a DC control voltage (AGC) for
the RF input FET to improve the front end overload capabilities. The main chain is again amplified
by a differential IC and block filter providing a wide pass band with steep sides.

A differential IC with a built-in constant current source provides partial limiting, this passing
through an double tuned IF transformer into the final multi-stage IC which provides hard limiting
characteristics for the ratio discriminator, which provides the composite audio output for the
multiplex. A second side chain amplifier and rectifier monitors the input to the first IC to

provide a signal strength control command as well as driving the signal strength meter. At the