Luxman t 88 v service manual

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h T- 88V
SERVICE MANUAL

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The mains input to the tuner goes via power switch to the mains transformer which has two windings:

1. A 12 volt winding for the panel lamps.

2. Q225 (2513235) transistor is used as a regulator to obtain from 17 volt to a 12 volt DC supply
for AM and FM tuner section.

A superhet design using a mixer/local oscillator and two stages of IF amplifications at 455K112.
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 2 gang tuning condenser, the
last winding feeding the base of the mixer transistor.

The oscillatOr operates at ASSKHz above the incoming signal to produce the intermediate frequency,
which is passed through a ceramic filter and then amplified by two further transistor IF stages.
The audio is then recovered by a D216 germanium diode detector and passed on to the pre-amp via
selector switch, and signal strength meter is driven from a D215 germanium diode detector. An
AGC voltage which is detected at the D217 and D2l8 controls the gain of the first IF amplifier,
two diodes D213 and D214 are used for increasing the AGC effect. This method used provides an
audio output relatively constant with varying RF signal strength.

FM 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 capacitors and consists of a FET for the tuned RF amp feeding, via a two
section transformer, the bi-polar transistor "Colpitts" oscillator operates at -10.7MHz above the
incoming signal. C115, C116, (3117 and 6118 are negative temperature coefficient condensers to
stabilize the oscillator to less than 25Kl-Iz per 10 deg. Celsius. The output is then fed via 3pF
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 response.
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The IF strip is contained on the same printed circuit board as the AM, the stereo multiplex
decoder and muting circuit. The L0.7MHz IF intermediate frequency is amplified by a transistor
(2214 then passed through a ceramic filter with a side chain AGC amplifier to provide a DC control
voltage for the RF input FET to improve the front-end overload capabilities. The main chain is
again amplified by a further transistor and ceramic filter providing a wide pass band with steep
sides.

At the audio output when the receiver is off-tuned from centre either a positive or negative

[)0 will appear which is monitored both by a centre tune meter and a bi-phase detector using one
NPN and one PN'P transistor which with another NPN forms an "AND" gate for one of two "shmitt"
trigger circuits. The other shmitt trigger receives a command from the signal strength circuit
previously mentioned. The collectors of the final transistor in each circuit form a "wired OR"
gate which via the muting "on-off" switch on the front panel controls the gate on the FET audio
mute circuit if it is enabled. The composite audio passes on to the PLL stereo demodulator, which
will derive the L and R audio output, the PLL stereo demodulator also is connected directly to the
stereo indicator lamp.

The Left and Right go through L.C, type rejection filters to remove the 19KHz pilot. These are

both in one moulding. Finally, a one transistor amplifier is used in each channel to raise the
level to 1.6V for the audio preamp.