Lexicon LXP-5 Service Manual

LXP-5 Service Manual
Circuit
Description
Battery Backup is required in the LXP-5 to maintain the non-volatiie function
of the processors’ associated SRAMs. A 3 volt lithium battery (BAT1)
maintains the minimum power requirement of the SRAM IC’s, U17 and U20,
while power is disconnected from the unit.
ZRST is used for controlling the Battery Backup circuitry, and drives the base
of Q4 through R16. During power up, ZRST goes low which turns Q4 off. In
turn, Q5 becomes active in its emitter follower configuration. The base of Q5
is biased by R20 and CR4, causing its emitter to ramp to +5 volts as C42
charges. This voltage is applied to the VCC pins of the SRAMs (U17 & U20,
pin 28). CR16 is reverse biased, preventing the +5 volts from reaching BAT1.
Upon a power fail condition, ZRST goes high which turns Q4 on and Q5 off.
C42 discharges until CR16 becomes forward biased, which provides +3 volts
from the lithium battery for the SRAM VCC.
The Battery Backup circuitry, along with the RCE signal command, prevents
stored data from being destroyed during power failure.
Separate unbalanced 1/4” phone jacks (J7 and J8) are provided for left and
right input signals. A single input source will be routed to both left and right
input stages by applying the signal to either input jack, thereby maintaining the
proper operating level.
Ferrite beads (FB8 and FB9) and capacitors (C71 and C73) are found at the
inputs to prevent unwanted high frequency interference from entering or
leaving the LXP-5 through the input cables. DC isolation is incorporated by
bipolar capacitors (C72 and C74) in line with the signal path. The left and right
signals feed a dual potentiometer (R77) which controls the input signal level
to the preamplifier stage. A dual op amp (U42) acts as the preamp by
providing 30 dB of gain. This enables the LXP-5 to perform satisfactorily for
nominal input levels as low as -27 dBV.
The left and right outputs of the preamp are applied to the stereo Mix control
(R41) through AC coupling capacitors (C68 and C69). The preamp output
signals also terminate at resistors R69 and R73 which sum the left and right
signals. This summed signal is applied to a passive 7 pole 15 kHz low pass
filter (U41) through an AC coupling capacitor (C99). A pre-emphasis function
is performed by an op amp (U40), whose output drives the sample and hold
circuit. The pre-emphasis is calibrated at the factory, and offers no provisions
for user calibration.
The pre-emphasis amplifier also drives the signal detect circuitry. The signal
passes through R63 afterwhich adiode (CR20) removesthe negative portion
of the audio signal and develops an average DC level across capacitor C96.
A sufficient DC voltage level will turn on transistor Q10, which illuminates a
green LED (LED3) by conducting +5VUNREG through R80. This LED can be
found on the front panel, and will illuminate at an input signal level of -34 dBV
or greater, with the input level control set at maximum.
Battery Backup
Analog Circuitry
Input Stage
3-5

Lexicon LXP-5 Specifications

General

Brand	Lexicon
Model	LXP-5
Category	Recording Equipment
Language	English