Sheet 8; Power Fail/Reset; Midi I/O; S/Pdif Out - Lexicon MPX 110 Service Manual

Sheet 8:

POWER FAIL/RESET

Reset signaling is controlled by the +5UNREG voltage. If the +5UNREG voltage at the input of the +5VD
regulator (U1) is high enough to create a 2 volt or greater drop across the regulator, then the differential
between the voltage divider (R1 and R2) at the emitter of Q1, and the regulated +5VD at the base of Q1 will
be enough to turn on Q1. As Q1 turns on, it charges C7 through R3. The voltage across R3 and R4 goes
from 0V to about +6V. R3 is also used as a voltage divider to generate RESET/ at a 5V level taking
Lexichip 3 out of reset. C7 is discharged through D5 on power down.
The MPX 110 power supply provides three regulated DC output voltages: +5VD for digital circuits, +5VA
and -5VA for analog circuits.
AC power is provided by an external transformer rated at 9VAC @ 1.9A. The transformer output is
terminated with a 5-mm/2.5-mm barrel type connector (J1), with its mating input jack located at the rear
panel of MPX 110. A .1 uF capacitor, C1 is connected across the AC input to help prevent noise spikes
from entering the unit. In addition, C2 (470pf) stop circuit generated RFI from radiating through the power
line.
All three regulated supplies (+5VDC analog, -5VDC analog and +5VDC digital) consist of a single diode
(D1, D2 and D3) used as a half wave rectifier to produce the unregulated 5 volt supply (approximately 10
VDC) across each supply's filter capacitor (C3, C4, and C5). The supplies are post-regulator filtered with 22
uF capacitors (C6, C50, C53). The +5VUNREG supply is monitored by the reset circuit for power up and
power fail conditions.
Voltage regulation is handled by three TO-220 packaged ICs:
+5VD digital circuits - U1 (LM2940)
+5VA analog circuits - U15 (LM2940)
-5VA analog circuits - U16 (MC7905)
Current limiting and short circuit protection are incorporated into the internal circuitry of these ICs.
The MPX 110 power supply also provides two unregulated DC output voltages: +V and -V for the analog
output stage at U18. The +V and -V power supplies were added to help isolate the output stage from the
rest of the power supply, and to delay the turn-on during power up to minimize the power on thump
characteristics at the analog output.
The unregulated +V supply (C49, C51, R46, R47 & Q6) and -V supply (C52, C54, R48, R49 & Q7) are both
derived from capacitance multipliers which multiplies the value of the capacitor by the beta of the transistor
for a higher (but poorly regulated) output voltage.
Twenty +5VD bypass caps, five +5VA bypass caps and five -5VA bypass caps are represented on page 8
of the schematic.

MIDI I/O

The MIDI interface utilized by MPX 110 complies with the MIDI specification. It incorporates 5 pin, female
DIN connectors for input and output (J4 and J3). MIDI IN is opto-coupled for ground isolation through U3 to
the UART (in Lexichip 3). The MIDI OUT signal is provided by Lexichip 3 and is fed to current loop driver
Q4 and out J3. FB1 and FB2 are used to reduce RFI radiation.

S/PDIF OUT

The S/PDIF output signal is generated using a pair of 74VHCT14 gates U7. Each gate has to source about
6.25 mA??, well within its capabilities. The resistors are selected so that the voltage across R29 is 0.5Vpp,
Lexicon
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