Lexicon 200, Service Manual

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MPX 200 Service Manual
6-10
+5VA
This is a filtered and ferrite bead isolated version of the +5VD supply. The current demands on this rail are
barely appreciable on U25. FB1 and C64 provide noise isolation and charge reserve for this rail, which is
used to power much of the analog domain in the system.
+3.3VD
U12 provides a low current supply for the AK4112 (U13). This is the only device that requires a 3.3 Volt
supply voltage.
-5VA
U11 and C84 regulate the half wave rectified voltage from the power transformer to -5 Volts. This rail is
used to power most of the circuitry in the analog domain in the system.
Rectifier
D10, D12, C122, C127, and C128 comprise a classic half wave rectifier circuit of the power transformer
secondary AC voltage. The positive side of the rectifier (D12, C127, and C128) uses a much larger filter
capacitor scheme because the current draw from this side of the rectifier is much higher than on the
negative side.
Sheet 8
This sheet shows the AC power entry, power transformer, voltage select switch, front panel power switch,
and chassis grounding scheme.
Power is brought onto the PCB via IEC connector J9. Digital ground is tied to the Earth ground lug of this
connector via R173. This same point ties the front panel to Earth by way of a keystone bracket that
mechanically attaches to a threaded stud on the front panel. C130 provides a common mode filter across
the LINE and NEUTRAL AC lines.
F1 is a 250mA slo-blo fuse in series with the LINE and power switch lines (J10 and J11).
SW2 is a voltage select switch that configures the dual winding interconnections on the power transformer
(T1) primary. When in the 120 position, the windings are connected in parallel and the unit is suitable for
use in countries where the line voltage is between 90VAC and 120VAC. When SW2 is in the 240 position,
the windings are connected in series, and the unit is suitable for use in countries where the line voltage is
between 220VAC and 240VAC.
The secondary windings of T1 are wired in series, with one end tied to power ground. This boosts the other
end in voltage.
Front Panel Board
Schematic Walkthrough
Sheet 1
This sheet shows the front panel connector, the Octal Registers, and the column drivers used to activate
the LEDs on the front panel.
Front Panel Connector
Just like on the Main PCB, this is not strictly speaking a connector. Rather, it is a 16 x 1 array of solder
pads for the mounting of a semi-rigid ribbon cable. This is the only interface scheme between the Front and
Main PCBs. This cable provides power (+5VD), ground (DGND), three address decoded register strobes