Lexicon 200, Service Manual

Page: 42 / 90

MPX 200 Service Manual
6-6
= 0: None of the above switches have been pressed.
ENC_3:0:
This is a straight read of the rotary encoder switch SW1.
Possible values range from 0000 to 1111.
Resistors R158 through R161 provide pull-ups for signals ENC_3:0, ensuring that a switch open condition
is properly read back as a logic ‘1’.
R172 and C121 are provisions for either AC or DC coupling the mechanical shell of SW1 to digital ground.
In this instance, it is DC coupled through R172, and C121 is left unpopulated.
Sheet 4
This sheet shows the MIDI IO, the Footswitch circuitry, power-up reset circuit, and the front panel
connector. The MIDI and footswitch connectors provide DC connection of the digital ground plane to
chassis ground on the back panel.
MIDI I/O
The MPX200 MIDI interface complies with the MIDI specification. It incorporates 5-pin female DIN
connectors for input, thru and out (J7 and J6). J6 is shared for thru and out, according to how the Lexichip3
is programmed. MIDI INPUT is brought in on J7 and is opto-coupled for ground isolation through U24. R164
provides current limit protection of U24 and D7 provides input voltage protection by clamping the input
signal to no more than 0.7V at the input of U24. The output of U24 is open collector, necessitating pull-up
resistor R163. R162 and C116 form a first order low pass filter that outputs a sinusoidal equivalent of the
MIDI signal. This signal is then “squared up” by U14 and presented to the MIDI UART within the Lexichip3.
The filter and U14 are essential in that the opto-isolator by itself does not provide sufficient drive capability
to allow daisy chaining more than two or three units together.
The MIDI OUTPUT signal is generated by the Lexichip3 and is fed to current loop driver Q4 and out J6.
FB4 and FB5 and the connector shield ground connection reduce RFI. R149 provides an input bias current
to Q4 while C111 and C110 reduce RFI. C109 and R147 are provisions for AC or DC coupling the
mechanical shell of J6 to chassis ground. Currently this is DC coupled through R147 with C109 left
unpopulated. R150 and C112 provide a de-coupled supply voltage to the MIDI cable via FB5 and pin 4 of
J6.
Footswitch
The tip and ring of the footswitch ¼” phone jack (J8) connect to FOOT_TAP/ and FOOT_BYPASS/,
respectively, through current-limiting resistors R168 and R170. Capacitors C123 and C124 filter out RFI.
D8 and D9 help protect from over voltage or static discharge. Pull-up resistors R169 and R171 default the
non-active switch state to logic high. FOOT_TAP/ and FOOT_BYPASS/ are fed to U23 on the previous
page, which allows software to determine the footswitch state. C119, C120, and R166 are provisions for AC
or DC coupling the mechanical shell of J8 to chassis ground. Currently this is DC coupled through R166
with C119 and C120 left unpopulated.
Power Up Reset
The +10V_UNREG voltage controls reset signaling. If the voltage at the input of the +5VD regulator (U25
sheet 7) is high enough to create a 2 volt or greater drop across the regulator, then the differential between
the voltage divider R60 and R61 at the emitter of Q3 and the reg5VD at the base of Q3 will be
enough to turn on Q3. As Q3 turns on, it charges C66 through R62 and D3. The voltage across R63 and
R62 goes from 0V to about +6V. The voltage divider formed by R62 and R63 presents a TTL compatible
signal to the first section of U14, at which point it becomes inverted. The second stage of U14 re-inverts it
back to an active low signal, thereby generating a master reset signal for the front panel PCB, Lexichip3,