Scanport Interface To Pitch And Mod Wheels; Volume Control And Control Sliders; Audio Differential Amplifiers For Left/Right Output Jacks; Amplifier For Headphones Jack - Kurzweil sp5-8 Service Manual

Printed Circuit Boards
Function Overview

ScanPort interface to Pitch and Mod wheels

(page 1, zone B4–C5)
The pots in the Pitch/Mod wheel assembly turn through only a small part of their
rotation range. U20 and associated components amplify the resulting small voltage swing
approximately 4X to make a full 0-5V swing for the ScanPort analog multiplexor (described
below). Additionally VR3 is used to center the range of the pitch wheel. If the scanner
diagnostic reports a center reading less than 120 or more than 136, VR3 should be adjusted to
make it as close to 128 as possible.

Volume Control and control sliders

(page 2, zone D2–D6)
These sliders connect between the +5.1V reference voltage and ground. Each wiper is filtered
by a 0.1uF capacitor and the resulting voltage passed to the ScanPort analog multiplexor
described below.

Audio differential amplifiers for Left/Right output jacks

(page 2, zone A5–C1)
Stereo audio from the Engine PCB comes in as a low-level, unfiltered, balanced signal. U18A
(left) and U18B (right) lowpass filters and amplifies this and converts it to single-ended left
and right signals with typically 50dB of common-mode rejection. U19A and U19B phase-
inverts the single-ended signal then present that plus the non-inverted signal to the output
jacks (J14 and J15) as a balanced output signal. Transistors Q7–Q10 mute the outputs during
power-up and power-down to prevent clicks. D43, D44, D47–D52 protect against static
discharge damage.

Amplifier for Headphones jack

(page 2, zone A1–A2)
U17 amplifies the single-ended left and right audio signals 2X then presents them to J16 for
driving headphones. U17 is a special high-current op-amp to better drive low-impedance
headphones. Q11 and Q12 mute the phones during power transitions while D53–D56 protect
the phones amplifier from static discharge.

ScanPort analog multiplexor

(page 2, zone D2–D4)
There are 8 different sources of analog voltage that are selected one at a time by U15 then
passed to the balanced output amplifier U14 which then drives the selected source voltage
through the Control Panel Bridge connector to the ScanPort on the Right Front Panel PCB.
The balanced outputs are symmetrical around 1/2 of the reference voltage as set by R42, R46,
and C32. U16 holds the address of the analog voltage being read during A-to-D conversion
by the Engine PCB.
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