Headphone Outputs; Qs7/Qs8/Qsr Aux Outputs; Power Up Mute; Digital Signal Paths - Alesis QS Series Service Manual

2.22 Headphone Outputs

The signal from the volume slider is sent through a X10 X3 X3 gain stage U5A, U5B
U2A, U2B U1B, U1A, current limited R12, R13 R3, R2 R1, R2 and output through the stereo
jack J2 J1 J1.

2.23 QS7/QS8/QSR Aux Outputs

The Aux outputs are completely isolated from the main outputs by virtue of a separate
DAC U3 U4. The differential output of the DAC is anti-alias filtered and buffered by U3C U3C
(left) and U3D U3D (right). This signal is current limited (R31, R32 R31, R32), R.F. Filtered
(C35, C36 C33, C34), and output via the ¼ " jacks (J8, J9 J5, J4).

2.24 Power Up Mute

The Power Up (PUP) mute circuit ensures that no sound is output during the critical
stages of powering the unit up and down. Most often noise is created due to the uneven power
demands placed on the positive and negative rails, especially at power up. There also may be
noise due to the random power up states in the digital hardware prior to the software
establishing correct values. Muting is accomplished with the use of the microprocessor reset
line (See Section 2.31A).
The positive and negative supplies are fed to the final analog output stages via
transistors Q2, Q4 Q1,Q3 Q1, Q3. R48, R50 and R51 R13, R18 and R19 R13, R18 and R19
are used to bias these transistors into saturation when the circuit is switched on. Q3 Q2 Q2
provides the necessary current to switch the supply transistors on, as well as providing a
threshold value for timing purposes. Since power up noise may extend past the time necessary
for the digital hardware to physically reset, a further time delay is required before the output is
allowed to turn on. This is accomplished with an RC circuit comprised of R49 and C30 R20 and
C14 R21 and C13. D15 D3 D1 provides a quick discharge path for the capacitor so that the
circuit functions correctly even if power is cycled off and on rapidly.
Note that in the QS6 it was found that this circuit did not load down the RST line (see
Section 2.24), causing power up reset to take longer than it should. The extra inverter was
eliminated in the QS7/QS8/QSR.

2.30 Digital Signal Paths

2.31 H8/510 Microprocessor

The H8 Microprocessor (U16 U21 U20) is the primary device in control of all information
flow in the QS Series. At the appropriate times, the H8 accepts input from various sources
(Keyboard, MIDI, etc.) and determines the correct response (play sound, output data to MIDI,
etc.). All of this information flow is under the direct control of the software stored in the
EPROM (U19, U18, U17).
Although a complete discussion of H8 programming is beyond the scope of this manual,
it is appropriate at this time to state the importance of understanding overall unit operation
including the interactions between hardware and software. Not only is it possible to
troubleshoot usage errors (i.e. keyboard not loading DataDisk files because Sys-Ex is
disabled), it can also make troubleshooting hardware easier as well. For example: it is easier
to troubleshoot a MIDI output problem by sending longer files out through MIDI while examining
the MIDI output circuit with an oscilloscope. The longer the file, the more time there is to look at
the circuit before having to send the file again. While this is a relatively simple example, it
Alesis QS Series Keyboards Service Manual V1.00
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11/19/06