CAUTION: The vacuum tube power supply voltage is 220VDC. There is a danger of electrical
shock if you touch components in this circuit. Use caution when testing this circuit, damage will
occur to the Main board and other ICs If you short the high voltage circuit to other components on
the board. Please note the capacitor in the high voltage circuit will maintain a charge beyond
100V even after the power is off for over 1 hour.
8. Mixer Board (MXH-35A)
This board is an audio mixer board; it consists of Op Amps, Muting relays, Leslie control
transistors, voltage regulators and audio jacks. The six audio signals from MGH-91A enter the
board and are conditioned by integrated circuits U1, U2 and U3. U1 and U2 act as no gain low
pass filters. They have a cut off frequency at 20kHz this is to filter out the Digital noise. After
leaving U1 and U2 the HF, LF, and a PEDAL signals are sent to PAH-72A. After being processed
by the overdrive board the signals return to the mixer board and are mixed with any signal from
the send/ return loop by U4A. The signal is then sent out of the board to the A/D converter on
MGH-91A. After the DSP signal processing and D/A conversion The Left and Right audio signals
return to the mixer board to U3A and U3B. Op amps U3 provide a cut off frequency of 20kHz or
more and boost of frequencies 100Hz or less about 10dB. This signal leaves the board and goes
through the dual volume control at J206 and return to U6 and the output is sent to a headphone
jack and lineout jacks. The main Rch signal is sent to U5 for the Leslie output it goes to the 11-pin
connector through J204. U7 and Q7 control the muting relay at the time of power On/Off. There is
a 3 to 5 seconds delay to the relay turning on after the power supply is energized; the turn off time
is short. Q3 is power ON transistor for external Leslie. When organ is turned on and Leslie is
connected to organ this transistor pulls pin 6 of Leslie 11 pin plug to ground thus turning on
Leslie. Transistor Q1 is for Fast speed and Q2 is for slow. These transistors receive control voltage
from MGH-91A they pull either pin 7 (Fast) or pin 8 (Slow) of 11 pin plug to ground causing Leslie
motors to operate at proper speed. U9 and U10 are voltage regulators they Regulate the + and –
18volts to 15 volts for the op amps on the board. This regulated voltage is also supplied to other
boards through J208.
