upper, lower and pedal drawbars. The output of these ICs is fed directly into an A/D port of the
CPU U1 on the MGH-91A board from connector J103. U4 is an analog switch used to scan 4
channels of analog data from pitch wheel, modulation wheel, tone control and overdrive level.
This analog data comes from J508 on the SWH-503A board. This data is also fed to the A/D port of
the Main CPU on MGH-91A.
5. Switch boards (SWH-503A, 504A, 505A)
These three boards contain the tab switches and LED indicators and controls for the entire organ.
The SWH-503A contains the decoder and LED driver circuits for all of the switches on the XK-3.
The SWH-504A and SWH-505A boards are wired to the SWH-503A board and utilize the decoder
and LED driver circuits on the SWH-503A.
The SWH-503A board sends and receives switch and LED signals to main board (MGH-91A) on
connectors J503 and J502
6. Vibrato Mode Switch VRH-29A
This board is the rotary switch which selects the vibrato / chorus mode of C1-V3. It Connects to
J105 of MGH-91A. If you need to change the board, first remove the 4 screws holding SWH-503A
LED indicator board under the switch assembly. Then pull out the knob and remove the nut on
the rotary switch’s shaft. The switch assembly will then drop down and out of the control panel.
7. Vacuum Tube Overdrive Board (PAH-72A)
The Tube overdrive board consists of opamp (U1), dual VCA ICs (U2, U3), Tubes (V1, V2) and a
220V dc power supply circuit (D1-4) and (Q1) for vacuum tubes plate voltage.
The signals from MXH-35A enter the board at J301. There are 3 signals HF, LF and Pedal. There
are 2 parallel audio paths through tube circuits one is for (HF) high frequency and one is for (LF)
low Frequency. The Pedal signal does not pass through the Tube overdrive circuit; it bypasses it
and is mixed back in at U1B. The U3 vca controls gain of overdrive level of the vacuum tubes.
The overdrive gain is controlled by the DC voltage (0-5V) control signal from MGH-91A J120. The
U3 output signals are amplified by tubes V1 and V2. The U2 vca attenuates the overdrive signals
back to the original input levels. The signal is then mixed with pedal signal at U1B. The signal
then leaves the board at J301 and goes to MXH-35A. The control signal is programmed to rectify
according to an overdrive level by HF OUT-V and LF OUT-V this comes from MGH91A CPU. VR1
(HF) and VR2 (LF) are TRIM. Pots in order to revise volume level of TUBE ON or OFF. Please
refer to the TEST & ADJUSTMENT for the adjustment method. The Q2 is transistor is an analog
switch, if TUBE AMP ON thru signal is stopped. When TUBE AMP Off then U2 gain turned off by
MGH-91A.
