Hammond-Suzuki Leslie 122A Repair Journal and Photos
by Chad Hauris, Retro Electronics and Audio Lab, Midland, TX. 7/7/07
Please click the photos for larger views.
This 1992 model Hammond-Suzuki Leslie 122A belongs to Faith Temple
Church of God in Christ in Midland. We repaired a brand new
Hammond-Suzuki Leslie for them which had bad connections at the 6550
tube filament leads. They also had this Hammond-Suzuki Leslie from 1992
which needed repair.
When we got the unit, we found that the power leads had been
disconnected from the motors and there were bad capacitors in the amp
and the crossover unit.
This is the rear view of the Hammond-Suzuki Leslie 122A. This model
differs from the older 122 models in that it uses 2 single motors for
drum and horn rotation. The older models used four motors, two for each
rotating device. Smaller motors with a reduction drive provided through
a rubber drive wheel were used to provide the "low" or "chorale" speed.
In these newer models, an integrated circuit oscillator is used to
pulse the motors on and off at different rates to provide the fast and
slow speeds. Tachometers mounted on each motor, consisting of a slotted
rotating wheel and an LED/phototransistor unit, provide feedback to the
The oscillator signals activate MOC 3010 optocouplers, which control TRIACs connected to each motor.
The amplifier is essentially the same as the earlier models, consisting
of two 6550 output tubes, 12AU7 driver and relay control tubes, and a
This is the amp, before repairs. The filter capacitor for the output tube cathodes was leaky, as you can see at the top.
This particular Hammond-Suzuki 122A had many inter-related problems.
First of all, we found that the power leads were disconnected from the
motors. When we applied 120 volt AC to the motor coils with an
alligator clip cord, the motors would only hum and not rotate. When we
removed the motors from the cabinet, we could see that the coils looked
burned and that the rotation was not smooth when the rotors were turned
It turns out that the motors were likely destroyed by bad TRIACs. When
the control circuit board was removed, the areas around the TRIACs were
burned. The failure mode was probably caused by part of the TRIAC going
open. The TRIAC is similar to two diodes connected back-to-back which
can be turned on with a control signal at the gate. If one of the
diodes opens, the TRIAC acts as a rectifer and sends DC to the motors,
causing them to overheat. We found some confirmation of the bad TRIAC
syndrome in this Google Groups message.
Also, we found that the capacitors on the crossover board had completely blown apart.
Here are the old, bad capacitors.
To begin the repairs, we replaced the bad capacitor in the amp. We
discovered that the cathode resistor in the 6550's appeared open, so we
replaced it as well.
To repair the circuit board, we ordered new TRIACs from Mouser. New
wiring was added to complete the circuits as some of the PC board
traces were burned away by the bad TRIACs.
This is the control circuit board after repairs. The new TRIACs can be
seen at the upper left. The heatsink is for the 15 volt regulator. The
supply voltage for the regulator is provided from the 6550 output tube
Here is the new wiring on the circuit board. New TRIACs were installed,
and all of the traces leading to the TRIACs were duplicated with wire.
Here is the repaired amp. We replaced the output tube cathode capacitor
and resistor, and added a new fuse holder for the 25 volt supply to the
Repaired crossover unit.
When the system was re-assembled, the motors would not start up. The
TRIACs were tested by hooking 100 ohm resistors across the
phototransistors in the optocouplers, causing the TRIACs to turn on
properly. Then,we checked the power supply to the circuit board and
found no voltage at the regulator.
Checking the output tube cathodes, there was only 5 volts. This was
also causing the output tubes to overheat. The screen and plate
voltages also measured low.
After some more head-scratching, we found the output tube cathode
voltage returned to normal when the 12AU7 relay control tube was
removed. This was a "National" brand tube made in 1992 and when we
tested it on the tester, it showed shorted. Replacing with an RCA tube
brought back normal operation.
The 25 volt cathode voltage from the 6550's is also used in the relay
control tube circuit as well as the motor control circuit board. When
the bad tube was replaced, 25 volts was restored to the circuit board
and it worked fine.
The Leslie 122 schematic is avilable free online from "Captain Foldback".
The Hammond-Suzuki 122A circuit is very similar, with the changes
mainly being just that the motor control relay does not directly switch
the motor voltages but provides control logic to the circuit board.
Also, 25 volt DC from the output tube cathodes is provided to the
circuit board. In addition, AC is tapped off of the power transformer
high voltage winding and is padded through resistors and fed to the
circuit board. This voltage is rectified on the circuit board with two
additional diodes but it does not appear to be used in this model
circuit board. The board has lots of holes for extra components which
are not used on this particular model.
Electronics Home Page