Motor Drive Amplifier
4840393
RTZ Professional Audio, LLC
4
6. OFFSET TRIMMER ADJUSTMENT
FOR USE WITH DTC-1200 DIGITAL
TRANSPORT CONTROLLER
Trimmers R3 and R4 are 10-turn precision
trimmers that allow adjusting the input reference
voltage to the op-amp motor drive servo loop.
These trimmers allow adjusting the starting point
at which the motors first begin to develop torque
and begin to spin. This alignment procedure is
best done with the DTC-1200 diagnostic MDA
DAC zero alignment procedure available in the
DTC TTY configuration console.
Open the DTC-1200 serial TTY configuration
console window, select the diagnostic menu and
then select option #5
“MDA DAC zero adjust” to
output the DAC reference values. Make sure no
tape is loaded on the machine before running
this diagnostic procedure as the reel motors will
spin.
Advance the DAC output level to 75 at the
prompt and adjust each trimmer until the motor
just begins to develop torque and spins as
slowly as possible. Adjust each motor to spin
closely to the same speed. Do not adjust the
trimmers to either end stop extreme. In general,
you should not need to adjust the trimmer in
either direction more that 2-3 turns. This
calibrates DAC level 75 as the starting torque
point for the constant tension servo loop.
7. OFFSET TRIMMER ADJUSTMENT
FOR USE WITH LEGACY AMPEX
CONTROLLER
The RTZ MDA card works with the original
Ampex controller and is optimized to work with
the RTZ DTC-1200 Digital Transport Controller
card. It includes offset adjust trimmers to allow
adjustment of the starting torque point for each
reel motor.
The MDA error inputs are 5mA current loop
interfaces. The offset trimmers allow adjusting
the point at which the motor first begins to
develop torque for a given input current. The
motor should drive full torque at 1mA or less,
and no torque at 5mA current present at the
error input.
Remove the MDA card from the machine and
adjust multi-turn trimmers R3 and R4 both to the
mid-scale position (approximately 5-turns). You
can measure the trimmer resistance (across the
wiper and an end stop pad on the PCB) with a
DVM. Adjust the trimmer for 1.1K ohm center
scale with the MDA card removed from the
machine.
The RTZ MDA offset adjust trimmers can be
calibrated by removing the transport controller
card and connecting a couple 4.8K resistors to
the MDA take-up
and supply “error input” pins to
ground. Connect a resistor lead to MDA edge
connector pin 8 or J and the other lead to
ground. Connect another resistor to pin 10 or L
and ground. Note you can also access the MDA
error inputs via TP2 & TP3 on the RTZ MDA
card.
The 4.8K shunt resistors provide 5mA current to
the MDA error input to signal no torque from the
motor. At 1mA and lower, the motor should run
at full torque. The Ampex transport control card
extender is another easy way to gain direct
access the MDA current loop error input signals.
Once you have the shunt resistors connected,
fire up the machine and observe the reels for
any motion. If no motion, adjust the
corresponding offset trimmer counter clockwise
up to 1-3 turns and motion should start. Adjust
just until till no motion is present. If the motor is
already running, adjust clockwise 1-3 turns till it
stops. Generally, the offset trimmer should have
around 1.1k resistance (or nearby).
Do not turn the trimmer to end stop extremes. If
unsure, kill the power, remove the MDA card
and measure across the trimmer center and end
stop and adjust for around 1.1k if necessary.
Basically, we want to apply 5mA current loop
load to the MDA inputs, and then adjust till
motion just stops fully. No further adjustment
should be required on the MDA unless major
components or the line voltage change. This
completes the MDA balance and input level
calibration procedure. Go back and align the
Ampex controller overall gain and readjust all
the tension settings as usual.