...should have adjusted the BAL pot to correct drift.
...from Glentek GA370 manual which is about the same for all DC Servo systems
snip
5.3 VELOCITY LOOP PHASING:
For proper servo operation it is necessary for the amplifier to receive negative feedback from the tachometer. If the
tachometer leads are reversed (positive feedback), the amplifier will run away. To check the phasing of the motor and
tachometer proceed as follows:
1. Make sure Current Limit potentiometer, RV5, is full CCW (as shipped from factory).
2. Make sure that nothing is connected to the Signal Input (J1-2) or the Auxiliary Signal Input (J2-1 or J2-2).
3. Apply the main power for DC Buss and the 120 volt power for the fans.
4. Slowly turn the Current Limit potentiometer, RV5, CW. If the motor starts to run away turn the Current
Limit potentiometer full CCW and reverse the motor armature leads. Again, slowly turn the Current Limit
potentiometer CW. The motor should be stopped or rotating slowly.
5. Set Current Limit potentiometer, RV5, to the desired peak current for the remaining adjustments and
operations.
Typical settings for Current Limit potentiometer, RV5 (20 turn potentiometer).
3 turns CW = 30% peak current 10 turns CW = 85% peak current
6 turns CW = 60% peak current 14 turns CW = 95% peak current
5.4 TACH GAIN ADJUSTMENT:
1. At this point the motor will be rotating slowly. Adjust the Balance potentiometer, RV6, until the motor
rotation is stopped.
endsnip
At this point after reading your description of all the things you have changed/tried/swapped around. I would guess the Original runaway problem was/is the Fadal 1010 X axis board, but first need to get the X Amp adjusted correctly. Which AMP's do you have ? Do you have the manual for your Modal number?