At first I replace the bushing IGUs that has been damaged and cause one

At first I replace the bushing IGUs that has been damaged and cause one axis in the z axis only moves one of the right side only, or switch to the left, apparently after replacing the result is still the same.

I decided to take off the motor on the z axis of the rails of the x-axis and 2 smooth rod. Then I use this G1 Z50 F2000 and homing, it turns out the two motors on the z axis rotates either the right or left, the part that stops just vibrate and make a sound, and sometimes the two motors that spin as usual, I tried to give the lubricant and the result was still the same ,

How do I fix this so that the two motors can work properly? I need your help.

Are they spinning the same direction? If not disconnect all power and switch one motor connector.
You might need to increase the power on the stepper driver with the small potmeter.

It is 2 motors connected in series, and they rotate in the same direction. I will try suggestions from you and I’ll let you know the results, thank you.

I’ve been turning the potentiometer, this may relate to one another at this address: http://reprap.org/wiki/RepRapPro_Setting_Motor_Currents, I get that voltage reference, I think also include, for the z-axis motor is 0.6V, but after I measuring the voltage of only 0,1V and I’ve been turning the potentiometer to maximum.

Is my Melzi PCB is damaged and must be replaced with new ones, or there are other ways to fix it?