FANUC "Yellow Cap" servo motors -buzzing noise

[brgrii]

I have a 1980's vintage Knee mill with FANUC 3M control and the "yellow cap" servo motors. It works like a charm with one niggling problem.
Usually (especially when it is cold), on start up it gives an error that basically says a motor is not responding properly to the start signal (servo 02 error). After several minutes, something warms up and everything starts up ok. Another symptom that concerns me a bit more is that one (or maybe two) of the yellow cap motors occasionally starts emitting a buzzing noise. If I move the axis a bit, the buzzing stops.
I have to believe these two items are related.
I have been thru FANUC troubleshooting (by the way, those people are GREAT - no charge, and as much help as you need) to the point where the next step is for them to send out a tech to start swapping out $1000.00+ compenents until they find the problem, and at that time I will have 3 times what the machine cost me into a repair.

So helpful CNC-Zoners, any ideas?

I am thinking of just sending all the motors in to a re-fitter to check them all out.
I have already replaced one of the VC units.
I have dismounted and cleaned the motors and encoders.
The guy I bought the VC unit from thought maybe it was a bad encoder, but I did not find anything obviosly wrong with any of them.

I hope this is enough info for someone to point me in a direction that will not cost me my firstborn to implememt.

Thanks!!

[L. Sakthivel]

1) It seems to be a cold start problem, I think it will not be a Motor problem, Pl. check the Drive output voltage during switch ON from cold state & compare with normal state. This is not a big issue, In drives components may be week & capacitors got discharged during switch off condition thats why it took time to become normal position.

2) Humming Noise due to the axis movement not reached the command position. This may be due to the Tightness / over load of the slide or Torque of the motor is less or gain value is more. Pl. check up the position deviation of the Axes.

[mif73]

Check that the coupling between the motor and encoder is tight, if not sure, then spread the coupling with a screw driver and reassemble, should help get rid of the buzzing (hunting) noise.
you can confirm this by checking the diagnostic screen for "servo error".
*make sure you mark the position of the motor and encoder before you remove the encoder and the position of the encoder and motor shaft so you can return it to same position or the zero return point may shift by half the pitch of the ball screw.
see how you go...