Alarm 421 on Fanuc 6M

[vfsi]

I replaced the Y axis Velocity Control unit with a used known good (?) unit from a different machine.
After setting the jumpers and Trim POTs RV1, RV2, RV3, RV4, RV5 to the same position as the removed VCU, the machine is up and running.

But I get the occasional Alarm 421 while rapid traveling at 100%, it does not occur at 50%.

The Fanuc 6M repair (?? the cover is missing so I don't know the real title) manual lists the jumper and Trim pot settings for the Velocity Control Unit. For trim Pot RV4, the Tacho-generator compensation, it says:
1) normally 5 scales
2)Use for fine adjustment of loop gain. Refer to maintanance manual of NC

My question is "Which manual should I refer to?, The machine manufacturers?, Some other Fanuc manual?

Or "How do I adjust this issue out of my machine?"

The 421 does sound like an issue with the Tacho gain and over shoot.

I have an Oscilloscope and the ability to use it, just not the VCU knowledge.

Any help, comments, questions, condolences or snide remarks would be apperciated.

vfsi

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[cncdiag]

Hello
Sounds like you may have a bad motor.
Check the brushes and Megger the motor.

[vfsi]

cncdiag,

Thanks for the suggestion, but I just had a good motor shop replace the brushes and test the motor. That all checks fine.