[sliptonic]

I have a Yeong Chin Supermax (Bridgeport clone) circa 1989. It has servo dynamics motors and amps.
motors (MT30U4-42)
amps SDF-1525-12

The machine has been retrofit to linuxcnc using Mesa 5i25/7i77 control. It's been running this way for a couple years.

Recently the Y axis started making a loud growling sound and tripping the RMS protection on the amp. I've disconnected the motor and the table moves freely. No binding or lack of lube.

I swapped the X and Y amps and the problem stays with the motor.

Running the motor disconnected from the lead screw, it vibrates badly and acts like the PID loop is horribly tuned. Without the power on, the motor turns smoothly by hand.
Using the linuxcnc halmeter and watching the encoder output while I turn it by hand I see normal encoder output (as far as I can tell).

I pulled the brushes and they seem to have life in them but I honestly don't know what they should look like.

I'm running out of ideas. Anyone point me in a direction?

[sliptonic]

A bit more playing around yielded another clue. If I run the motor fast in one direction for a while, it gets smoother. Stop and restart and it's rough again.

[machinehop5]

A bit more playing around yielded another clue. If I run the motor fast in one direction for a while, it gets smoother. Stop and restart and it's rough again.

...check Tachometer feedback from motor. Most likely dirty brushes on Tach side

[sliptonic]

I think you nailed it! I pulled brushes and put a shop vac on one side with a filter. I pulled out a shaving the size of a fingernail clipping. Motor running smooth so far. Thank you!

[sliptonic]

My problem started up again. Motor running rough, especially on startup. The previous fix (cleaning brushes on the tach side) made such a difference I decided to repeat the process.
This time I took the motor out completely and disassembled it. Lots of graphite dust in the housing which I cleaned up and thoroughly blew out the tach housing and the stator.

I put it back together and it moves smoothly by hand. I had taken pictures of all the connections and reconnected it carefully. I checked the encoder outputs in linuxcnc and I'm getting good counts.
But when I enable the motors, it spins violently in one direction and then throws a following error in linuxcnc.

I double-checked the tach connections and output from them all the way to the amps and it looks good. I can't think what else I could have messed up.

Any ideas?

[machinehop5]

page 16 ...out of Phase tach wiires backwards or motor wires