MESA 7i76E burning out issue

A while ago, my MESA 7i76E broke; one channel just stopped working. I wrote about that here . I then found someone who repaired the card, after which the machine ran fine for half a year. He said he "replaced the resistor network" of the channel.

Today, the exact same issue appeared again. Joint 2 stopped working and after a restart it worked for a minute and went away again. This is exactly the behavior I saw last time.

This time though I was better prepared and had a spare 7i76E ready to replace the other one. After replacing it the machine ran fine - but after an hour the new card displays the same behavior, too! Clearly something is going on that breaks this channel... sometimes.

What would you do to diagnose this? Replace the JMC servo (with the built in controller) and hope for the best?

If Step/Dir resistor networks or driver chips are being damaged
it means a voltage considerably outside the 0 to 5V range is being
applied to the Step/Dir pins.

First thing I would check is drive grounding and Step/Dir wiring
Routing step/dir lines parallel to servo motor wiring might also
cause enough EMI to damage the drivers.

An intermittent short to 24V wiring is another possibility.

Thank you, that makes it more clear. I will measure all the cabling and ground connections.

Nope, 3 axis 4 joint CNC. I doubt EMI could be it, that data cable is shielded, less than a meter long and separate from power, too. But I'll check every wire.

Alright, I found out that though the error looks identical, this time the MESA card is NOT the problem. The other good news is that the problem has become reproducible and the Joint Amplifier Failure now always happens immediately during the (attempted) homing procedure.

I can see during the homing attempt that the joint that has the amplifier error attempts to move - the other joint does not. In other words, I think that the servo with the alarm is the good one, and it goes into alarm because it fails to drag the other servo along.

It is still a puzzle though. By swapping port/cables/motors it seems that the common denominator is a port/cable:

 

The red path marks the one that has amplifier error in the different scenarios that I tested.

What puzzles me is that the common denominator in all failure scenarios are panel ports 2 and 3. However, I have measured the every wire several times with an ohm meter (as shown by the green lines in the image), and they are all fine. Since they are also not moving cables I cannot imagine it is a flapping connectivity issue.

So I am a little clueless as to what else I could do now. It seems every potential source of the problem was excluded.

Maybe EMI could have something to do with it, but... why would that suddenly become a reliably reproducible problem after running years with no issue?
Anyway, I also measured all the grounds on the shields of each cable, and they also seem fine/low impedance. (star configuration off of the panel, no loops)

Any ideas what else I can check?

Perhaps an output is marginal because of previous damage

I would verify full 5V swing on all step/dir pins.

Here is a simple script to toggle all stepgen step and direction pins slowly:

 


Download then

chmod +x stepgens.txt

and

./stepgens.txt to run

Note that this needs a relatively recent mesaflash

github.com/LinuxCNC/mesaflash

Thanks a lot for your help!

Now that I hooked up an oscilloscope, the problem disappeared completely again - can't reproduce it anymore, at all.

That made generating pulses easier of course, I simply jogged the thing along. The levels of the step+/- pins on the motor side are around 4.8 volts, and -4.8 on a pulse.

I also noticed that the level between STEP- and the cable shield (GND) is around 40 volts AC (50 Hz). I can't say I understand why that would be so. Is that normal/intended?

Ack!! 40V is a real issue and indicates a grounding problem. This likely
what damaged the step/dir outputs (accidental contact to shield ground would do it)

I would verify that the drive frame grounds connect to earth ground and that the
7I76E ground (24V logic power ground) is also connected to earth ground.

Ah-ha! The 24V field power GND is not connected to earth ground, I was not sure that it should be. That probably explains it then.

Just weird that the machine ran fine for so long.

Thanks, I was getting frustrated with the constantly changing symptoms. I will connect those grounds tomorrow and test again.