dead Outputs on Mesa 7i76E and blown up 7i85S

Hello everyone,

Yesterday, I managed to blow up two Mesa boards within just a few minutes, and I’m still not sure why it happened or how to avoid this in the future.

The machine is a retrofitted Schaublin 125CNC lathe, which had been running flawlessly—until last Friday’s coffee break. When I came back, I found the central lubrication pump had burned out, apparently because it had been running continuously.

I discovered that output 6 and output 7 were permanently high as soon as the card was powered up. These outputs are connected to 400V 3-phase contactors (for the lubrication and coolant pumps), each with a 24V coil and a flyback diode directly across the coil. Each contactor draws about 275 mA.

I’m not sure which output failed first—or whether both failed at the same time. The coolant pump is already configured in HAL but hasn’t yet been wired to its contactor, so I can only confirm that the lubrication pump was not being controlled by LinuxCNC at all—it isn’t connected in HAL.

After powering down the machine and disconnecting the wire between the lubrication pump’s contactor and output 6, I turned the power back on. While sitting in front of the control cabinet and moving some wires around, I saw smoke coming from the 7i85S board next to the 7i76E. It appears that capacitors C22 and C24 have blown, and one IC also looks suspicious.

I’m only using the encoder inputs of the 7i85S, and those still seem to be functional. I suspect that one of the unconnected step/dir outputs may have been damaged.

While I can accept that switching contactors directly might not have been ideal (even if within spec), I don’t understand what could have caused the damage on the 7i85S—especially since nothing was connected to any of its outputs.

During my investigation, I also realized that I had used the VIN pin on the field power connector instead of pins 1, 2, 3, or 4—so field power was supplied through the internal jumper. Could that have caused issues?

By the way, this part of Norbert’s connection sheet (page 2) may be a bit misleading or unclear in that regard.

Both boards are still working, aside from the two damaged outputs and possibly one or more differential outputs on the 7i85S. Also worth noting: the 24V supply to the cards is not switched on the DC side; the PSU is switched on the AC side.

Should I replace the boards anyway, or are they still safe to use going forward?

Thanks in advance for any help or insights!

— Max
 
         

Not sure about the 7I76E outputs, but the 7I85S seems to have been damaged by an overvoltage on the the 5V
supply, possible related to the original fault. It appears that both the IS32174 step/dir driver and U11, the LMV342
have been damaged.

7I85 U11 is part of the encoder circuitry when TTL mode is used, it will not affect differential mode encoders
so the 7I85s will probably continue to function for encoder inputs if they are differential, BUT the 5V situation
needs to be checked.

On the 7I76E, the damaged driver chip (NCV7608) should be replaced as it may fail further and perhaps cause
further damage.

Thank you for your response, Peter.
Is the NCV7608 still available? I only found one listing on Aliexpress, which makes me wonder if it’s becoming hard to source.

While taking a closer look at the board, I found a small piece of cut wire lying around in that area:
 
 

Could this have caused the issue?If not, what else could I check to resolve the 5V overvoltage?
The 7i85S is powered via the 25-pin connector from the 7i76E.
Thanks again for your help!
–
Max

Picture 2, that looks like arcing across those two parts, moisture can do that, but the rest of the boards look like there is no moisture in quantities to cause that.

Thanks, Tommy.
The shop is heated and very dry, especially during the German winter, so moisture is probably not the issue.
-
Max

That wire is on a 5V regulator pin, so maybe related, hard to tell.

Yes, NCV7608s are obsolete, we do have spares if you need a few.