Blown 7i84

Hi Peter,
I blew a component on my 7i84 on my Maho mill.

History :
I had a loose screw in my pendant shorting a 24V wire, dropping the CB for the 24V rail. I removed the screw. The machine worked normally for a week.

I was using the machine to set up a job, and when I switched my spindle on it,  it dropped a CB on one of the main 3 phase CB's. That was poor judgement...

At that time  I was commanding a different spindle speed, so the gearbox comp would have tried to twitch the spindle back and fourth.  These are the pins for the twitching relays...
net activate-spindle-twitch-cw  mh400e-gearbox.twitch-cw           => hm2_5i25.0.7i84.0.2.output-06
net activate-spindle-twitch-ccw mh400e-gearbox.twitch-ccw          => hm2_5i25.0.7i84.0.2.output-07

However it could also have been one of these three...
# --- SPINDLE-CW ---
net spindle-cw   motion.spindle-forward => hm2_5i25.0.7i84.0.2.output-12
# --- SPINDLE-CCW ---
net spindle-ccw  motion.spindle-reverse => hm2_5i25.0.7i84.0.2.output-11
# --- SPINDLE-ON ---
net spindle-on   motion.spindle-on      => hm2_5i25.0.7i84.0.2.output-13

The damage is pretty clear...
 

 

1/ Do you have specific recommendations what I need to be looking as a root cause? I suspect it would be a short in the output connected to the sixth pin from the LH on the top row of that IC. Which output would that be please? Any additional troubleshooting suggestions I would appreciate.
2/ Any chance of fixing this? Any guidance appreciated.

Cheers,
Mark
 

Yeah, an instant dead short on the 24V rail could damage the output chips.
It may be repairable by replacing (probably both) NCV7608s
Unfortunately they are obsolete so a bit hard to find, I can probably
mail a couple to you if you want to try.

To make sure the damage is limited to the output chips, so worth repairing,
you could remove the output chips and see if you still have communication
with the 7I84.

Other option would be to replace it with a 7I84U (which is fully compatible
and a bit more rugged)

I had burned up a few outputs on my 7i84, was able to replace the chip, its the same one as above it in the top row. Worked ever since. As for the blown chip, do the relays have diodes? Is there any potential for a spark from AC to travel back to the mesa from the relay? I personally use mosfet output protectors to protect the expensive mesa cards

All I can add is a bit of advice on replacing the chip.
My method of removal is to carefully cut the leads on the chip, not too much pressure just lightly with an exacto knife. Then clean up the pads with desoldering braid.
There's also the flood method, but it requires some very deft action with the soldering iron.
Replacement is pretty easy, lookup the "drag method" on YouTube.
If you have experience with a proper desoldering station and hot air gun, ignore what I've said as you probably realised I'm a ham fisted Aussie.

Thanks a lot Peter and guys.
I wonder why it dropped the CB on one phase of the 3Ph power this time, rather than dropping the 24V DC CB?

The Mesa 7i84 is driving relays on the original MAHO 28A1 card. I don't see any diodes on that board, and I didn't add any.

You can sort of see the diagonal trace the current took through the IC. Which Mesa output number connects to those pins please, so I can concentrate where I am looking for the short?
Cheers;
Mark

Yes, I would cut off the pins of the IC too. Even if you have hot air, you can fry the connector.
I think these are reverse diodes. Remove the chip, check with a multimeter for a short, and power up the board.
 

3 phase CB should be 3 pole, not 3 individual 1 pole breakers, to prevent single phasing. I would also recommend installing a phase monitor and connecting to the main contactor to cut power if one phase is lost from the power grid.

Thanks a lot.

I just rechecked. You are right. There is no single phase breaker on 3Ph power in this cabinet. It was Q2, a 3ph 16A breaker which tripped.

 
Downstream of this, the 24V rail of line 204 is powered through the 2A 6F2 breaker, which did not trip this time.

 

The field power of the 7i84 is powered off line 204 (incorrectly marked here as 200).
 
It is the output-03 (TB3-20) which appears to have blown, according to the damage trace on the housing of the IC.
In my search for root cause, I dont understand why a 16A 3Ph breaker would trip if there was a short in a downstream DC circuit protected by a 2A CB.

After dinner I will work back from the 24V control relay 11K6, moving back along the wiring until I get to Q2 looking for shorts. Any other trouble shooting recommendations are much appreciated.
Cheer,
Mark

 

Is it not possible that the blown 7i84 output resulted in a constantly on state, possibly causing the fwd and rev 3 phase contractors being active at the same time? That would cause the 3 phase CB to trip.

The relays should however be hard wire interlocked to prevent this from being possible

I really appreciate you guys looking at and commenting on this. I would not have thought of that, but I never touched that part of the Maho wiring, so it still has the hardware inhibits on the main 3Ph contactors.
 

Output-03 only runs one of the 24V relays to control 24V to one of the little gearbox shifter motors.