Recently I worked on a FANUC I/O PCB, Type A20B-1001-0550/058 from a company in Nevada. Upon initial inspection I found the 6.3AMP fuse (designated F2) was blown and the fuse holder was destroyed. Traces were blown off the pcb in the vicinity of a FANUC Hybrid Module Part Number NFZ2BC6WO (designated DVA04). And the NFZ2BC6WO Hybrid Module was scorched on one side. As I was going through the pcb in search of other problems, I also found an MB74LS240 IC (designated IC7) with a shorted pin 18. This pin drives one of the inputs to the Hybrid Module at position DVA04. In addition to this the Hybrid Module Input Enable circuit had two PNP Transistors that were shorted. This board was hit hard. Luckily, though, the control logic section was intact, as I was able to power up the pcb, and found life in this area. And no IC was drawing a large current and getting hot.
Before I fixed any of the found problems, I had to determine if the NFZ2BC6WO FANUC Hybrid Module was serviceable. I reverse engineered the Hybrid Module connections on the pcb to find the inputs and outputs. From this and consultation with our lead engineer, we determined that the outputs were TRIAC based. And the inputs were Opto-Coupled. I removed the suspect Hybrid Module, and set up a test circuit. The Hybrid module passed our test. It survived the explosion.
Inside the 6.3A fuse holder (F2) is a blown fuse switch. This switch was melted. One side of the switch goes to a red LED (designated LED3) that indicates a blown fuse when illuminated. The other side of the switch is tied to IC7's ground. What appears to have happened is that when the fuse blew (due to external circumstances), the Higher Voltage arced over to the fuse-blown switch and travelled into IC7 through the ground. The trace between the fuse-blown switch and IC7's ground was blown off the pcb.
I removed and replaced the shorted IC and Transistors. I repaired the blown traces and re-installed the Hybrid Module. And I replaced the fuse holder and installed a new fuse to fix the FANUC I/O PCB. I then tested the other TRIAC Hybrid Modules good as well as the Hybrid Module Input Enable circuit.
We shipped the FANUC I/O PCB back to the customer and when they received it they installed it in their machine. Afterwards, they gave us a call to let us know that it was working and they were very pleased.