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2025-09-14: This is a quick note for a very specific repair on Fuji Electric FRENIC MEGA Variable Frequency Drives(VFD).
Background: Customer had a remote keypad wired to the drive via the RJ-45 jack and an ethernet cable running about 50 feet. This cable got damaged (wires were shorted) and the keypad no longer worked at all - even when connected directly to the drive. The drive was operated/monitored from this remote keypad and a new keypad did not work either. On these drives, parameters can be easily transferred via the keypad, but since that was not working, there were some questions raised about being able to install a new drive without knowing the original parameters... (this was also time-sensitive - they needed to get running again as soon as possible!) A control board swap from a brand new drive was proposed just to see if the drive was ok. Everything, including the remote keypad, worked... but no parameters.
Here is an image taken while putting everything back together. Screw terminals are off-image on the bottom. The RJ-45 jack is partially visible on the blue-green board at the top center of the image. This jack has a 6-wire cable that plugs into the white header (CN3) on the main circuit board.
And this is where a sharp eye caught something important that made nearly everyone happy! Take a quick look at the green box in the image. These are a set of 3 fuses or zero-ohm resistors... and they're burned open. A quick check (while powered) shows 5VDC on the left side and no power on the right side. The side that doesn't have power leads to Pin 1 of CN3 where the RJ45 jack is plugged in. Surprise! It is only an unlabeled fuse problem!
A soldering iron was quickly retrieved from the truck. Three fuses were removed. A wire was stripped and carefully soldered in as a jumper. (image above) Board was reinstalled and powered up. Yes - there was 5VDC at Pin 1 (of CN3)! While I replaced & tested the cable, the customer ran the machine.
And yes, the repaired board is still in the VFD. The customer was warned about the additional damage potential. The new cable is routed properly inside conduit. We didn't have to take educated stabs at parameters and waste hours setting up a drive from new. Customer didn't have to pay for a replacement drive (or for the extra hours of labor making a new drive work properly) We (and the customer) now have a parameter backup just in case! Sounds like a happy ending all around. Now if only they knew who damaged the cable in the first place...