Previously, you found your transformer was overloaded. You moved enough loads to correct that, but you still have problems with circuit board failures. Will this nightmare ever end? Yes, but not before you know what's going on with harmonics in your system. To do that, you need a harmonics analyzer or other instrument capable of providing harmonic content information (for example, a power analyzer).
Perhaps the easiest place to connect to the load side of that transformer is at the panel it serves. What you first see on the display may look far worse than it actually is, especially if you're viewing everything out to the 50th harmonic. If we were troubleshooting a 480V distribution system, we'd mostly be concerned with the 5th and 7th harmonics, but for single-phase distribution, we need to focus on the 3rd harmonic. You can ignore the others.
The 3rd harmonics present on the load side add together in the neutral and then circulate in the primary of the transformer. That can mean enough additional heat to effectively reduce the rating of the transformer. With enough harmonics, an otherwise properly specified transformer runs in saturation — resulting in wave flat-topping and other power quality problems. If harmonics are above the normal tolerance of 5% THD (per IEEE-519), then you'll have to look at the primary side.
