Question: I have an old Xermac model HP power supply and I am getting more power than I should. My surface finish is rougher than normal and the overburn is greater than it should be. How can that be? How can I debug this?
Answer: It is of course possible that one of the peak current switches is stuck in the ON position, even though the toggle switch is physically in the off position. However, it is more likely that one of the peak current relays has failed. The Xermac model HP power supply used open frame 2-pole relays. Several different manufacturers have made this relay. The current replacement is Magnecraft model #199AX-14. This unit has 40A rated contacts. The original relays built back when the HP series was in production were Potter Brumfield with only 30A rated contacts. The Magnecraft relays are the preferred modern substitute. A common failure mode occurs when repeated cycling of the contacts under load degrade the contact surface. At some point, the contacts will weld together and you will see excessive power, poor finish and increased overburn. The relays are located on the side of the hotbox inside the rear door or mounted to the sides in the rear of the enclosure. To debug, turn power off to the machine (following proper lock-out/tag-out procedure) and physically inspect the relays. If there is no motion in the contacts, there are likely welded shut.
EDM Zap offers tuning and regular maintenance for Xermac equipment.