Nels

There is a distinct difference between the earlier model 3500 Transient Attack and the HA 3500 in that being the power transformer. The 3500 TA has a shell-core transformer. The HA 3500 has a toroidal transformer. They aren't interchangeable because of physical design and wiring, and the shell core is unavailable, while the HA model replacement transformer is available from Compass Systems online.