zook:
A hi-freq. (HF) is used to superimpose a high voltage-low amperaqe current into the welding circuit to initiate an arc and or maintain a continuous HF circuit when using alternating current. This unit is used to modify an existing piece of CC arc welding equipment so that scratch starting is no longer required.
This unit will enable your machine to weld aluminum or magnesium, etc. I highly recommend you contact Lincoln Electric (easy to find on the web). I'm 99.9% sure that HF unit will work on your old Idealarc power source.
Good luck and have fun.