I have a hot springs prodigy spa 110v - I believe it's 1990. After draining and refilling i plugged in the spa and it began to spark where the heater plugs into the box (receptacle). I unplugged the spa and saw that the heater plut and the J &J recepticle had melted. I unplugged the heater from the control box and everything else seems to be working fine. I took to box out to see if anything inside had melted but everything looks okay.
Inside are 3 relays marked 1,2,3. The heater is number 3. The contacts on this seem a little more worn and pitted than the other two and are blackish compared to the others which are white. Any ideas what might have happened here? I just acquired a used heater and new recepticle. Should I just replace and go or could this be the relay's fault?
I can post photos if that would help.

Probably a power surge of some kind. I would go ahead and replace the relay just in case.
It's not unusual to see the heater receptacles burnt.
Look in your equipment area for a serial number sticker. It will start with an H. The 1st number after the H is the quarter, and the 2nd number is the year made (H101431) is 1st quarter 1990.
The relay part number for a 1990 is 70297.

Pete