Not sure which "underneath" you mean, but my 630TS had arcing under the little platform that holds the 1B3GT rectifier tube:
It's worth cleaning that entire area with isopropyl alcohol, whether or not that's the immediate source of arcing.
This article has other restoration notes that may be helpful in a general way:
http://antiqueradio.org/RCA630TSTelevision.htm
Regards,
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html