[Arran]

If that is a CRT brightener and not an isolation transformer I would get rid of it if you can. It was a a common quick and dirty fix for a dim picture, not necessarily a weak picture tube, and if they stayed on there too long they would destroy whatever coating was left on the cathode. Sometimes it could simply be a case of the set's B+ being too low and hence the high voltage as well.
Regards
Arran