[Welsh Anorak]

Well, it's definitely to do with the focus. As you can't see into the ceramic base, it's just possible the discharge is happening beghind the tube base insulator. Carefully prise it off and have a look there. Clean it then (again carefully!) put the CRT base back witout the insulator in place. Ive had them do this and appear innocent and this will prove it. If no good then it's tme to turn the lights out and look in the CRT neck which may well confirm your worst fears.